Evolutionary Biology

Covariation constrains and biases the evolution of morphological traits, leading to similar phenotypes appearing repeatedly in certain clades. Here, I test whether this phenomenon can explain the evolution of locomotor adaptations in the forelimbs of apes, which share a suite of traits thought to have evolved in parallel despite the locomotor diversity of extant apes. Three hypotheses have been proposed: (1) apes have a shared, derived pattern of covariance that biases their evolution along evolutionary lines of least resistance; (2) apes have greater autonomy and increased modularity that improves their ability to evolve into novel areas of morphospace; and (3) the large number of potentially homoplastic traits in apes are really a smaller, more parsimonious number of trait complexes. I find that apes have higher evolvability and respondability but lower autonomy and flexibility than do monkeys. This is true both along hypothetical evolutionary trajectories and when averaged across random selection vectors. Evolvability along trajectories is lower than average evolvability in apes, indicating that these trajectories are not evolutionary lines of least resistance. Apes are generally best represented by models of modularity that include more modules than models that best represent monkeys, but several models of modularity receive comparable support across all taxa. Overall, all three hypotheses receive some support but the predictions of none are fully met. Instead, these results may indicate a partial breakdown and realignment of the P matrix in apes in response to directional selection, demonstrating that the relationship between integration, selection, and evolution is complex and nonlinear.

While speciation diversity is widely acknowledged, significant debate exists regarding the universality of hybrid speciation. An 18 year systematic investigation of all Populus taxa on the Qinghai-Tibet Plateau resulted in the identification of 23 recorded taxa and 8 newly identified taxa. Morphologically, these taxa can be categorized into three groups, one of which includes 11 taxa with distinct characteristics of P. simonii. Whole-genome resequencing of 150 genotypes across 31 taxa yielded 2.28 million SNPs. Among these, P. ranwuensis, P. baibaensis, P. qingdensis, and P. erlangshanensis were identified as new species. These species, along with six other taxa displaying characteristics of P. simonii, originated through natural hybridization, with P. simonii serving as a common direct ancestral species. The another direct ancestral species of P. yunnanensis and P. trinervis is P. szechuanica, that of P. ranwuensis, P. qingdensis, P. kangdingensis, P. przewalskii, and P. schneideri var. tibetica is P. xiangchengensis, that of P. qamdoensisis is atypical P. curviserrata, and that of P. baibaensis is P. pseudoglauca. Natural hybridization is identified as a dominant driver of species diversification in Populus on the Qinghai-Tibet Plateau and a critical factor contributing to the disappearance of ancient species in the northern, western, and central regions of the plateau. The universality and evolutionary significance of hybrid speciation appear to have been substantially underestimated.

Ancestral state reconstruction is a phylogenetic comparative method that involves estimating the unknown trait values of hypothetical ancestral taxa at internal nodes of a phylogenetic tree. Ancestral state reconstruction has long been, and continues to remain, among the most popular analyses in phylogenetic comparative research. In this review, I illustrate the theory and practice of ancestral state reconstruction for both discretely and continuously-valued phenotypic traits. For discrete characters, I focus on the Mk model and describe the distinction between marginal and joint reconstruction, as well as between local vs. global estimation. For continuous traits, I describe ancestral state reconstruction under a model of Brownian motion evolution. I highlight several use cases of ancestral state reconstruction via a set of empirical examples, ranging from diel activity pattern in primates, to environmental tolerance in lizards. Finally, I discuss and explore the statistical properties of ancestral state estimation as well as its limitations. By illustrating ancestral reconstruction under the hidden-rates and threshold models (for discrete traits), and bounded Brownian motion (for continuous characters), I demonstrate the considerable sensitivity of ancestral reconstruction to model misspecification. Although ancestral state reconstruction is virtually certain to retain its popularity into the future, I conclude by recommending considerable caution and circumspection in the use and interpretation of ancestral reconstruction in empirical evolutionary research.

Chemical cues released upon injury play a crucial role in mediating antipredator responses in many aquatic species. Prey populations capable of exploiting information from different sources may enhance their fitness, although the factors determining this ability are not always clear. In this study, we investigated the antipredator responses to both conspecific and heterospecific injury cues (i.e., alarm cues) in three brown frog species: Rana dalmatina, Rana latastei, and Rana temporaria. We recorded two tadpole defensive behaviours: the proportion of time spent inactive and the distance covered. Responses to a native odonate predator were used as a positive control. Tadpoles of R. dalmatina showed a clear decrease in activity in response to both con- and heterospecific cues, similar to their reaction to predator cues. Tadpoles of R. latastei slightly decreased activity in response to alarm cues from both agile frog species but did not react to common frog cues. Finally, R. temporaria exhibited high sensitivity to conspecific cues and a weak response to heterospecific cues. The phylogenetic-relatedness hypothesis seems to explain the observations for both R. latastei and R. temporaria, while for R. dalmatina, the ecological coexistence hypothesis, given its overlapping distribution with the other two species, seems to better explain the data. However, since the invoked hypotheses are not mutually exclusive, further analysis of other populations of these species is necessary to confirm the generality of these findings and to better understand the role of environmental factors in shaping these antipredator responses.

New mutations compete with their unmutated progenitors for limited places in the germ line. This process favors genetic variants with functions that are easily broken by mutation. Genetic recombination, by contrast, favors robust interactions among parts that must maintain function in many different combinations. Therefore, fragile epistatic interactions are predicted among closely-linked sites with more robust interactions predicted among sites that recombine freely on the time-scale of mutation. Genes that function in DNA replication and repair are predicted to accumulate features that challenge their own abilities because such individualized ‘stress tests’ place their own loss-of-competence mutations at a selective disadvantage while at the same time testing the overall competence of all loci involved in replication and repair. Conserved fragile sites that undergo breakage under conditions of replicative stress may persist in the genome because they test the competence of the machinery of DNA replication and repair. An intriguing possibility is that cellular selection in the germ line may be able to maintain a lower germline mutation rate than individual selection acting alone.

Siphons in bivalves have been postulated as a key adaptive trait, enabling modes of life inaccessible to asiphonate lineages, that afford better protection from predation and dislodgement, thereby enhancing their taxonomic diversification. To test the impact of siphons on diversity, we compared two bivalve clades with similar shell forms and life positions that differ in the presence/absence of this supposed key trait: the asiphonate Archiheterodonta (origin ~ 420 Myr ago) and the siphonate Veneridae (origin ~ 170 Myr ago). We measured three characters relevant to burrowing (shell length, cross-sectional area, and proportional shell volume) in these two groups, finding that siphonate venerids occupy more modes of life than archiheterodonts because they can live at a greater range of distances from the sediment–water interface, with the thinnest shells occurring in the deepest-burrowing groups. Asiphonate taxa have thicker shells, perhaps as a compensatory adaptation in response to the potential for exposure and attack because they are limited to shallower depths of burial. The lack of siphons may have impeded morphologic and taxonomic diversification in archiheterodonts. In contrast, siphons are consistent with a key adaptive trait in the Veneridae, evidently enabling taxonomic diversification into a greater range of morphologies.

We studied ontogenetic variation in skull morphology in tiger snakes sampled from mainland South Australia (Coffin Bay), mainland Western Australia (Herdsman Lake), Franklin Island, and Hopkins Island. The snakes from the two islands were selected because of their unusually large absolute body and head size, likely attained as a result of dietary constraints and strong selective pressure during thousands of years of isolation on islands. We performed geometric morphometric analysis of the skulls from these snakes to identify and precisely quantify morphological differences. Island snakes are born larger (trunk length) than mainland snakes, and consequently also tend to have larger absolute head size at birth. The heads of island snakes also show a faster relative growth rate (positive allometry) compared to those of mainland snakes, likely to cope with the large prey items found on these islands. However, the snakes from the two islands differ from each other in terms of morphological change associated with ontogeny, especially with regard to a palatal bone (ectopterygoid), which, compared to mainland snakes, is relatively longer in the snakes from Hopkins Island but shorter in the snakes from Franklin Island. Some morphological traits potentially associated with promoting a larger gape were also observed in one of the mainland populations (Herdsman Lake), consistent with standing variability for selection. Our results reveal that discernible changes in morphology and allometry in skull bones have evolved in island populations of tiger snakes in less than 10 000 years, and suggest that the large heads of island tiger snakes might develop via slightly different paths through morphospace on different islands.

Although physiology ensures homeostasis and fitness in a particular environment, and ecological shifts cannot be realized without physiological changes, metabolic transformations during animal adaptive radiations still remain unexplored. We present a study of energy reserve storage in the salmonid assemblage inhabiting a cold-water Lake Kronotskoe. This assemblage diversified from Salvelinus malma and includes eight distinct ecomorphs with contrasting lifestyles and trophic specializations. We hypothesized that ecomorphs differ in energy storage and expenditure, and that their metabolic phenotypes should be among the primary targets of natural selection. To test this hypothesis, we compared the stored amount and ratio of carbohydrates, lipids, circulating peptides that supply the citric acid cycle, as well as proxy indicators of metabolic rate, the blood levels of plasma proteins (including albumin) and hemoglobin. Among ecomorphs, numerous significant differences in physiological parameters were found, closely related to the composition of food, the depth of habitat, and determined by internal factors, probably genetics. Each ecomorph has a specific metabolic phenotype corresponding to its tropho-ecological specialization and lifestyle. Metabolically advanced predators accumulate lipids; littoral insectivorous morphs grow slower and accumulate glycogen; amphipod feeders do not accumulate spare substances; the deepwater consumer of silt benthos differs in the most divergent physiological characteristics. We assume a specific selection on endocrine regulators of energy metabolism during the adaptive radiation of the assemblage, among which the most plausible candidates are thyroid hormones and leptin.

Enteromius Cope, 1867 is a species-rich genus of small cyprinids endemic to Africa, which includes the ‘sawfin barbs’. This study explored the species diversity of this group within the Lake Edward system, including adjacent areas that belong to the Lakes Albert and Victoria systems. We used a multifaceted approach encompassing mitochondrial and nuclear DNA analyses, including a molecular clock analysis, and morphometrics. Additionally, broader regional relationships were investigated by including ‘sawfin barbs’ from other parts of the East Coast ichthyofaunal province and the Nile Basin, and from the Congo Basin, into the molecular analyses. In contrast to the previously reported three species from the Lake Edward system and adjacent areas, the results showed a fourfold increase in the number of species, thereby indicating that the three species actually constituted species complexes. Within these complexes, a consistent geographic pattern unfolded: if one species occurred at higher altitudes of the Lake Edward system, another closely related species occupied lower altitudes near Lakes Edward and George. This geographic consistency suggested an allopatric mode of speciation. Intriguingly, the revealed Pliocene-Pleistocene origin of nearly all species of ‘sawfin barbs’ from the Lake Edward system and neighbouring regions largely predated the important geological events that reshaped the hydrology in the western rift. This study offers a more detailed insight into the evolutionary patterns of the African small barbs representing a very high and unrecognized species diversity, accompanied by little morphological but high genetic divergence between species, indicating intriguingly old species origins.

Plants have different strategies to avoid selfing and buffer its negative consequences on plant fitness. One strategy is the arrangement of petals and the disposition of the reproductive structures (RS) inside the flowers, allowing the development of different pollination mechanisms. In Calceolaria L. species two possible floral phenotypes can be found: short RS protected by the upper corolla lip (nototribic flowers) and long RS resting in the lower corolla lip (sternotribic flowers), the latter being hypothesized to favor selfing. We selected 13 Calceolaria taxa and characterized their floral phenotype as nototribic or sternotribic, measured RS length and herkogamy, and performed hand-pollination treatments to determine the number of seeds produced by self- and cross-pollination to test whether floral phenotype influences inbreeding. GLMs analysis was performed to determine the differences between the sizes of RS and both floral phenotypes, and LMM was performed to evaluate the relationship between the RS and inbreeding with both floral phenotypes. We found a relationship between stamen length and herkogamy in both floral phenotypes, where sternotribic flowers have a higher stamen length and lower herkogamy, whereas the opposite occurred in taxa with nototribic morphology. Stamen length significantly influences the inbreeding with sternotribic flowers having a higher inbreeding depression by geitonogamous self-pollination than nototribic ones. Our results suggest that plants may evolve different reproductive mechanisms to cope with pollination unreliability. Thus, floral phenotype may favor the development of geitonogamy selfing, which may explain the two floral phenotypes present in this specialized oil-secreting genus.

Studies of differences in modularity and integration among the related species can provide important information on their evolution. We argue that evolutionary changes in the modularity patterns might be adaptations. We studied a set of 30 craniometric measurements of bats from the Myotis myotis – Myotis blythii species group (Vespertilionidae, Chiroptera, Mammalia): M. m. myotis, M. b. oxygnathus, M. b. omari, M. b. blythii, and M. b. altaicus. Besides, M. dasycneme that is phylogenetically, morphologically, and ecologically rather distant from this group was studied for comparison. We use innovative approach based on the network community detection to analyze covariance structures of these OTUs and test an array of modularity hypotheses for each OTU. We show that covariance structures of M. m. myotis, M. b. oxygnathus, M. b. omari, and M. b. blythii are more similar to each other, while M. b. altaicus and M. dasycneme structures are more different from one another and from other OTUs. The skull modularity decreases with the increasing skull size in the studied OTUs; this pattern is in good agreement with the hypothesis of skull integration increase in evolution of M. myotis–M. blythii species group with increasing size that makes it possible to prey on large hard-shelled insects. The studied OTUs demonstrate both the effect of the covariance structure on evolutionary change and the restructuring of the covariance structure in evolution by selection.

Geometric Morphometrics can be used to describe morphology as a series of coordinates after the effects of variation in translation, rotation, and scale have been removed. This can be further divided into the notion of shape and form, where the latter excludes the scaling procedure from analyses. Dimensionality reduction in Geometric Morphometrics is necessary for the representation of this data into a reduced, more manageable set of dimensions, while preserving as much of the original variation as possible. The purpose of this study is to explore a new means of performing dimensionality reduction on Procrustes landmark data. Here we present a new mathematical model that can be used to enhance dimensionality reduction techniques such as Principal Component Analyses. Integrated into a new R library, the GraphGMM framework uses elements of geometric learning and graph theory to aggregate and embed (project) morphological information from Procrustes coordinates into a new set of transformed coordinates. We validate this model through the use of theoretically constructed, as well as open source, datasets. We finally present a pilot case study using great ape radii to show how these transformed landmarks efficiently capture morphological information, prior to dimensionality reduction, leading to a more efficient construction of a final representation of a morphological coordinate space. Graph-based Geometric Morphometrics thus provides a new insight into the study of morphological patterns, that can be used as an additional source of information in bioanthropological studies.

Body size is a fundamental trait in evolutionary and ecological research, given that it varies allometrically with several relevant features, such as life-history and physiological traits. Although previous studies uncovered many intriguing patterns, finding general principles of body size evolution in vertebrates has been elusive. In this study, we take advantage of recent advances in phylogenetic comparative methods and the availability of large-scale datasets to explore body size evolution in terrestrial vertebrates. Ancestral character estimation and disparity-through-time plots showed considerable variation in body size evolution, both across lineages and over time. In addition, regardless of the corresponding taxon, posterior predictive simulation demonstrated several consistent ways in which body size evolution in those groups departed from constant-rate models, namely: (1) there was considerable rate heterogeneity within each taxon, (2) there was a positive relationship between body size and its rate of evolution (i.e., large-bodied animals evolved faster than small-bodied ones), and (3) faster evolutionary rates near the present. Finally, geographical mapping of body mass and evolutionary rates revealed some similarities across taxa, but no clear latitudinal trends. Overall, these results indicate that there may be general patterns in the body size evolution on large scales in terrestrial vertebrates, with some intriguing taxon-specific differences.

The environmental transformations associated with cities are expected to affect organisms at the demographic, phenotypic, and evolutionary level, often negatively. The prompt detection of stressed populations before their viability is compromised is essential to understand species’ responses to novel conditions and to integrate urbanization with biodiversity preservation. The presumably stressful conditions of urban environments are expected to affect organisms’ developmental pathways, resulting in a reduction of the efficacy of developmental stability and canalization processes, which can be observed as increased Fluctuating Asymmetry (FA) and Phenotypic Variance (PV), respectively. Here, we investigated whether patterns of phenotypic variation of urban populations of a fully terrestrial salamander, Salamandra salamandra bernardezi, are affected by urban settings compared to surrounding native forest populations. We sampled populations within and around the city of Oviedo (northern Spain) and used geometric morphometrics to compare morphological differentiation, head shape deviance from the allometric slope, PV, and FA. We also compared morphological patterns with neutral genetic and structure patterns. We observed increased levels of differentiation among urban populations and in PV within certain of them, yet no differences in allometric deviance and FA were detected between habitats, and no morphological measures were found to be correlated with genetic traits. Our results do not support a clear negative impact of urban conditions over salamander populations, but rather suggest that other ecological and evolutionary local processes influence morphological variation in this urban system.

Phenotypic change plays diverse roles in species’ colonization, but most invasion studies target single species. To compare ecomorphological changes among co-invading species with overlapping niches, we examined three lizards on the island of O‘ahu (Anolis carolinensis, A. sagrei, Phelsuma laticauda). Using specimens from three decades of unfolding invasions obtained through museum collections and contemporary field work, we quantified shifts in three traits: snout vent length (SVL), forelimb-, and hindlimb-length (limb lengths relative to SVL). We hypothesized that competition among these three species has led to ecological shifts that will be detectable through morphological change. Overall, we found that unique patterns of phenotypic change were both species-specific and sex-specific within species: (1) male A. sagrei, female A. carolinensis, and male P. laticauda increased in SVL and (2) relative hindlimb length increased in female A. carolinensis since the 1980s. The observed changes involve traits that may be consequential to invasion dynamics. This study illustrates how museum- and field-based research can be integrated to document nuanced temporal patterns in the phenotypes of co-invading species that share similar niches in native ranges, raising questions about the underlying process(es) driving species- and sex-specific change in co-invaded systems.

Understanding the evolution of evolvability—the evolutionary potential of populations—is key to predicting adaptation to novel environments. Despite growing evidence that evolvability structures adaptation, it remains unclear how adaptation to novel environments in turn influences evolvability. Here we address the interplay between adaptation and evolvability in the peacock fly Tephritis conura, which recently underwent an adaptive change in ovipositor length following a host shift. We compared the evolvability of morphological traits, including ovipositor length, between the ancestral and the derived host race. We found that mean evolvability was reduced in females of the derived host race compared to the ancestral host race. However, patterns of multivariate evolvability (considering trait covariances) were very similar in both host races, and populations of the derived host race had diverged from the ancestral host race in directions of greater-than-average evolvability. Exploration of phenotypic integration patterns further revealed relatively high levels of independent variation in ovipositor length compared to other measured traits, allowing some degree of independent divergence. Our findings suggest that adaptation to novel environments can reduce mean evolvability without major changes in patterns of variational constraints, and that trait autonomy helps facilitate divergence of functionally important traits.

The goal of evolutionary biology is to explain the diversity of the entire sweep of the natural world; population biology only examines tiny slices of time of a few individuals of single species. What gives the tiny scale of population biology its relevance to evolutionary biology is the following assumption: processes identical or similar to those observed in a given population biology study are operative in unexamined individuals in the same species, have been operative throughout the history of the species, and are operative in other species. Without this assumption, population biology studies are just very detailed descriptions of a handful of individuals of a species. Population biology lacks the means to test its jusifying assumption. It is tested by the comparative method, studies of convergent evolution across species. The comparative method has its own blind spots, mainly its inability to examine intraspecific variation, heritability, and fitness directly, exactly the purview of population biology. Population and comparative biology thus provide complementary sources of direct evidence regarding evolutionary process. Both, along with optimality models, evo-devo studies of the variants that can or can’t be produced in development, together with assumptions about unseeable ancestral populations, make up essential parts of a maximally well-supported evolutionary explanation. Recognizing this essential epistemic interdependence shows why it is necessary to select sources of evidence from across population, comparative, optimality, and developmental studies, leading to collaboration rather than criticism across these fields, and stronger explanations accounting for the evolution of diversity in organismal form and function.

Measurement error is present in all quantitative studies, and ensuring proper biological inference requires that the effects of measurement error are fully scrutinized, understood, and to the extent possible, minimized. For morphometric data, measurement error is often evaluated from descriptive statistics that find ratios of subject or within-subject variance to total variance for a set of data comprising repeated measurements on the same research subjects. These descriptive statistics do not typically distinguish between random and systematic components of measurement error, even though the presence of the latter (even in small proportions) can have consequences for downstream biological inferences. Furthermore, merely sampling from subjects that are quite morphologically dissimilar can give the incorrect impression that measurement error (and its negative effects) are unimportant. We argue that a formal hypothesis-testing framework for measurement error in morphometric data is lacking. We propose a suite of new analytical methods and graphical tools that more fully interrogate measurement error, by disentangling its random and systematic components, and evaluating any group-specific systematic effects. Through the analysis of simulated and empirical data sets we demonstrate that our procedures properly parse components of measurement error, and characterize the extent to which they permeate variation in a sample of observations. We further confirm that traditional approaches with repeatability statistics are unable to discern these patterns, improperly assuaging potential concerns. We recommend that the approaches developed here become part of the current analytical paradigm in geometric morphometric studies. The new methods are made available in the RRPP and geomorphR-packages.

Interspecific hybridization has been historically neglected in research and conservation practice, but it is a common phenomenon in nature, and several models have been developed to characterize it genetically. Even though Trichechus inunguis (Amazonian manatee) and T. manatus (West Indian manatee) exhibit large morphological, karyotypic, and molecular differences, a hybrid zone was identified on the northern coast of South America, from the Amazon River estuary toward the Guianas coastline. Two major populations or evolutionarily significant units (ESUs) within T. manatus, namely, the Caribbean and Atlantic, were separated and their differentiation was likely promoted or reinforced by the interspecific hybridization zone. We used nuclear and mtDNA sequences to reconstruct manatee speciation, population diversification through time and space, and secondary contact, which resulted in a hybrid zone. In this hybrid zone, the genetic contribution of each parental species was estimated, and different models for generating the current scenario were tested using statistical phylogeographic tools. All the results suggest a long hybridization history, during which a stable and structured hybrid swarm is generated. The coastline hybrid zone is composed of individuals with a lesser genomic contribution from T. inunguis; this zone works as a genetic sink that restricts gene flow between neighbouring Atlantic (Brazil) and Caribbean (all others) T. manatus populations, which further reinforces the isolation and differentiation of the Brazilian manatees.

Two macroecological and evolutionary rules are strongly related to the body size of organisms: Bergmann’s and Rensch’s rules. Bergmann’s rule states that organisms are larger in colder regions (high latitudes). Rensch’s rule states that sexual size dimorphism increases when males are larger. Organisms with widespread distribution and resource-mediated growth—such as hermit crabs and their gastropod shells—become excellent models for investigating these patterns. This study is the first to address macroecological and evolutionary patterns in body size among populations and also among sex of the three species of hermit crabs of the genus Clibanarius: C. antillensis, C. sclopetarius and C. symmetricus throughout their distribution. This research included systematic review of data from specialized literature along with primary data by traditional morphometrics of intersexual and populational average body size of the three different species. Regression models were designed to assess the rules separately and altogether. We have observed that the three species of Clibanarius showed interpopulational clines consistent with Bergmann’s rule. Surprisingly, our findings showed a gender-specific divergence from females as a response to latitudinal gradients, suggesting that latitude (as a proxy) increased the interpopulational body effect only in females. We suggest that phenotypic plasticity due to decreased selective pressure with higher latitude (decreased temperature) and greater productivity may affect the bias of these rules. Our data also suggest that female body size variation is modulated by the selection of fecundity in body size.

The ability of wildlife to endure the effects of high temperatures is increasingly important for biodiversity conservation under climate change and spreading urbanization. Organisms living in urban heat islands can have elevated heat tolerance via phenotypic or transgenerational plasticity or microevolution. However, the prevalence and mechanisms of such thermal adaptations are barely known in aquatic organisms. Furthermore, males and females can differ in heat tolerance, which may lead to sex-biased mortality, yet it is unknown how sex differences in thermal biology influence urban phenotypic divergence. To address these knowledge gaps, we measured critical thermal maxima (CTmax) in male and female agile frog (Rana dalmatina) tadpoles captured from warm urban ponds and cool woodland ponds, and in a common-garden experiment where embryos collected from both habitat types were raised in the laboratory. We found higher CTmax in urban-dwelling tadpoles compared to their counterparts living in woodland ponds. This difference was reversed in the common-garden experiment: tadpoles originating from urban ponds had lower CTmax than tadpoles originating from woodland ponds. We found no effect of sex on CTmax or its difference between habitats. These results demonstrate that aquatic amphibian larvae can respond to the urban heat island effect with increased heat tolerance similarly to other, mostly terrestrial taxa studied so far, and that phenotypic plasticity may be the main driver of this response. Our findings also suggest that heat-induced mortality may be independent of sex in tadpoles, but research is needed in many more taxa to explore potentially sex-dependent urban thermal responses.

The original exposition of the method of “Cartesian transformations” in D’Arcy Thompson’s On Growth and Form (1917) is still its most cited. But generations of theoretical biologists have struggled ever since to invent a biometric method aligning that approach with the comparative anatomist’s ultimate goal of inferring biologically meaningful hypotheses from empirical geometric patterns. Thirty years ago our community converged on a common data resource, samples of landmark configurations, and a currently popular biometric toolkit for this purpose, the “morphometric synthesis,” that combines Procrustes shape coordinates with thin-plate spline renderings of their various multivariate statistical comparisons. But because both tools algebraically disarticulate the landmarks in the course of a linear multivariate analysis, they have no access to the actual anatomical information conveyed by the arrangements and adjacencies of the landmark locations and the distinct anatomical components they span. This paper explores a new geometric approach circumventing these fundamental difficulties: an explicit statistical methodology for the simplest nonlinear patterning of these comparisons at their largest scale, their fits by what Sneath (1967) called quadratic trend surfaces. After an initial quadratic regression of target configurations on a template, the proposed method ignores individual shape coordinates completely. Those have been replaced by a close reading of the regression coefficients, accompanied by several new diagrams, of which the most striking is a novel biometric ellipse, the circuit of the trend’s second-order directional derivatives around the data plane. These new trend coordinates, directly visualizable in their own coordinate plane, do not conduce to any of the usual Procrustes or thin-plate summaries. The geometry and algebra of the second-derivative ellipses seem a serviceable first approximation for applications in evo-devo studies and elsewhere. Two examples are offered, one the classic growth data set of Vilmann neurocranial octagons and the other the Marcus group’s data set of midsagittal cranial landmarks over most of the orders of the mammals. Each analysis yields intriguing new findings inaccessible to the current GMM toolkit. A closing discussion suggests a variety of ways by which innovations in this spirit might burst the current straitjacket of Procrustes coordinates and thin-plate splines that together so severely constrain the conversion of landmark locations into biological understanding. This restoration of a quantitative diagrammatic style for reporting effects across regions and gradient directions has the potential to enrich landmark-driven comparisons over either developmental or phylogenetic time. Extension of the paper’s quadratic methods to the next polynomial degree, cubics, probably won’t prove generally useful; but close attention to local deviations from globally fitted quadratic trends, however, might. Ultimately there will have to emerge a methodology of landmark configurations, not merely landmark locations.

Body size is a pivotal ecological and evolutionary trait, as it can significantly impact both survival and reproductive success. To understand how human-mediated disturbances influence body size, we conducted a temporal analysis of body mass index (BMI) variations in 2788 individuals spanning six South American rodent species to describe their seasonal and yearly fluctuations between 2005 and 2009. Additionally, we used microsatellite genotyping to estimate genetic pedigrees for individuals from two of these species (Akodon azarae and Calomys musculinus). This enabled us to dissect the phenotypic variation of body size, offering insights into the evolutionary dynamics of that variation. We report significant increments of BMI across years in three species (A. azarae, Calomys venustus, and Oxymycterus rufus). In addition, we observed moderate and similar levels of narrow-sense heritability in A. azarae and C. musculinus, suggesting that part of the variation in this trait is attributable to additive genetic effects. Furthermore, the phenotypic variance, additive genetic variance, and evolvability of BMI were higher in C. musculinus when compared to A. azarae. These findings suggest that BMI in C. musculinus has the potential to exhibit a more rapid response to equivalent selection pressures than in A. azarae. The heritability and evolvability values also imply that the annual changes in BMI may be influenced, at least in part, by natural selection, probably in response to shifting environmental conditions within intensively managed agroecosystems. However, a long-term study is necessary to understand and predict the role of selection in the evolutionary dynamics of body size variation among rodents inhabiting agroecosystems.