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Genetic and phenotypic correlation in natural a population of song sparrows

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Abstract

We estimated heritabilities, and genetic and phenotypic correlations between beak and body traits in the song sparrow (Melospiza melodia). We compared these estimates to values for the same traits in the Galápagos finches, Geospiza (Boag, 1983; Grant, 1983). Morphological variance is low in the song sparrow, and our results show that genetic and phenotypic correlations are considerably lower than correlations in the morphologically more variable Geospiza. Comparison using a larger sample of Galapagos populations confirms the existence of an association between variance and correlation for phenotypic values. We suggest two possible explanations for this association. First, most traits studied are functionally related, and the joint evolution of variance and correlation may have resulted from stabilizing selection about a line of optimal allometry between traits. Alternatively, introgression between populations and species could have caused correlation and variance to evolve jointly. Both selection and introgression were probably influential in producing the observed pattern, but it is not possible to estimate their relative importance with current data. Genetic and phenotypic correlations were correlated in the song sparrow, but heritabilities of traits varied greatly. As a result, the genetic variance-covariance matrix for traits is not simply a constant multiple of the phenotypic matrix. Evolutionary response to natural selection cannot, therefore, be predicted from the measurement of phenotypic characteristics alone.

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... All measurements (in o.u.) were firstly log e -transformed to remove dependence of variances on means and finally corrected to remove sexual dimorphism in the mean value of each trait. This cor-rection was done by adding to male measurements the difference between female and male means (Schluter & Smith, 1986). ...
... where 00 refers to mid-parent and 0 to offspring values. The average of r 1 and r 2 is the best estimate of the genetic correlation (Schluter & Smith, 1986;Lofsvold, 1986;Merila & Gustafsson, 1993). Since the relative similarity between r 1 and r 2 can be used as a measure of their robustness (Schluter & Smith, 1986;Merila & Gustafsson, 1993), Mantel's test (see below) was used to verify similarity between both genetic correlation matrices estimated with r 1 and r 2 values. ...
... The average of r 1 and r 2 is the best estimate of the genetic correlation (Schluter & Smith, 1986;Lofsvold, 1986;Merila & Gustafsson, 1993). Since the relative similarity between r 1 and r 2 can be used as a measure of their robustness (Schluter & Smith, 1986;Merila & Gustafsson, 1993), Mantel's test (see below) was used to verify similarity between both genetic correlation matrices estimated with r 1 and r 2 values. An overall similarity between these matrices was verified (Mantel's test: r = 0.7, P = 0.01). ...
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Article
Genetic variation in correlations among size-related traits of head, thorax and wings was examined in Drosophila buzzatii, by comparing the correlation pattern of the phenotypic correlation matrix (CP-Rp) between inversion karyotypes of the second chromosome. CP-Rp differed between some karyotypes in a natural population. CP-Rp in homokaryotypic classes of wild-reared flies, but not in heterokaryotypes, differed from the whole population represented by laboratory-reared flies. Similarity in CP-Rp was highly significant for a same homokaryotype in two populations. In one of them, the chromosome is polymorphic for four inversions. In the other population, one of the inversions is almost fixed. CP-Rp was significantly similar between these populations, illustrating that similarity of CP-Rp may even occur between populations which have greatly diverged in frequencies of some genotypes affecting correlation patterns. It is suggested that chromosomal inversions are factors affecting genetic correlations among traits known to be phenotypically correlated with fitness components.
... All measurements (in o.u.) were firstly log e -transformed to remove dependence of variances on means and finally corrected to remove sexual dimorphism in the mean value of each trait. This cor-rection was done by adding to male measurements the difference between female and male means (Schluter & Smith, 1986). ...
... where 00 refers to mid-parent and 0 to offspring values. The average of r 1 and r 2 is the best estimate of the genetic correlation (Schluter & Smith, 1986;Lofsvold, 1986;Merila & Gustafsson, 1993). Since the relative similarity between r 1 and r 2 can be used as a measure of their robustness (Schluter & Smith, 1986;Merila & Gustafsson, 1993), Mantel's test (see below) was used to verify similarity between both genetic correlation matrices estimated with r 1 and r 2 values. ...
... The average of r 1 and r 2 is the best estimate of the genetic correlation (Schluter & Smith, 1986;Lofsvold, 1986;Merila & Gustafsson, 1993). Since the relative similarity between r 1 and r 2 can be used as a measure of their robustness (Schluter & Smith, 1986;Merila & Gustafsson, 1993), Mantel's test (see below) was used to verify similarity between both genetic correlation matrices estimated with r 1 and r 2 values. An overall similarity between these matrices was verified (Mantel's test: r = 0.7, P = 0.01). ...
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Article
Recent studies have shown that body size is a heritable trait phenotypically correlated with several fitness components in wild populations of the cactophilic fly Drosophila buzzatii. To obtain further information on size-related variation, heritabilities as well as genetic and phenotypic correlations among size-related traits of several body parts (head, thorax and wings) were estimated. The study was carried out on an Argentinean natural population in which size-related selection was previously detected. The genetic parameters were estimated using offspring-parent regressions (105 families) in the laboratory G2 generation of a sample of wild flies. The traits were also scored in Wild-Caught Flies (WCF). Laboratory-Reared Flies (LRF) were larger and less variable than WCF. Although heritability estimates were significant for all traits, heritabilities were higher for thorax-wing traits than for head traits. Phenotypic and genetic correlations were all positive. The highest genetic correlations were found between traits which are both functionally and developmentally related. Genetic and phenotypic correlations estimated in the lab show similar correlation pattern (r = 0.49; P = 0.02, Mantel's test). However, phenotypic correlations were found to be typically larger in WCF than in LRF. The genetic correlation matrix estimated in the relatively homogeneous lab environment is not simply a constant multiplicative factor of the phenotypic correlation matrix estimated in WCF.
... In hotter environments, larger bills are hypothesized to aid in thermoregulation by dissipating more dry heat in response to high ambient temperatures compared to smaller bills (Tattersall et al., 2016). The high heritability of bill dimensions (Boag, 1983;Grant, 1983;Jensen et al., 2003;Keller et al., 2001;Schluter & Smith, 1986) suggests the bill may readily evolve in response to selection and provides further justification for examining how climate may result in local adaptation in bills. However, changes in the avian bill represent just one of many potential evolutionary mechanisms to cope with thermal stress and climate variation, and selection probably operates on a suite of traits involved in thermoregulation. ...
... Our estimates of SNPbased heritability are relatively high (SNP-based h 2 mean =0.34) and support the hypothesis that population differences are not strictly a result of plasticity. These estimates are comparable to those in other song sparrow populations (Schluter & Smith, 1986;Smith & Zach, 1979) and to contemporary estimates of bill heritability in other passerines (Jensen et al., 2003;Keller et al., 2001;Lundregan et al., 2018). Moreover, given that we certainly missed many loci associated with bill morphology due to a relatively low density of loci ...
Article
Disentangling the effects of neutral and adaptive processes in maintaining phenotypic variation across environmental gradients is challenging in natural populations. Song sparrows (Melospiza melodia) on the California Channel Islands occupy a pronounced east‐west climate gradient within a small spatial scale, providing a unique opportunity to examine the interaction of genetic isolation (reduced gene flow) and the environment (selection) in driving variation. We used reduced representation genomic libraries to infer the role of neutral processes (drift and restricted gene flow) and divergent selection in driving variation in thermoregulatory traits with an emphasis on the mechanisms that maintain bill divergence among islands. Analyses of 22,029 neutral SNPs confirm distinct population structure by island with restricted gene flow and relatively large effective population sizes, suggesting bill differences are likely not a product of genetic drift. Instead, we found strong support for local adaptation using 3,294 SNPs in differentiation‐based and environmental association analyses coupled with genome‐wide association (GWA) tests. Specifically, we identified several putatively adaptive and candidate loci in or near genes involved in bill development pathways (e.g., BMP, CaM, Wnt), confirming the highly complex and polygenic architecture underlying bill morphology. Furthermore, we found divergence in genes associated with other thermoregulatory traits (i.e., feather structure, plumage color, and physiology). Collectively, these results suggest strong divergent selection across an island archipelago results in genomic changes in a suite of traits associated with climate adaptation over small spatial scales. Future research should move beyond studying univariate traits to better understand multidimensional responses to complex environmental conditions.
... Published genetic covariances for ecologically relevant morphological traits were obtained for the Galapagos medium ground finch Geospiza fortis (Boag 1983), song sparrow Melospiza melodia (Schluter and Smith 1986a), collared flycatcher Ficedula albicollis (Merila et al. 1994), and Peromyscus mice (Lofsvold 1986). (Co)variances were based on In-transformed traits in most cases; in the remaining cases I used the squared coefficient of covariation instead. ...
... I consider this second alternative less likely than the others for the following reasons. Not all species in taxa sampled hybridize currently, for example, the focal sparrow species does not (Schluter and Smith 1986a; gene flow between geographically differentiated song sparrow populations could, however, produce the same effect if the morphological axis of population differentiation is similar to that between closely related species). The stickleback species used here (Enos Lake limnetic) possesses a unique allele that is not present in its sympatric congener (McPhail 1984), suggesting that gene flow is low (but unidirectional gene flow from the benthic to the limnetic is not ruled out by this observation). ...
Article
Are measurements of quantitative genetic variation useful for predicting long-term adaptive evolution? To answer this question, I focus on gmax , the multivariate direction of greatest additive genetic variance within populations. Original data on threespine sticklebacks, together with published genetic measurements from other vertebrates, show that morphological differentiation between species has been biased in the direction of gmax for at least four million years, despite evidence that natural selection is the cause of differentiation. This bias toward the direction of evolution tends to decay with time. Rate of morphological divergence between species is inversely proportional to θ, the angle between the direction of divergence and the direction of greatest genetic variation. The direction of greatest phenotypic variance is not identical with gmax , but for these data is nearly as successful at predicting the direction of species divergence. I interpret the findings to mean that genetic variances and covariances constrain adaptive change in quantitative traits for reasonably long spans of time. An alternative hypothesis, however, cannot be ruled out: that morphological differentiation is biased in the direction gmax because divergence and gmax are both shaped by the same natural selection pressures. Either way, the results reveal that adaptive differentiation occurs principally along "genetic lines of least resistance."
... G. scandens and G. fortis: weight, wing cord, tarsus length, four bill dimensions (Boag, 1983). M. melodia: wing length, tarsus length, three bill dimensions (Schluter & Smith, 1986). Schluter & Smith, 1986). ...
... M. melodia: wing length, tarsus length, three bill dimensions (Schluter & Smith, 1986). Schluter & Smith, 1986). The correlations between the genetic and phenotypic correlations, however, show a wide variation (Fig. 5, Table 2). ...
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Article
The estimation of genetic correlations is central to the study of evolutionary change in populations. However, sample sizes required to achieve a small standard error are typically enormous. This precludes large-scale comparative analyses. Cheverud has conjectured that in some circumstances the phenotypic correlation can be substituted for the genetic correlation. This suggestion is examined using a large set of morphological traits in the sand cricket, Gryllus firmus. In this case the difference between the two estimates is very small. Further, by simulation it is shown that the phenotypic correlations are as good as, or better than, the estimated genetic correlations as estimates of the true genetic correlations. Examination of other data sets of morphological traits suggests that the phenotypic correlation may, in general, be a suitable substitute for the estimated genetic correlation. However, because the number of such examinations is still small, a protocol is suggested in which two sets of genetic analyses are undertaken to confirm the assumption in a large comparative analysis.Keywords: bias, evolution, genetic correlation, Pearson product-moment correlation, phenotypic correlation
... An implicit assumption we have made throughout this paper is that phenotypic correlations/covariances are at least qualitatively similar to the underlying genetic correlations. This assumption is founded on the fact that several studies, including an analysis of 41 pairs of phenotypic/genetic correlation matrices (Cheverud, 1988), have found high similarity between phenotypic and genetic correlations (Schluter & Smith, 1986b; Merila È & Gustafsson, 1993; Koots & Gibson, 1994; Roff, 1995). For example, a re-analysis of avian morphometric data indicated close to 1:1 relationship between phenotypic and genetic correlations (Roff, 1995). ...
... Furthermore, our main conclusion remains unchanged even if the correspondence between genetic and phenotypic correlations is not perfect, given that they have a similar sign, which is likely to hold true (e.g. Cheverud, 1982; Schluter & Smith, 1986b; Merila È & Gustafsson, 1993; Roff, 1995). ...
Article
Theory predicts that genetic and phenotypic correlations among traits may direct the process of short-term evolution by limiting the directions of variation available to natural selection to act on. We studied correlations between 14 skeletal traits in 10 geographically distinct and relatively young greenfinch (Carduelis chloris) populations to unravel whether the divergence among populations has occurred into directions predicted by the within- population correlations (cf. drift/correlated responses models), or whether it is better explained by 'adaptive' models, which predict no necessary association between within- and among-population correlations (allometries). We found that the within-population character correlations (or covariances) did not predict character divergence between populations. This was because the first eigenvector of the among-population correlation/covariance matrix, summa- rizing the major dimension of divergence, was a bipolar body:beak dimension, and distinct from the ( isometric) first eigenvector of within-population matrix. Hence, as the divergence among greenfinch populations cannot be satisfactorily accommodated by drift/correlated response models, an adaptive basis for divergence is suggested. The second major axis of within-population variation was a classical 'group size' factor revealing that beak size was more or less free to vary independently of body size. Consequently, even if the divergence among populations cannot be simply accommodated to expecta- tions of drift and correlated response models, it is striking that the most pronounced size-independent (nonallometric) changes had occurred along the second largest dimension of variance. This could mean that selection pressures which shape integration within populations are the same as those that cause divergence among populations. A relaxed beak:body integration could also occur as a result of species level selection favouring taxa in which independent evolution of beak and body is made possible.
... The partition ofshape from size provided by the path model allows novel tests of several important hypotheses in the study of morphological evolution. We may ask, for example, whether stabilizing selection for particular allometric relationships is common (e.g., Schluter and Smith, 1986) or whether developmental programs regulating allometry are usually conserved under selection. Similarly, the model encourages tests of the hypothesis that directional selection often occurs purely on general size, which could lead to allometric maladaptation (e.g., Gould, 1974), in comparison with selection jointly on general size and shape allometry. ...
Article
This paper describes a path model for the analysis of phenotypic selection upon continuous morphological characters. The path-analysis model assumes that selection occurs on unmeasured general size and shape allometry factors that summarize linear relations among sets of ontogenetically, phylogenetically, or functionally related traits. An unmeasured factor for general size is considered the only aspect of morphometric covariance matrices for which there is an a priori biological explanation. Consequently, selection coefficients are derived for each measured character by holding constant only a general size factor, rather than by using multiple regression to adjust for the full covariance matrix. Fitness is treated as an unmeasured factor with loadings, representing directional selection coefficients, computed as the covariances of the size-adjusted characters with the measured fitness indicator. The magnitudes and signs of the selection coefficients, combined with biological insight, may suggest hypotheses of selection on one or more shape allometry factors. Hypotheses of selection on general size and shape allometry factors are evaluated through cycles of measurement, analysis, and experimentation, designed to refine the path diagram depicting the covariances among the measured characters, the measured indicator of fitness, and unmeasured factors for morphology and fitness. The path-analysis and multiple-regression models were applied to data from remeasurement of Lande and Arnold's (1983) pentatomid bugs and to Bumpus's (1899) data on house sparrows. The path analysis suggested the hypothesis that variation in bug survivorship was an expression of directional selection on wing loading. Bumpus's data are consistent with a hypothesis of stabilizing selection on general size in females and directional selection for small wing size relative to body size in males.
... In two decades studies of natural animal and addition to the direct effects of her genes, plant populations have demonstrated that the mother may also affect the offspring, for many morphological, behavioral and life-example, through cytoplasmic factors, egg history traits often are heritable and genet-size, lactation performance, or parental care ically correlated (Arnold, 1981;Gustafsson, (Falconer, 1965Janssen et al., 1988;Mitchell-Olds and Rutledge, 1986;Sinervo, 1990;Mousseau and Dingle, 1991). Palmer and Dingle, 1986; Schluter and Such maternal effects will affect the resem- Smith, 1986; Boag and van Noordwijk, blance between relatives and hence bias es-1987; Mousseau and Roff, 1987; Brodie timates of heritabilities and genetic corre-1989). To understand the genetic con-lations (Cheverud, 1984; Kirkpatrick and straints in evolution more fully, however, Lande and Price, 1989). ...
Article
We present heritability estimates for final size of body traits and egg size as well as phenotypic and genetic correlations between body and egg traits in a recently established population of the barnacle goose (Branta leucopsis) in the Baltic area. Body traits as well as egg size were heritable and, hence, could respond evolutionarily to phenotypic selection. Genetic correlations between body size traits were significantly positive and of similar magnitude or higher than the corresponding phenotypic correlations. Heritability estimates for tarsus length obtained from full-sib analyses were higher than those obtained from midoffspring-midparent regressions, and this indicates common environment effects on siblings. Heritabilities for tarsus length obtained from midoffspring-mother regressions were significantly higher than estimates from midoffspring-father regressions. The results suggest that this discrepancy is not caused by maternal effects through egg size, nor by extra-pair fertilizations, but by a socially inherited foraging site fidelity in females.
... Field studies have helped to create a new consensus that strong selection is not uncommon (Endler 1986;Kingsolver et al. 2001). Information on selection is being combined with quantitative genetic techniques to make detailed predictions about the course of evolution (Price et al. 1984;Smith 1986a, 1986b;Stratton 1994;Campbell 1996;Dudley 1996aDudley , 1996b. However, tests of those predictions are difficult and have rarely been carried out. ...
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Article
The study of natural selection in laboratory systems undergoing experimental evolution can provide important insights into the relationship between natural selection and adaptation. We studied selection on the norm of reaction of age at first reproduction in a laboratory population of Drosophila melanogaster. This population had been selected on a discrete generation schedule in the laboratory for more than 600 generations. Using genetically marked strains, we studied development time, size, female fecundity, and viability of flies that began development at different times relative to the initiation of each bottle. Only flies that began development within 30 h of the initiation of the bottle were reliably able to eclose before the next transfer. Theory predicts that flies initiating development around this critical time should decrease size at maturity to ensure eclosion by the 14‐d deadline, but late flies are not smaller. This result suggests an unknown constraint on response to selection on age at maturity in this population. Ultimately, laboratory systems provide the best opportunity for the study of natural selection, genetic variation, and evolutionary response in the same population.
... The similarity between P and both A and D is moderate to strong, and reflects a general correspondence between the phenotypic and genotypic patterns of morphological integration . This is consistent with the findings of most other studies investigating phenotypic and genetic covariance structure of morphology (e.g., Bailey 1956; Leamy 1977; Atchley et al. 1981; Boag 1983; Lofsvold 1986; Schluter and Smith 1986; Merila and Gustafsson 1993 ). The eigenvector analysis indicates that this similarity is due primarily to a single functional component of body morphology, the defensive complex. ...
Article
Phenotypic, genetic, and environmental variances and covariances for 33 morphometric traits were estimated for a population of threespine stickleback, Gasterosteus aculeatus, from the Brush Creek drainage, California, by sib analysis of laboratory-bred families. Heritabilities of the morphometric traits ranged from −0.28 to 0.78, and were moderately low (mean h2 = 0.26); the mean and range of heritabilities for five phenotypic eigenvectors were similar. The average coefficient of genetic determination of the traits and eigenvectors was high (0.57 and 0.63, respectively), indicating a substantial genotypic contribution to variation in body morphology. The defensive complex, a functional set of bony armor structures, was genetically and environmentally integrated: genetic factors (e.g., pleiotropy) are reinforced by environmental factors to produce a functional phenotype. Other components of morphology, including body form, were environmentally, but not genetically, integrated. Given the importance of genetic factors to evolutionary change under natural selection, these results implicate natural selection in the evolution of the defensive complex; the role of natural selection in the evolution of other components of morphology is equivocal. Genetic integration of functionally (phenotypically) independent traits suggests that stochastic processes or pleiotropic mutation also have played a role in the evolution of morphology in this population of sticklebacks.
... Bortolotti 1984, Francis and Wood 1989). However, measurement error can be assessed properly only when it is evaluated relative to variation among individuals in a sample (Schluter andSmith 1986, Bailey andByrnes 1990). ...
... Further, average correlations among traits was only 0.22 in this population, and the two first principal components accounted for only about 50% of the total variance, which is lower than in many other species (e.g. Schluter and Smith, 1986). This too, is expected under this model. ...
Article
We investigated the possible causes of the evolution of sexual size and shape dimorphism in the great tit (Parus major) by using two different approaches. First, we used the equilibrium approach, i.e. analysing current selection to see whether it was possible to find directional selection in the direction of the dimorphism, or stabilising selection maintaining dimorphism at its current level. Second, we used the historical approach, i.e. putting the degree of dimorphism in a phylogenetic perspective to analyse what kind of changes (if any) have occurred. This was carried out in the following way: (i) we described the level of sexual dimorphism in a population of Swedish great tits by means of path model. (ii) We used the path model design to analyse survival and reproductive selection in this population. (iii) We compared the level of dimorphism in relation to size in the great tit with that of the closest congener, the blue tit P. caeruleus. (iv) We compared the amount of interspecific morphological variation with that which would be expected under a drift model.
... In some organisms, single-generation experiments can be done relatively easily in the field. In monogamous birds, for example, offspringparent regression has often been carried out with natural matings in natural populations (e.g., Schluter and Smith 1986;reviewed in Boag and van Noordwijk 1987). Since the magnitude of genetic variance and covariance can be strongly affected by the environment (genotype by environment interaction), confining the experiments to unnatural environments can be a serious shortcoming if quantitative estimates of variance are of interest. ...
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Article
Artificial selection has been practiced by humans since the dawn of agri-culture, but only recently have evolutionary ecologists turned to this tool to understand nature. To perform artificial selection, the phenotypic trait of interest is measured on a population, and the individuals with the most extreme phenotypic values are bred to produce the next generation. The change in the mean of the selected trait across each generation is the response to selection, and other traits can also evolve due to genetic correlations with the selected trait. Artificial selection can directly answer the question of how quickly a trait will evolve under a given strength of selection. This kind of result can help ecologists determine whether range or niche boundaries are determined by a lack of variation for a key phenotypic trait or trade-offs due to genetic correlations with other fitness-related traits. In a related approach, controlled natural selection, the organisms are not selected according to their values for a given trait, but rather are allowed to evolve for one to several generations under experimentally imposed environmental treatments such as temperature, light, nutri-ents, presence or absence of predators or competitors, etc. The results of this kind of study can tell us how quickly a population can adapt to a given environmental change, either natural or anthropogenic. Finally, artificial selection can create more variation for mea-surements of natural selection or can be coupled with QTL mapping; both these combi-nations provide new insights into adaptation. I discuss advantages and disadvantages of these approaches relative to other kinds of studies and highlight case studies showing how these tools can answer a wide range of basic and applied questions in ecology, ranging from niche and range boundaries and character displacement to climate change and invasive species.
... In addition, divergence in covariance structure among greenfinch populations, despite generally conservative morphological patterns across cardueline finches (Bjö rklund 1993(Bjö rklund , 1994Bjö rklund and Merilä 1993), was associated with low integration between bill traits and body traits (Merilä and Bjö rklund 1999; see also Schluter and Smith 1986;van den Elzen et al. 1987;Merilä 1997;Badyaev and Martin 2000a). Decoupling of bill and body trait evolution in finches may be enabled by differential phenotypic and genetic patterns of growth between these groups of traits (Boag 1984;Bjö rklund 1993;Badyaev and Martin 2000b). ...
Article
Abstract Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change by limiting the amount and form of genetic variation available to natural selection. We studied evolution of morphological variance-covariance structure among seven populations of house finches (Carpodacus mexicanus) with a known phylogenetic history. We examined the relationship between within- and among-population covariance structure and, in particular, tested the concordance between hierarchical changes in morphological variance-covariance structure and phylogenetic history of this species. We found that among-population morphological divergence in either males or females did not follow the within-population covariance patterns. Hierarchical patterns of similarity in morphological covariance matrices were not congruent with a priori defined historical pattern of population divergence. Both of these results point to the lack of proportionality in morphological covariance structure of finch populations, suggesting that random drift alone is unlikely to account for observed divergence. Furthermore, drift alone cannot explain the sex differences in within- and among-population covariance patterns or sex-specific patterns of evolution of covariance structure. Our results suggest that extensive among-population variation in sexual dimorphism in morphological covariance structure was produced by population differences in local selection pressures acting on each sex.
... This issue is most troublesome in cases of recent or rapid divergence where retention or incomplete sorting of ancestral polymorphisms, leading to a lack of reciprocal monophyly among taxa in the gene tree, is more prevalent (Nigel and Avise 1986; Pamilo and Nei 1988; Avise 1989; Takahata 1989; Avise and Ball 1990; Wu 1991; Maddison 1997; Hudson and Coyne 2002; Rosenberg 2002; Funk and Omland 2003; Forister et al. 2008; Niemiller et al. 2008). It is well known that adaptive divergence can dramatically outpace lineage sorting of neutral loci (Lande and Arnold 1983; Schluter and Smith 1986; Reznick and Ghalambor 2001 ). Thus, some of the most interesting units of biological diversity might be missed by uncritical application of genealogical species concepts, and good species might be denied protection under, for example, the Endangered Species Act (ESA). ...
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The growing use of molecular systematics in conservation has increased the importance of accurate resolution of taxonomic units and relationships. DNA data relate most directly to genealogies, which need not have perfect relationships with species limits and phylogenies. We used a multilocus gene tree approach to elucidate the relationships between four endangered Central American iguanas. We found support for the proposition that the described species taxa correspond to distinct evolutionary lineages warranting individual protection. We combined gene trees to estimate a phylogeny using Bayesian Estimation of Species Trees (BEST), minimizing deep coalescence, Species Trees from Average Ranks (STAR), and traditional concatenation. The estimate from concatenation conflicted with the other methods, likely owing to the disproportionate effect of mtDNA on concatenated analyses. This illustrates the importance of appropriate treatment of multilocus sequence data in phylogenetics. Our results indicate that these species have gone through recent and rapid speciation, resulting in four closely related narrow-range endemics. Keywords Ctenosaura -BEST-Minimizing deep coalescence-Estimating species trees-Species designation-Gene trees
... Measurement errors underestimate heritability estimates and nestling measures of e.g. tarsus length may be particularly sensitive to such errors (Schluter & Smith, 1986). However, a previous analysis of measurement repeatabilities among the members of the blue tit research group, showed a rather high between-individual repeatability of tarsus length (r = 0.96; Rå berg et al., 2005). ...
Article
We report the first study with the aim to estimate heritability in a wild population, a nest box breeding population of blue tits. We estimated heritability as well as genetic and phenotypic correlations of resting metabolic rate (RMR), body mass and tarsus length with an animal model based on data from a split cross-fostering experiment with brood size manipulations. RMR and body mass, but not tarsus length, showed significant levels of explained variation but for different underlying reasons. In body mass, the contribution to the explained variation is mainly because of a strong brood effect, while in RMR it is mainly because of a high heritability. The additive variance in RMR was significant and the heritability was estimated to 0.59. The estimates of heritability of body mass (0.08) and tarsus length (0.00) were both low and based on nonsignificant additive variances. Thus, given the low heritability (and additive variances) in body mass and tarsus length the potential for direct selection on RMR independent of the two traits is high in this population. However, the strong phenotypic correlation between RMR and mass (0.643 +/- 0.079) was partly accounted for by a potentially strong, although highly uncertain, genetic correlation (1.178 +/- 0.456) between the two traits. This indicates that the additive variance of body mass, although low, might still somewhat constrain the independent evolvability of RMR.
... In addition, divergence in covariance structure among greenfinch populations, despite generally conservative morphological patterns across cardueline finches (Bjö rklund 1993(Bjö rklund , 1994Bjö rklund and Merilä 1993), was associated with low integration between bill traits and body traits (Merilä and Bjö rklund 1999; see also Schluter and Smith 1986;van den Elzen et al. 1987;Merilä 1997;Badyaev and Martin 2000a). Decoupling of bill and body trait evolution in finches may be enabled by differential phenotypic and genetic patterns of growth between these groups of traits (Boag 1984;Bjö rklund 1993;Badyaev and Martin 2000b). ...
Article
Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change by limiting the amount and form of genetic variation available to natural selection. We studied evolution of morphological variance-covariance structure among seven populations of house finches (Carpodacus mexicanus) with a known phylogenetic history. We examined the relationship between within- and among-population covariance structure and, in particular, tested the concordance between hierarchical changes in morphological variance-covariance structure and phylogenetic history of this species. We found that among-population morphological divergence in either males or females did not follow the within-population covariance patterns. Hierarchical patterns of similarity in morphological covariance matrices were not congruent with a priori defined historical pattern of population divergence. Both of these results point to the lack of proportionality in morphological covariance structure of finch populations, suggesting that random drift alone is unlikely to account for observed divergence. Furthermore, drift alone cannot explain the sex differences in within- and among-population covariance patterns or sex-specific patterns of evolution of covariance structure. Our results suggest that extensive among-population variation in sexual dimorphism in morphological covariance structure was produced by population differences in local selection pressures acting on each sex.
... Increased relatedness of survivors might be a normal outcome of bottlenecks in nature if traits that promote survival under the extreme environmental conditions are heritable. For example, wing length was correlated positively with female survival in 1989 and is heritable on Mandarte (h 2 ˆ 0:31 (Schluter & Smith (1986)). Thus, selection on wing length could lead to the survival of relatives. ...
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Article
Population bottlenecks are often invoked to explain low levels of genetic variation in natural populations, yet few studies have documented the direct genetic consequences of known bottlenecks in the wild. Empirical studies of natural population bottlenecks are therefore needed, because key assumptions of theoretical and laboratory studies of bottlenecks may not hold in the wild. Here we present microsatellite data from a severe bottleneck (95% mortality) in an insular population of song sparrows (Melospiza melodia). The major findings of our study are as follows: (i) The bottleneck reduced heterozygosity and allelic diversity nearly to neutral expectations, despite non-random survival of birds with respect to inbreeding and wing length. (ii) All measures of genetic diversity regained pre-bottleneck levels within two to three years of the crash. This rapid recovery was due to low levels of immigration. (iii) The rapid recovery occurred despite a coincident, strong increase in average inbreeding. These results show that immigration at levels that are hard to measure in most field studies can lead to qualitatively very different genetic outcomes from those expected from mutations only. We suggest that future theoretical and empirical work on bottlenecks and metapopulations should address the impact of immigration.
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Many organisms show distinct morphological types. We argue that the evolution of these alternate morphologies depends upon both fitness differences between morphs within each sex and the genetic correlation between sexes. In this paper, we examine the evolution of alternate morphologies using wing dimorphism in insects as a model system. Many insect species are wing dimorphic, one morph having wings and being capable of flight, the other lacking functional wings. While there is a well established trade-off in females between macroptery and reproduction, there are few data on the possible costs in males. We examine trade-offs between macroptery and life-history traits in male sand crickets, Gryllus firmus, and estimate the genetic correlation of wing dimorphism between the sexes. Macropterous males develop faster than micropterous males and are either larger or the same size depending upon rearing conditions. There is no difference in absolute or relative testis size at eclosion or 7 d thereafter. Finally, there is no difference between macropterous and micropterous males in relative success at siring offspring. Thus, with respect to the above traits, there are no costs associated with being winged in male G. firmus. It is possible that there may be a trade-off between calling rate and macroptery. A comparison of the relative frequency of macroptery between males and female across different orders of insects supports this hypothesis. The genetic correlation of wing dimorphism between the sexes is high (r8 = 0.86), and hence the frequency of macroptery in males may be strongly influenced by selection acting on females.
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Microevolution of quantitative traits in the wild can be predicted from a knowledge of selection and genetic parameters. Testing the predictions requires measurement of the offspring of the selected group, a requirement that is difficult to meet. We present the results of a study of Darwin's finches on the Galápagos island of Daphne Major where this requirement is met. The study demonstrates microevolutionary consequences of natural selection.
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The genetic correlation is a central parameter of quantitative genetics, providing a measure of the rate at which traits respond to indirect selection (i.e., selection that does not act upon the traits under study, but some other trait with which they have genes in common). In this paper, I review the pattern of variation among four combinations of traits: life history × life history (L × L), morphological × morphological (M × M), life history × morphological (L × M), and behavioral × behavioral (B × B). A few other combinations were investigated, but insufficient data were obtained for separate analysis. A total of 1798 correlations, distributed over 51 different animal and plant species, were analyzed. The analysis was conducted at two levels: first by dividing the data set solely by trait combination, and second by blocking the data by trait combination and species. Because selection will tend to fix alleles that show positive correlations with fitness traits faster than those that are negative and because the latter are expected to arise more frequently by mutation, correlations between life-history traits are predicted to be more often negative than those between morphological traits. This prediction was supported, with the ranking in decreasing proportion of negative correlations being: L × L > L × M > B × B > M × M. The mean magnitude of the genetic correlation shows little variation among morphological and life-history combinations, and the distribution of values is remarkably flat. However, the estimated standard errors and the coefficient of variation (SE/rG ) are large, making it difficult to separate biological factors influencing the pattern of dispersion from experimental error. Analysis of the phenotypic and genetic correlations suggest that for the combinations M × M and L × M, but not L × L or B × B, the phenotypic correlation is an adequate estimate of the genetic correlation.
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We present estimates of standardized selection (directional and quadratic) differentials via reproductive success on eight morphological traits in a newly founded long-term study population of great reed warblers Acrocephalus arundinaceus in Sweden. In order to predict the evolutionary response to selection in these traits we present estimates of heritabilities (h2), phenotypic (rP) and genetic (rA) correlations among the same traits. We also examined the extent of parental effects in the expression of the phenotypic traits. Overall, the population is subject to low levels of directional selection and higher levels of stabilizing selection. This makes us predict that the population is changing very little if anything, even though h2 in many cases are considerable. Midparent-midoffspring h2 ranged from 0.14 and 0.94 (mean 0.58) and were significant for seven of eight traits. We found indications of positive maternal effects in tarsus length. Phenotypic correlations between traits ranged from 0.02 to 0.43 (mean 0.15) and showed generally much lower values than the corresponding genetic correlations that ranged between 0.08 and 1.04 (mean 0.46). Overall, the correlation between rP and rA was significant, although moderate, but they tended to differ in magnitude, possibly due to overestimation of additive covariance between traits.
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David Lack proposed that parental feeding ability ultimately limited clutch size in bird species in which the young were dependent upon their parents for food. However, many species can raise broods that are larger than their normal clutch size. Based on nine years of experimental results from an individually marked population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York (USA), I test six hypotheses that have been proposed as explanations for why birds fail to lay larger, seemingly more productive clutch sizes. I modified brood sizes by adding or removing 1-2 nestlings when broods were 1-3 d old and then documented the effects of brood size and manipulated brood size on nestling size and survivorship, offspring recruitment, adult survival, and future adult reproduction. Most first clutches of the season held three eggs (62% of 503 clutches), but the proportion of young to hedge did not vary with brood size (1-5 young), and as a result, broods of five were the most productive. Lack's basic food-limitation model was thus rejected. Although nestling mass and ninth-primary length at fledging declined with brood size, offspring survival during the immediate 10-12 d period after fledging was unrelated to nestling mass or lengths of the tarsus or ninth primary. The findings that the underweight young in broods of four and five did not suffer disproportionate mortality and that they were just as likely to appear as recruits in future years led to a rejection of the extended version of Lack's food-limitation model. Comparisons of annual variation in the relationship between productivity and brood size showed that productivity increased with brood size in eight of nine years (significant in six years). Thus, high temporal stochastic variation in conditions for rearing young (the "bad-years" hypothesis) is unlikely to explain the relatively small clutch size of kingbirds. Predictions of two other hypotheses that predict asymmetrically low survivorship of young in large broods (the "cliff-edge" and "brood-parasitism" hypotheses) were also rejected. On the other hand, evidence suggested that females individualize clutch size such that each female lays a clutch that matches her individual feeding ability. Although fledgling production was not adversely affected by experimental increases in brood size, most enlarged broods lost young during the 10-12 d immediately after fledging. Thus, enlarged broods ultimately produced no more independent young than did control broods that began with the same number of eggs, Fledgling deaths were not related to nestling mass or size, and recruitment was independent of manipulations. Survival and fecundity costs of reproduction also existed for females. Male survival (68%) was independent of the number of young that had been raised (0-5 young), and future breeding efforts were not compromised by elevated effort in the past year. However, females that raised broods of five were less likely to return to breed in the following year (42%) than were females that raised 2-4 young (62%). Among the survivors, females that raised enlarged broods in the preceding year also experienced more hatching failure and fledged fewer young than females that raised reduced broods in the preceding year. I suggest that costs of reproduction probably set the ultimate limit to clutch size in Eastern Kingbirds. I did not test the hypothesis that high rates of nest predation favor the evolution of small clutch size, but given that predators destroyed similar to 50% of nests each year, it is also likely that nest predation has contributed to the evolution of the current clutch size of kingbirds. Whether a female produces a clutch of three or four eggs is probably determined by individual differences in parental ability, which may be related to either intrinsic properties of the female or territory quality.
Article
Morphological consequences of hybridization were studied in a group of three interbreeding species of Darwin's finches on the small Galápagos island of Daphne Major in the inclusive years 1976 to 1992. Geospiza fortis bred with G. scandens and G. fuliginosa. Although interbreeding was always rare (< 5%), sufficient samples of measurements of hybrids and backcrosses were accumulated for analysis. Five beak and body dimensions and mass were measured, and from these two synthetic (principal-component) traits were constructed. All traits were heritable in two of the interbreeding species (G. fuliginosa were too rare to be analyzed) and in the combined samples of F, hybrids and backcrosses to G. fortis. In agreement with expectations from a model of polygenic inheritance, hybrid and backcross classes were generally phenotypically intermediate between the breeding groups that had produced them. Hybridization increased additive genetic and environmental variances, increased heritabilities to a moderate extent, and generally strengthened phenotypic and genetic correlations. New additive genetic variance introduced by hybridization is estimated to be two to three orders of magnitude greater than that introduced by mutation. Enhanced variation facilitates directional evolutionary change, subject to constraints arising from genetic correlations between characters. The Darwin's finch data suggest that these constraints become stronger when species with similar proportions hybridize, but some become weaker when the interbreeding species have different allometries. This latter effect of hybridization, together with an enhancement of genetic variation, facilitates evolutionary change in a new direction.
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Random measurement error is ubiquitous in morphometric data, and it can cause serious statistical problems. We stress that measurement error is a potential problem primarily when true phenotypic variation in shape is relatively small, such as in studies of intraspecific variation in shape. A model for the partitioning of measurement error in landmark based morphometrics is presented. The impact of measurement error can be reduced in a number of ways, depending on the methods used to collect, process and analyse data, and we give some practical advice. We also recommend that repeated measures of all individuals are taken routinely in morphometric studies where measurement error may be a potential problem. This enables both a quantification, by estimating repeatabilities from analyses of variance, and a reduction, by averaging repeated measures, of the relative impact of measurement error. We perform an analysis of shape variation in a uniform sample of young perch (Perca fluviatilis), solely aimed at illustrating how different components of measurement error can be quantified, and demonstrate (a) that estimates of repeatability will only be informative of the error components that are actually repeated in each repeated measure, (b) that the relative impact of different components of measurement error can be partitioned and assessed by planned hierarchical repeated measurement protocols followed by nested analyses of variance, (c) that measurement error is unevenly distributed among different shape variables and (d) that the relative magnitude of ME in a given shape variable can be reduced to an estimable extent by averaging several repeated measures.
Article
Many organisms show distinct morphological types. We argue that the evolution of these alternate morphologies depends upon both fitness differences between morphs within each sex and the genetic correlation between sexes. In this paper, we examine the evolution of alternate morphologies using wing dimorphism in insects as a model system. Many insect species are wing dimorphic, one morph having wings and being capable of flight, the other lacking functional wings. While there is a well established trade-off in females between macroptery and reproduction, there are few data on the possible costs in males. We examine trade-offs between macroptery and life-history traits in male sand crickets, Gryllus firmus, and estimate the genetic correlation of wing dimorphism between the sexes. Macropterous males develop faster than micropterous males and are either larger or the same size depending upon rearing conditions. There is no difference in absolute or relative testis size at eclosion or 7 d thereafter. Finally, there is no difference between macropterous and micropterous males in relative success at siring offspring. Thus, with respect to the above traits, there are no costs associated with being winged in male G. firmus. It is possible that there may be a trade-off between calling rate and macroptery. A comparison of the relative frequency of macroptery between males and female across different orders of insects supports this hypothesis. The genetic correlation of wing dimorphism between the sexes is high (r<sub>8</sub> = 0.86), and hence the frequency of macroptery in males may be strongly influenced by selection acting on females.
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Heritability of the size of a hindlimb muscle, M. flexor cruris lateralis (FCRL), was analyzed in Eastern Kingbirds (Tyrannus tyrannus) to assess the potential of the trait to be influenced by evolutionary processes. Size of the FCRL appears to be highly heritable, although the birds are directionally asymmetrical and this asymmetry suggests that muscle size probably is not presently under rigid selective control. Heritabilities were recalculated after the removal of cases of doubtful parentage; the results lend support to a hypothesis of "quasi nest-parasitism" for Eastern Kingbirds.
Article
This paper describes a path model for the analysis of phenotypic selection upon continuous morphological characters. The path-analysis model assumes that selection occurs on unmeasured general size and shape allometry factors that summarize linear relations among sets of ontogenetically, phylogenetically, or functionally related traits. An unmeasured factor for general size is considered the only aspect of morphometric covariance matrices for which there is an a priori biological explanation. Consequently, selection coefficients are derived for each measured character by holding constant only a general size factor, rather than by using multiple regression to adjust for the full covariance matrix. Fitness is treated as an unmeasured factor with loadings, representing directional selection coefficients, computed as the covariances of the size-adjusted characters with the measured fitness indicator. The magnitudes and signs of the selection coefficients, combined with biological insight, may suggest hypotheses of selection on one or more shape allometry factors. Hypotheses of selection on general size and shape allometry factors are evaluated through cycles of measurement, analysis, and experimentation, designed to refine the path diagram depicting the covariances among the measured characters, the measured indicator of fitness, and unmeasured factors for morphology and fitness. The path-analysis and multiple-regression models were applied to data from remeasurement of Lande and Arnold's (1983) pentatomid bugs and to Bumpus's (1899) data on house sparrows. The path analysis suggested the hypothesis that variation in bug survivorship was an expression of directional selection on wing loading. Bumpus's data are consistent with a hypothesis of stabilizing selection on general size in females and directional selection for small wing size relative to body size in males.
Article
Microevolution of quantitative traits in the wild can be predicted from a knowledge of selection and genetic parameters. Testing the predictions requires measurement of the offspring of the selected group, a requirement that is difficult to meet. We present the results of a study of Darwin's finches on the Galapagos island of Daphne Major where this requirement is met. The study demonstrates microevolutionary consequences of natural selection. The population of medium ground finches, Geospiza fortis, experienced size-selective mortality during a drought in 1976-1977; large birds with deep beaks survived better than small birds. During another drought, 1984-1986, the population experienced selection in the opposite direction on beak traits. Changes in food supply were the apparent causes of selection on beak traits in both episodes. As expected from the high heritabilities of all measured traits, the effects of selection were transmitted to the next generation. Evolutionary responses to both episodes of selection were quantitatively well predicted in general. This allows us to conclude that, to a first approximation, targets of selection were identified correctly, and genetic parameters were correctly estimated. Nevertheless, not all responses of individual traits were equally well predicted. A search for possible reasons for the largest discrepancies revealed evidence of bias caused by environmental effects on growth and adult size of some traits, as well as possible selection on the offspring generation before their measurement. These findings illustrate an important assumption in the study of microevolution: that the environments experienced during growth to maturity by the parental and offspring generations are the same, for otherwise a measured difference between generations may have a partly environmental cause, thereby giving a misleading estimate of the evolutionary response to selection. Simple extrapolations from observed selection to long-term evolution may underestimate the total force of selection involved if it oscillates in direction or acts on the environmental variance.
Article
Daphne Major was studied from 1976-1992. The island supported breeding populations of Geospiza fortis (harmonic mean of 198 breeding individuals), G. scandens (H = 80), G. fuliginosa (H = 3) and, in the past 10 yr, G. magnirostris (H = 6). G. fortis hybridized with G. fuliginosa in 11 out of the 13 yr in which both species bred. G. fortis and G. scandens hybridized in six of the years. Hybridizing individuals constituted 1.8% of breeding G. fortis, on average, 0.8% of G. scandens, but 73.0% of the much rarer G. fuliginosa. F1 hybrids were viable and fertile. They rarely bred with each other to produce an F2 generation. Much more frequently they backcrossed to the common species, G. fortis and G. scandens. In all these cases hatching and fledging success were high, giving scarcely any indication of genetic incompatibilities in the F1, F2 or backcross generations. Introgression of genes could be a small factor contributing to the intermediate appearance of G. fortis on Daphne Major, ie between typically larger forms of this species elsewhere in the archipelago and the smaller G. fuliginosa. However hybridization with the larger G. scandens has the opposite directional effect on G. fortis. Introgression also contributes to the large morphological variation displayed by this and several other populations in the archipelago. Hybridization raises questions about how species of Darwin's finches should be defined and recognized. In terms of the broad biological species concept there are four species of finches on Daphne Major, neither completely independent evolutionarily on the one hand (except for G. magnirostris), nor approaching panmixia on the other hand. -from Author
Article
David Lack proposed that parental feeding ability ultimately limited clutch size in bird species in which the young were dependent upon their parents for food. However, many species can raise broods that are larger than their normal clutch size. Based on nine years of experimental results from an individually marked population of Eastern Kingbirds (Tyrannus tyrannus) breeding in central New York (USA), I test six hypotheses that have been proposed as explanations for why birds fait to lay larger, seemingly more productive clutch sizes. I modified brood sizes by adding or removing 1-2 nestlings when broods were 1-3 d old and then documented the effects of brood size and manipulated brood size on nestling size and survivorship, offspring recruitment, adult survival, and future adult reproduction. Most first clutches of the season held three eggs (62% of 503 clutches), but the proportion of young to fledge did not vary with brood size (1-5 young), and as a result, broods of five were the most productive. Lack's basic food-limitation model was thus rejected. Although nestling mass and ninth-primary length at fledging declined with brood size, offspring survival during the immediate 10-12 d period after fledging was unrelated to nestling mass or lengths of the tarsus or ninth primary. The findings that the underweight young in broods of four and five did not suffer disproportionate mortality and that they were just as likely to appear as recruits in future years led to a rejection of the extended version of Lack's food-limitation model. Comparisons of annual variation in the relationship between productivity and brood size showed that productivity increased with brood size in eight of nine years (significant in six years). Thus, high temporal stochastic variation in conditions for rearing young (the 'bad-years' hypothesis) is unlikely to explain the relatively small clutch size of kingbirds. Predictions of two other hypotheses that predict asymmetrically low survivorship of young in large broods (the 'cliff-edge' and 'brood-parasitism' hypotheses) were also rejected. On the other hand, evidence suggested that females individualize clutch size such that each female lays a clutch that matches her individual feeding ability. Although fledgling production was not adversely affected by experimental increases in brood size, most enlarged broods lost young during the 10-12 d immediately after fledging. Thus, enlarged broods ultimately produced no more independent young than did control broods that began with the same number of eggs. Fledgling deaths were not related to nestling mass or size, and recruitment was independent of manipulations. Survival and fecundity costs of reproduction also existed for females. Male survival (68%) was independent of the number of young that had been raised (0-5 young), and future breeding efforts were not compromised by elevated effort in the past year. However, females that raised broods of five were less likely to return to breed in the following year (42%) than were females that raised 2-4 young (62%). Among the survivors, females that raised enlarged broods in the preceding year also experienced more hatching failure and fledged fewer young than females that raised reduced broods in the preceding year. I suggest that costs of reproduction probably set the ultimate limit to clutch size in Eastern Kingbirds. I did not test the hypothesis that high rates of nest predation favor the evolution of small clutch size, but given that predators destroyed ~50% of nests each year, it is also likely that nest predation has contributed to the evolution of the current clutch size of kingbirds. Whether a female produces a clutch of three or four eggs is probably determined by individual differences in parental ability, which may be related to either intrinsic properties of the female or territory quality.
Full-text available
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The differing amounts of adaptive divergence on continents and remote archipelagoes are considered as a source of evidence for ecological character displacement. A classical example is the accelerated evolution of morphological differences between finch species on the Galapagos and Hawaii, throught to result from an absence of competitor taxa on the islands. The amount of divergence is greater in the Galapagos and Hawaiian finches, even when only beak-size divergence in the granivorous component is considered. Studies of seed availability and use, of biochemical differences between species, and of natural selection and the quantitative genetics of size on islands and continents support the hypothesis of character displacement. Several alternative explanations are considered (greater resource diversity on continents, predation on continents, reduced gene flow, and founder events on islands), but although each may play a role, little evidence suggests that they are sufficient to account fully for the pattern. -from Author
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The frequency and heritability of 13 behavioural acts of hatchling Japanese quail,Coturnix japonica, were measured in a laboratory experiment to examine their relationship to juvenile and adult dominance. The heritability of similar acts in adult quail were also quantified to determine how heritability estimates changed with age. Juvenile dominance rank, determined by frequency of winning conflicts, was uncorrelated with the frequency of any hatchling behaviour or hatchling body size but contained significant additive genetic variance in both males and females. Dominance rank in adult females, measured similarly, also had a high heritability estimate. Dominance in adult males, measured as frequency of initiating a fight and frequency of mounting, had only low heritability. Dominance ranking in adults was uncorrelated with the dominance ranking of juveniles. Dominance may be heritable but may not respond to natural selection because of the interaction between the genotype and the environment.
Article
Are measurements of quantitative genetic variation useful for predicting long-term adaptive evolution? To answer this question, I focus on g(max), the multivariate direction of greatest additive genetic variance within populations. Original data on threespine sticklebacks, together with published genetic measurements from other vertebrates, show that morphological differentiation between species has been biased in the direction of g(max) for at least four million years, despite evidence that natural selection is the cause of differentiation. This bias toward the direction of evolution tends to decay with time. Rate of morphological divergence between species is inversely proportional to theta, the angle between the direction of divergence and the direction of greatest genetic variation. The direction of greatest phenotypic variance is not identical with g(max), but for these data is nearly as successful at predicting the direction of species divergence. I interpret the findings to mean that genetic variances and covariances constrain adaptive change in quantitative traits for reasonably long spans of time. An alternative hypothesis, however, cannot be ruled out: that morphological differentiation is biased in the direction g(max) because divergence and g(max) are both shaped by the same natural selection pressures. Either way, the results reveal that adaptive differentiation occurs principally along ''genetic lines of least resistance.''
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I studied the effects of food deprivation on the growth, final size, and inheritance of body size in nestling tree swallows (Tachycineta bicolor). Each experimental brood was divided into deprived and fed nestlings. Deprived nestlings showed relatively poor growth during the experimental stage (days 5–8), but were not significantly different in size from either their nestmates or control young at day 16. Parents of experimental broods did not compensate by increasing their food delivery rate. Heritability estimates of tarsus length were similar for deprived and fed nestlings. For control nestlings, tarsus length was significantly heritable at days 9 and 16, and bill length was heritable at day 16. There were no significant heritability estimates for wing length or body mass. Small sample sizes and single-parent–offspring regression likely inflated the standard errors associated with the heritability estimates. The mechanism that allowed deprived nestlings to grow as large as control and fed nestlings remains unknown.
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Ecomorphologists have thus far paid little attention to phenotypic plasticity of anatomical structures. In this paper we present four cases of phenotypic plasticity in African cichlids, which we suppose to be adaptive responses to environmental changes. On the basis of these cases we hypothesize that the response of a plastic anatomical structure to an environmental change may be constrained by surrounding structures. Alternatively, the surrounding structures may be affected by a plastic structure which changed as a direct response to environmental alterations. We also analyze the morphological and ecological implications of the potentials of growth retardation and accelaration on the phenotypic plasticity in organisms. The flexibility of the reaction norm of an organism may depend on these potentials of heterochrony.
Chapter
Most of the characteristics studied by avian ecologists relate to the external appearance of birds¡ªmorphology, life history, behavior, and in some cases, physiology: the very characteristics upon which natural selection operates. These traits are likely to be the products of tens or hundreds of loci, and their expression can be modified by numerous environmental factors (Lynch and Walsh, 1998; Falconer and Mackay, 1996). The study of the emergent and evolutionary properties of this type of multifactorial inheritance is known as quantitative genetics, and it is based on indirect statistical inference rather than direct observation of specific loci and their gene products. The theory of quantitative genetics has been subject to considerable empirical scrutiny, and its successful application in plant and animal breeding testifies to its practical utility (Lynch and Walsh, 1998; Falconer and Mackay, 1996; Hill and Mackay, 1991; Mather and Jinks, 1982)
Article
The genetic correlation is a central parameter of quantitative genetics, providing a measure of the rate at which traits respond to indirect selection (i.e., selection that does not act upon the traits under study, but some other trait with which they have genes in common). In this paper, I review the pattern of variation among four combinations of traits: life history x life history (L x L), morphological x morphological (M x M), life history x morphological (L x M), and behavioral x behavioral (B x B). A few other combinations were investigated, but insufficient data were obtained for separate analysis. A total of 1798 correlations, distributed over 51 different animal and plant species, were analyzed. The analysis was conducted at two levels: first by dividing the data set solely by trait combination, and second by blocking the data by trait combination and species. Because selection will tend to fix alleles that show positive correlations with fitness traits faster than those that are negative and because the latter are expected to arise more frequently by mutation, correlations between life-history traits are predicted to be more often negative than those between morphological traits. This prediction was supported, with the ranking in decreasing proportion of negative correlations being: L x L > L x M > B x B > M x M. The mean magnitude of the genetic correlation shows little variation among morphological and life-history combinations, and the distribution of values is remarkably flat. However, the estimated standard errors and the coefficient of variation (SE/r(G)) are large, making it difficult to separate biological factors influencing the pattern of dispersion from experimental error. Analysis of the phenotypic and genetic correlations suggest that for the combinations M x M and L x M, but not L x L or B x B, the phenotypic correlation is an adequate estimate of the genetic correlation.
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We consider a possible example of rapid evolution of character displacement, involving two honeyeater species on Long and neighboring islands off New Guinea. Long's caldera collapsed about three centuries ago in one of the largest volcanic explosions of recent millennia. The present trees, mammals, and people of Long suggest recent overwater origins. The avifaunas of Long and the two nearest islands, Tolokiwa and Crown, also betray recent origin: Long has a deficit of the expected large-island species, and all three islands have an excess of supertramps (small-island specialists) and other species noted for overwater dispersal. Among the nine pairs of congeners on Long and its neighbors, two supertramp honeyeaters, the larger Myzomela pammelaena from the northern Bismarcks and the smaller M. sclateri from the southern Bismarcks, constitute the sole pair of species that are sympatric nowhere else and that presumably first met on the Long group. These are also the sole two bird populations of the Long group...
Article
We present estimates of standardized selection (directional and quadratic) differentials via reproductive success on eight morphological traits in a newly founded long-term study population of great reed warblers Acrocephalus arundinaceus in Sweden. In order to predict the evolutionary response to selection in these traits we present estimates of heritabilities (h2), phenotypic (rP) and genetic (rA) correlations among the same traits. We also examined the extent of parental effects in the expression of the phenotypic traits. Overall, the population is subject to low levels of directional selection and higher levels of stabilizing selection. This makes us predict that the population is changing very little if anything, even though h2 in many cases are considerable. Midparent-midoffspring h2 ranged from 0.14 and 0.94 (mean 0.58) and were significant for seven of eight traits. We found indications of positive maternal effects in tarsus length. Phenotypic correlations between traits ranged from 0.02 to 0.43 (mean 0.15) and showed generally much lower values than the corresponding genetic correlations that ranged between 0.08 and 1.04 (mean 0.46). Overall, the correlation between rP and rA was significant, although moderate, but they tended to differ in magnitude, possibly due to overestimation of additive covariance between traits.
Article
Four external skeletal and three feather dimensions were measured on adult collared flycatchers (Ficedula albicollis) and their adult offspring. By using mid-offspring-midparent regressions, all traits were found to be heritable with an arithmetic mean heritability of 0.46. Heritability estimates from full-sib analyses were about 1.5 times higher (mean 0.67), indicating that variation in traits was affected by shared nest environment among full-sibs. The overall body size as measured by principal component one (PC1) was found to be heritable (h2 = 0.40). However, this multivariate measure of heritability was not significant in offspring-father comparison, while highly so in offspring-mother comparison (h2 = 0.60). Low offspring-father resemblance was evident also in univariate estimates of heritability. Possible causes of this (extra-pair copulations, maternal effects, sex-linked variance) are discussed. Genetic correlations among seven traits were estimated to be low (mean 0.22), and of similar magnitude or higher than phenotypic correlations (mean 0.18). All genetic correlations were positive. Genetic and phenotypic correlations as well as covariances were fairly similar to each other (r = 0.85 and r = 0.87, respectively). Environmental correlations did not follow the pattern of genetic correlations (r = 0.11), but were more similar to phenotypic correlations (r = 0.60). Given the low genetic correlations and moderate heritabilities, the overall conclusion is that the external morphology of collared flycatchers is largely under additive genetic control and that there is a strong potential for evolutionary change in morphology even under complex multivariate selection.
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Hypotheses for divergence and speciation in rainforests generally fall into two categories: those emphasizing the role of geographic isolation and those emphasizing the role of divergent selection along gradients. While a majority of studies have attempted to infer mechanisms based on the pattern of species richness and congruence of geographic boundaries, relatively few have tried to simultaneously test alternative hypotheses for diversification. Here we discuss four examples, taken from our work on diversification of tropical rainforest vertebrates, in which we examine patterns of genetic and morphological variation within and between biogeographic regions to address two alternative hypotheses. By estimating morphological divergence between geographically contiguous and isolated populations under similar and different ecological conditions, we attempt to evaluate the relative roles of geographic isolation and natural selection in population divergence. Results suggest that natural selection, even in the presence of appreciable gene flow, can result in morphological divergence that is greater than that found between populations isolated for millions of years and, in some cases, even greater than that found between congeneric, but distinct, species. The relatively small phenotypic divergence that occurs among long-term geographic isolates in similar habitats suggests that morphological divergence via drift may be negligible and/or that selection is acting to produce similar phenotypes in populations occupying similar habitats. Our results demonstrate that significant phenotypic divergence: (1) is not necessarily coupled with divergence in neutral molecular markers; and (2) can occur without geographic isolation in the presence of gene flow.
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An exceptionally wet year followed by two very dry years severely perturbed the food supply of cactus finches Geospiza conirostris on Isla Genovesa, Galapagos. Long-billed birds had a survival disadvantage associated with the absence of Opuntia flowers and fruits, which they normally exploit. This selection event was followed by another favoring birds with deep beaks capable of extracting the only available food, arthropods from beneath the bark of trees and within Opuntia trunks and old pads. For young birds, the ability to develop (learn) techniques for extracting a new food item was a better predictor of survival than was bill shape. Among the group that acquired these skills, more of the birds with deep bills survived, as was the case for the adults. During 1984, males that obtained females had deeper bills than those that failed to do so. Because this mating advantage did not occur in years without natural selection, birds with deep bills probably had more energy available for courtship. A parallel case of natural selection is described in a sympatric congener, G. magnirostris in which birds with deep beaks capable of cracking large and hard Cordia lutea seeds were favoured. The population has undergone little or no evolutionary change in bill dimensions in response to selection during the last 9 yr despite the presence of high levels of additive genetic variance. Evolutionary change has not occurred because the action of selection on one trait in one direction has been counteracted by selection in the opposite direction on another, positively correlated trait. Changes in the proportions of dry-season niches among years cause changes in the proportions of birds with certain beak morphologies and feeding skills best suited to exploiting them. Therefore, the population tracks a moving peak in an adaptive landscape under environmental fluctuations, and there is more than one individual fitness optimum within the range of phenotypes in the population. -from Authors
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Many traits are genetically correlated with each other. Thus, selection that changes the mean value of one trait causes other traits to change as well. Recent comparative studies have emphasized the possible importance of such correlated responses in affecting the evolution of traits, including some behaviors, which are of little adaptive significance, or even maladaptive. However, it is also possible for traits with major effects on fitness, such as brain size, to evolve entirely by correlated response. Other traits that do not appear to have evolved at all may have been subject to much directional selection, simply to prevent their evolution by correlated response. The new interest in correlated responses reflects more rigorous attempts to consider the organism as a whole, rather than dissecting it into a number of questionably separable traits.
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To determine whether genetic constraints on adaptive evolution were operating in a laboratory population of a flour beetle, Tribolium castaneum, we first estimated the direct selection acting on each of several body size traits. Strong selection in males for an increase in pupal weight and a decrease in the ratio of adult to pupal weight occurred. In addition, a non-significant trend for a decrease in adult width was found. No significant selection on females was detected, although there were trends toward an increase in pupal weight and a decrease in adult width. These estimates were then combined with estimates of the genetic variances and covariances of the traits to predict the multivariate response to selection, that is, the evolutionary change in the traits across one generation. These projections showed only a small predicted change in male pupal weight in spite of the strong selection on pupal weight, and a relatively large predicted increase in width in spite of the possible negative direct selection on this trait. Both of these results were due in part to the positive genetic covariance between pupal weight and width, and they therefore suggest the possibility of genetic constraints on adaptive evolution of these traits.
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Additive genetic variances and covariances of quantitative characters are necessary to predict the evolutionary response of the mean phenotype vector in a population to natural or artificial selection. Standard formulas for estimating these parameters, from the resemblance between relatives in one or two characters at a time, are biased by natural selection on the parents and by maternal effects. We show how these biases can be removed using a multivariate analysis of offspring-parent regressions. A dynamic model of maternal effects demonstrates that, in addition to the phenotypic variance-covariance matrix of the characters, sufficient parameters for predicting the response of the mean phenotype vector to weak selection are the additive genetic variance-covariance matrix and a set of causal coefficients for maternal effects. These can be simultaneously estimated from offspring-parent regressions alone, in some cases just from the daughter-mother regressions, if all of the important selected and maternal characters have been measured and included in the analysis.
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Genetic variation in correlations among size-related traits of head, thorax, and wings was examined in Drosophila buzzatii, by comparing the pattern of the Phenotypic Correlation Matrix (Rp) between inversion karyotypes of the second chromosome. No similarity in Rp was observed between some karyotypes in a natural population. The pattern of Rp in wild-reared heterokaryotypes, but not in homokaryotypes, was similar to the whole population represented by laboratory-reared flies. While phenotypic correlations in wild-reared flies were found to be larger than in laboratory-reared flies, similarity in the pattern of Rp was very high for one homokaryotype reared in both environments: the relatively homogeneous lab environment and the more variable field environment. While no such a similarity across environments was detected between different karyotypes, the pattern of Rp was similar for a same homokaryotype in different populations. Thus, the lack of karyotypic similarity in Rp is, at least partially, genetic. These results indicate that chromosomal inversions are factors affecting genetic correlations among traits known to be phenotypically correlated with adult fitness components in this species.
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Hypotheses for divergence and speciation in rainforests generally fall into two categories: those emphasizing the role of geographic isolation and those emphasizing the role of divergent selection along gradients. While a majority of studies have attempted to infer mechanisms based on the pattern of species richness and congruence of geographic boundaries, relatively few have tried to simultaneously test alternative hypotheses for diversification. Here we discuss four examples, taken from our work on diversification of tropical rainforest vertebrates, in which we examine patterns of genetic and morphological variation within and between biogeographic regions to address two alternative hypotheses. By estimating morphological divergence between geographically contiguous and isolated populations under similar and different ecological conditions, we attempt to evaluate the relative roles of geographic isolation and natural selection in population divergence. Results suggest that natural selection, even in the presence of appreciable gene flow, can result in morphological divergence that is greater than that found between populations isolated for millions of years and, in some cases, even greater than that found between congeneric, but distinct, species. The relatively small phenotypic divergence that occurs among long-term geographic isolates in similar habitats suggests that morphological divergence via drift may be negligible and/or that selection is acting to produce similar phenotypes in populations occupying similar habitats. Our results demonstrate that significant phenotypic divergence: (1) is not necessarily coupled with divergence in neutral molecular markers; and (2) can occur without geographic isolation in the presence of gene flow.
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The study of natural selection in laboratory systems undergoing experimental evolution can provide important insights into the relationship between natural selection and adaptation. We studied selection on the norm of reaction of age at first reproduction in a laboratory population of Drosophila melanogaster. This population had been selected on a discrete generation schedule in the laboratory for more than 600 generations. Using genetically marked strains, we studied development time, size, female fecundity, and viability of flies that began development at different times relative to the initiation of each bottle. Only flies that began development within 30 h of the initiation of the bottle were reliably able to eclose before the next transfer. Theory predicts that flies initiating development around this critical time should decrease size at maturity to ensure eclosion by the 14-d deadline, but late flies are not smaller. This result suggests an unknown constraint on response to selection on age at maturity in this population. Ultimately, laboratory systems provide the best opportunity for the study of natural selection, genetic variation, and evolutionary response in the same population.
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Speciation involves both the differentiation of populations and the establishment of reproductive isolation. One current debate in evolutionary biology stems from differences in opinion about the extent to which the two processes are coupled, especially when population differentiation is largely a result of natural selection. Reproductive isolation can occur before, concurrently with, or after, any visible divergence. The establishment of reproductive isolation often involves some form of sexual selection or assortative mating. Indeed, sexual selection may often be the driving force in both morphological divergence and reproductive isolation, thereby coupling the two processes (Lande 1981).
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Expected density is usually a polymodal function of beak depth. Islands differ substantially in the shapes of their density functions. Mean beak sizes of species actually present on each island correspond to local maxima in expected density. However, 2 species never occupy the same or closely adjacent local maxima. Simple models incorporating the effects of both food supply and interspecific competition on assembly/evolution accurately predict observed morphological patterns. Results support the hypothesis that both food supply and interspecific competition have determined morphological properties in communities of these finches.-from Authors
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The repeatabilities of 7 external morphological characters were high for Geospiza fortis and G. scandens. Geospiza fortis mates displayed positive assortative mating with respect to morphology in 1976, but not 1978, while G. scandens showed no consistent assortative mating in any year. Heritabilities in G. fortis were high in both years, averaging .76 based on 3 types of parent-offspring regression and on full sib intraclass correlations. Heritabilities in G. scandens were lower, averaging .46. Genetic correlations were also high among the 7 characters in G. fortis, and had a multivariate structure like that seen in the phenotypic correlations. G. fortis morphological variation may be influenced by introgression with G. fuliginosa and frequency dependent, temporally fluctuating directional selection, while G. scandens morphology may reflect an evolutionary history predominated by stabilizing selection.-from Author
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Multivariate statistical methods are derived for measuring selection solely from observed changes in the distribution of phenotypic characters in a population within a generation. Selective effects are readily detectable in characters that do not change with age, such as meristic traits or adult characters in species with determinate growth. Ontogenetic characters, including allometric growth rates, can be analyzed in longitudinal studies where individuals are followed through time. Following an approach pioneered by Pearson (1903), this analysis helps to reveal the target(s) of selection, and to quantify its intensity, without identifying the selective agent(s). By accounting for indirect selection through correlated characters, separate forces of directional and stabilizing (or disruptive) selection acting directly on each character can be measured. These directional and stabilizing selection coefficients are respectively the parameters that describe the best linear and quadratic approximations to the selective surface of individual fitness as a function of the phenotypic characters. The theory is illustrated by estimating selective forces on morphological characters influencing survival in pentatomid bugs and in house sparrows during severe weather conditions.
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It has often been argued that the principles of random mutation and selection are insufficient to account for macroevolutionary phenomena, such as the origin of morphological novelty and directionality in evolution. A third, epigenetic, principle is said to be required and this principle is thought not to be included in microevolutionary theory. The third principle has most recently been identified as internal selection and/or non-random phenotypic effects of mutation. It is shown that the genetic variance/covariance matrix of quantitative genetic theory measures developmental constraints due to internal selection and non-random mutation. The genetic variance/covariance matrix causes the response to selection to deviate from the optimal rate and direction as specified by the selection gradient, which measures direct selection on the phenotypes. Therefore, microevolutionary theory takes account of developmental constraints on evolution by natural selection through the genetic variance/covariance matrix. Theories for predicting the pattern of genetic variance and covariance from stabilizing selection and the phenotypic effects of mutation are discussed.
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The adaptive significance of morphological traits can be assessed by measuring and identifying the forces of selection acting on them. Boag and Grant documented directional selection in a small population of Darwin's medium ground finches, Geospiza fortis, on I. Daphne Major, Galápagos, in 1977. Large beak and body size were favoured at a time of diminishing food supply and high adult mortality. We show here that in two subsequent periods of moderate to high adult mortality (1980 and 1982), the population was subject to the same selection. We have used a recently developed technique to ascertain the targets of direct selection. Beak depth and body weight were commonly under direct selection to increase but, surprisingly, beak width was directly selected to decrease, over all three periods of mortality. The results have implications for our understanding of evolutionary change in morphological traits of Darwin's finches.
Article
Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galápagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.
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Using information taken from studies of great tit in The Netherlands, examines 1) the heritability of dispersal, as an illustration of the difficulties in identifying the proper null hypothesis and as an example of resemblance of relatives that seems to be entirely caused by the sharing of environments; 2) within-clutch variation in egg size, exemplifying the problems encountered in investigating environmental variation in a case where c70% of the phenotypic variation is genetic; 3) the genetic correlation between egg size and body size; and 4) the heritability of nestling body size.-P.J.Jarvis
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We studied the inheritance of morphological variation in a population of song sparrows inhabiting Mandarte Island, B C., Canada. Song sparrow young reached adult size by the end of the eighth week after hatching. Males were consistently larger than females in body weight, beak depth and width, and beak, wing and tarsus length. The beak characters were less variable on repeated measurement than body weight and wing length, with tarsus length intermediate. All characters except tarsus length and beak depth were positively intercorrelated. There was a weak negative correlation between mate beak widths, implying some negative assortative mating, but pairing was random with respect to other characters. Female wing length and body weight varied with the season of hatching and beak width, tarsus length and female body weight varied among years. Mid-parent/offspring regression analysis on measurements corrected for sex differences showed that beak length, depth and width, and tarsus length were significantly heritable. Male and female parents contributed about equally to the mid-parent/offspring resemblance. Our results show that sufficient genetic variation exists in the population to allow natural selection to alter beak and tarsus phenotypes. This verifies an important assumption of modern microevolutionary theory.
Article
and in many instances the estimate of a phenotypic correlation is reported smaller in magnitude than that of the corresponding genetic correlation, e.g. with certain poultry records, in Lerner & Cruden [1948], sheep records in Morley [1951] and with certain dairy records in VanVleck [1960] and Searle [1961]. Such results may seem a little unexpected at first sight since phenotype includes genotype and one might anticipate the correlation between phenotypes to be larger than that between genotypes. When estimates have not followed this pattern the explanation is sometimes given that a phenotypic correlation less than a genetic correlation is the result of a negative environmental correlation in the records of the two traits. This paper investigates the relationship between these three correlations on the basis of a linear model, and demonstrates the situations in which this explanation is correct. Other comparisons are also made.
Article
Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galapagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.
Article
Static ("population") allometric coefficients were estimated for 17 osteometric (skull and jaw) characters in lines of rats selected for increased and decreased body weight gain, and in a control line, each of the three lines having two replicates. Bivariate allometric coefficients were estimated by logarithmic regression of the dependent variables on femur length, and they averaged .49. In analyses of covariance, population allometric coefficients showed significant line differences for one character, but replicate differences for three characters. Multiple comparisons testing of the population allometric coefficients in each sex also revealed significant heterogeneity between replicates of both selection lines (in about one-half of the characters), but not between replicates of the control line, or between any of the lines (replicates pooled) themselves. Covariance changes due to selection, rather than to random drift, were hypothesized to account for this pattern. Evolutionary allometric coefficients calculated from logarithmic regressions through the six line by replicate means in each sex were higher (average = .83) than the population allometric coefficients, but agreed quite well with theoretical values calculated from genetic covariances.
Article
The morphology of G. difficilis varies on different islands in the Galapagos (Fig. 2). Lack (1947) argued that beak and body size in this species are related to feeding, and noted that variation in morphology is associated with inter-island differences in the presence or absence of potential competitor species. We investigated the relationship between inter-island morphological variation in G. difficilis and in two potential competitor species, G. fuliginosa and G scandens, and their dry season diets. Our observations support some of Lack's general conclusions concerning the significance of morphological variation in G. difficilis. For example, populations of G. difficilis are similar in diet to G fuliginosa on islands where this species is absent. However, overall morphological distances between populations of G difficilis and the other two species did not successfully predict diet differences. There are at least two reasons for discrepancies. First, environments vary seasonally and annually, and populations may not all have been studied during the most critical periods of food limitation. Second, traits have not evolved independently in G. difficilis, and this complicates dietmorphology comparisons, for example when there are alternative morphological solutions to a single foraging problem. Consideration of these factors helps clarify the significance of variation among G. difficilis populations in certain morphological traits. Beak length and body size appear to be particularly important in relation to food. On the low island of Genovesa, G. difficilis has become smaller, and this is correlated with a diet of small seeds and flower nectar. On the low islands of Darwin and Wolf G. difficilis has a relatively longer beak than other populations have. This may be an adaptation to cactus-feeding, but some data indicate that it may have evolved to facilitate blood- and egg-feeding.
Article
The hypotheses that developmentally and functionally related cranial traits are relatively tightly morphologically integrated in the phenotype, genotype, and with respect to environmental factors were tested. Sets of developmentally and functionally related traits, F-sets, were generated from theoretical and experimental information concerning development and function in the cranium. Cluster analysis was used to form sets of relatively highly correlated traits in the phenotype (P-sets), genotype (G-sets), and with respect to environmental factors (E-sets). The theoretically derived F-sets were then compared to the P-, G-, and E-sets formed by cluster analysis in order to test the hypotheses listed above. The nearly complete agreement between F- and P-sets indicates that developmentally and functionally related traits are relatively tightly integrated in the phenotype. Analysis of F- and E-sets indicated that at least traits within the two major functional cranial components, the neurocranial and orofacial components, were relatively tightly integrated. The F- and G-sets showed very little agreement due to the occurrence of relatively tight genotypic integration of various neurocranial and orofacial traits. Further analysis indicated that within the neurocranial and orofacial components, developmentally and functionally related traits are relatively tightly integrated. Some theoretical and empirical support was presented for the presence of genetic correlations among developmentally and functionally related traits, possibly maintained by stabilizing selection along ridges or peaks in an adaptive topography. However, the existence of some strong genetic correlations among traits with no developmental or functional relationship was also noted. It is hypothesized that these non-functional correlations may arise through stochastic processes, such as founder effect and genetic drift, and that they can be important in determining the evolutionary possibilities open to a population, independent of any adaptive considerations.
Article
A new measure of morphological distance is used to estimate morphological and phylogenetic relations among the Darwin's finches. The measure, B, is based on a model for multivariate evolution (Lande, 1979), and it estimates the total net force of directional selection acting on characters that is required to bridge the differences between any two species. This force depends on the amount of genetic variance in traits, and on genetic correlations between traits. "Selection distance" between species is shown to be correlated with biochemical distance, and the method produces a phylogenetic tree similar to the one originally suggested by Lack (1947). The results indicate that, in addition to natural selection, genetic parameters have strongly influenced the direction and rate of morphological divergence in the Darwin's finches.
Article
Distribution, abundance, diet, and beak morphology of the six Geospiza finch species were studied at eight sites on seven Galapagos islands. The resulting information was used to test the theories of Lack and Bowman that interspecific competition (Lack) and floristic and food differences among islands (Bowman) determine the ecological and morphological characteristics of the finches. Both factors were found to be important in different aspects of finch ecology. Most of the 21 populations studied spent more time foraging for seeds and fruits than for insects. Most populations had generalized diets and nine out of 18 pairs of sympatric populations had diets overlapping by 50% or more. Diet breadth was positively correlated with variety of available foods (classified by an index comprising measures of size and hardness), but was not correlated with abundance or biomass of sympatric populations of finches (potential competitors). Competitive release, in the form of large population biomass on islands with few congeners, was not found. Nor was population biomass related to variety of available food. Overlap in diet between two species was greatest when the species were most similar in abundance and body size. Interspecific competition is indicated by the distribution and morphology of the finches. The absence of four combinations of species cannot be attributed to change: (1) G. conirostris and G. fortis, (2) G. conirostris and G. scandens, (3) G. difficilis and G. fuliginosa, and (4) G. scandens and G. difficilis. These absences cannot be attributed to the absence of suitable foods. and sites with similar floras and suites of food types do not necessarily have similar ground finch faunas. The ecological and morphological similarity of members of each of the four pairs suggests an inability to coexist for competitive reasons. In contrast, where only two species occur together on an island they tend to differ markedly in size (beak and body) and diet. Species with large beaks can eat larger and harder seeds and fruits than species with small beaks, and as a consequence they tend to have more diverse diets. Species with large beaks also eat medium-sized seeds and fruits quicker than species with small beaks. Species with small beaks may be able to deal with small soft seeds more efficiently than species with large beaks, but the data are not clear on this point. In addition, small species with small beaks have a metabolic advantage over large species with large beaks. Variety of available foods influences the large species more than the small species: the two largest species, G. magnirostris and G. conirostris, occur only on sites where plants producing large and hard seeds and fruits are plentiful. Food variety apparently does not influence the degree of diet overlap. The number of plant species on an island statistically determines the number of land-bird species in general, and Geospiza species in particular, which occur on that island. The influence is probably mediated through food supply, since 53% of the variation in Geospiza species diversity among the eight sites was accounted for in a multiple linear regression analysis by diversity of available seeds and fruits. We conclude that food supply and interspecific competition have jointly determined the ecological course of the radiation of Darwin's finches and the resulting pattern of species diversity. Interisland variation in vegetation favored the initial steps of differentiation. Competitive interactions among species influenced later stages by determining which ecological types could coexist on an island with a given array of foods.
Article
We documented temporal patterns of natural selection on beak and body traits in a song sparrow population. We looked for evidence of selection in association with reproduction and overwinter survival in order to identify the conditions under which size in beak and body traits is adaptive. We also attempted to identify the specific traits most closely associated with fitness under these conditions. Selection was observed in association with both survival and reproduction. Patterns of selection differed between the sexes. Selection on males was weak and stabilizing in association with over- winter survival. Selection on females was strong, was both stabilizing and directional, and was associated with both survival and reproduction. In females, traits that enhanced juvenile survival also reduced reproductive success; i.e., there was a trade-off between survival and reproduction. Patterns of selection in the song sparrow parallel those reported for the Galapagos finch, Geospiza fortis. However, in song sparrows, selection occurred mainly on tarsus length and beak length, and not on beak depth or width as in G. fortis. This difference may occur because most North American sparrows partition food resources by habitat, while most Galapagos ground finches partition food by seed size.
Article
The niche variation hypothesis is an adaptive explanation for variation within populations and for, the differences in variation between populations in morphological, physiological or behavioral traits. It has received only partial support from empirical tests and has been criticized on theoretical grounds. Recent quantitative genetic models have made an advance by exploring the effects of mutation, migration, mating pattern and selection on phenotypic variance. These models are reviewed and their most important features are integrated in a new model. In this model population variation is in a state of balance between the opposing forces of mutation and immigration, which tend to elevate variation, and selection and possibly genetic drift tending to decrease it. Populations exhibiting different levels of variation are interpereted as having different equilibrium points, and it is the task of empirical studies to determine the relative magnitudes of the opposing factors. An example is given from studies of Darwin's finches. Geospiza fortis varies more than G. scandens on Isla Daphne Major, Galápagos, in several morphological traits including beak and body size. This is explained, assuming equal mutation rates in the two species, as the result of more frequent genetic input to the G. fortis population, through occasional hybridization with immigrant G. fuliginosa, and relaxed stabilizing selection. Stabilizing selection is less intense on G.fortis than on G. scandens because the G. fortis population has a broader niche; there is both a within-phenotype and betweenphenotype component to the broad niche of G. fortis. The success of theory in explaining population variation is discussed, and it is concluded that empirical studies lag far behind theory.
Mass, wing length, tarsus length and four bill dimensions were measured on adult Large Cactus Ground Finches (Geospiza conirostris) and their offspring in a field study on Isla Genovesa, Galapagos. Heritabilities of these traits were determined by regressing family mean offspring values on midparent values. Principal components analysis was used to provide a multivariate characterization of size and shape, and component scores were used in heritability analysis. Morphological traits were highly heritable. All heritabilities of univariate traits exceeded 0.65, and the arithmetic mean was 0.84. Principal component 1 provides a measure of overall body size; this has a high heritability (0.79). Principal component 2, a bill-pointedness shape factor, also has a high heritability (0.92). Indirect evidence suggests the possibility of a small influence of genotype--environment correlation on the heritability estimates for two traits, mass and bill length. The overall conclusion is that there is a strong potential for evolutionary change in morphology in this population. The population may have been subject to disruptive selection recently, followed by stabilizing selection. Genetic correlations between measured traits were moderately large (mean 0.54) and all were positive. In several respects G. conirostris is intermediate between two other congeneric species, G. fortis and G. scandens. Heritabilities and genetic correlations appear to be higher in G. fortis and lower in G. scandens. These differences are associated with different feeding niches, different degrees of predictability of food resources and different types of selection pressures to which the species are subjected.
Article
A basic principle of natural selection on correlated characters is expressed as an adaptive topography for the vector of mean phenotypes in a population. Under some simple conditions on the pattern of phenotypic and genetic covariation within populations, selection only on body size, certain types of multivariate selection, and random genetic drift in a stochastic phylogeny are each expected to produce allometric evolution, i.e., straight lines or linear regressions on logarithmic coordinates. The orientation of these lines is determined by genetic parameters of the populations. Using this theory, phylogenetic or comparative information can be combined with experimental data on population genetic parameters to test hypotheses about past selective forces. Data from selection experiments on brain and body weights in mice support the conclusions that [1] the short-term differentiation of brain and body sizes in very closely related mammalian forms resulted either from directional selection mostly on body size with changes in brain sizes largely a genetically correlated response, or from random genetic drift; [2] during the long-term allometric diversification within most mammalian orders there has been more net directional selection on brain sizes than on body sizes. It is suggested that encephalization in primates decreased the genetic correlation between brain size and body size within populations, which facilitated further encephalization in the human lineage by avoiding antagonistic selection on brain and body sizes. The evolution of brain:body ontogeny is briefly discussed.
Article
A quantitative genetic analysis of growth from hatching in Geospiza fortis reveals 1) the presence of maternal effects influencing offspring size for the first few days after hatching; 2) high genetic correlations between adult and chick size from age day 3 onward; and 3) the presence of among-brood variance in shape from hatching onward. Selection on adult morphology is expected to produce strongly correlated responses of chick morphology. Geospiza species differ in size and shape at hatching and in relative growth rates of characters. These differences can be largely explained as arising from correlated responses to selection on adults. -from Authors
Article
Six bill dimensions, and wing, tarsus and hallux lengths were measured on almost all museum specimens of Geospiza species available, and up to 20 specimens from each population of the remaining species of Darwin's finches. The data were subjected to univariate and multivariate analyses in order to provide a quantitative description of size and shape differences among populations and between species. Each species of Geospiza varies among islands in size, and most of the remaining species do so as well. There is more variation in shape among species than among populations of the same species, especially in bill proportions. Allometric relations differ among species. Approximate morphological counterparts to the ground finch species, Geospiza, can be identified among the tree finches. There is a small amount of overlap in multivariate space between a ground finch species and a tree finch species (two cases), but no overlap between any two species within each group. Size variation among populations is not generally correlated with geographical variables such as latitude, longitude, island area or its degree of isolation. Nor do coefficients of variation show strong geographical trends. Several of the results confirm the findings of other workers from simpler and non-statistical comparisons. In addition we have shown that the tree finches have relatively long legs (tarsi), and that these finches as well as the ground finches which spend most time scratching on the ground or climbing in cacti also have a relatively long hallux (hind toe). To interpret the various morphological patterns a knowledge is required of inter-island variation in food supply, feeding habits and the incidence of genetic exchange between populations. Recent field studies have provided some of this needed information, which helps to explain, among other things, why several populations of Darwin's finches are so unusually variable.
Article
Atchley, W. R., C. T. Gaskins, and D. Anderson (Departments of Biological Sciences, Animal Sciences, and Biomedical Engineering and Computer Medicine, Texas Tech University, Lubbock, Texas 79409) 1976. Statistical properties of ratios. I. Empirical results. Syst. Zool. 25:137–148.—Results are presented of an empirical analysis on the statistical consequences of compounding ratios of continuous variables. Three commonly employed relationships among the ratio variables Y and Z were examined including 1) Y = X1/X2, Z = X3/X2; 2) Y = X1/X2, Z = X1; and 3) Y = X1/X2, Z = X2. Simulation studies with minimum sample sizes of 25,000 indicated large and systematic changes in both the structure and the underlying distributions of data when ratios and proportions were compounded between continuous variables. Ratio variables are skewed to the right and leptokurtic and the nonnormality is increased when magnitude of the denominator coefficient of variation is increased. Further, there is a pronounced increase in spurious correlation between variables when ratios are compounded and the magnitude of this spurious correlation is a function of the size of the denominator coefficient of variation. This spurious correlation may increase from r = 0.0 between the original raw variables to r = 0.99 between the derived ratio variables. Multivariate statistical procedures such as principal components analysis are greatly affected when the data upon which the analyses are based include ratios or proportions. In this case, there is often an inflation of the first eigenvalue together with large changes in the magnitude and direction of the coefficients on the various principal components. Several common applications of ratios in biological research are discussed. Contrary to a widely held belief, it is shown that in the scaling of data, ratios do not remove the effect of the scaling variables but rather increase the correlation between the ratio variable and the original scaling variable.
Article
A statistical genetic model of a multivariate phenotype is derived to investigate the covariation of pleiotropic mutations with additive effects under the combined action of phenotypic selection, linkage and the mating system. Equilibrium formulas for large, randomly mating populations demonstrate that, when selection on polygenic variation is much smaller than twice the harmonic mean recombination rate between loci with interacting fitnesses, linkage disequilibrium is negligible and pleiotropy is the main cause of genetic correlations between characters. Under these conditions, approximate expressions for the dynamics of the genetic covariances due to pleiotropic mutations are obtained. Patterns of genetic covariance between characters and their evolution are discussed with reference to data on polygenic mutation, chromosomal organization and morphological integration.
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Reproductive Success Chicago: Chicago University Press. population. Evolution
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Quantitative genetics in natural populations of birds illustrated with examples from the great tit, Parus major
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