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Microevolution in island rodents
Abstract and Figures
We perform a meta-analysis on morphological data from four island rodent populations exhibiting microevolution (>100 years). Data consisting of incidences of skeletal variants, cranial, and external measurements are from house mice (Mus musculus) on one Welsh and one Scottish island, black rats (Rattus rattus) on two Galapagos islands, and deer mice (Peromyscus maniculatus) on three California Channel islands. We report extremely high rates of microevolution for many traits; 60% of all mensural traits measured changed at a rate of 600 d or greater (max. 2682 d). The proportion of all mensural traits evolving at 600–800 d (23%) was idiosyncratic and departed from an expected negative exponential distribution. We argue that selection, rather than founder events, is largely responsible for the substantial shifts in morphology seen among insular rodents. Examining individual traits, there is a trend towards the nose becoming longer and wider, while the skull becomes shallower, shown by both rats and mice on five different islands. We found a significant correlation between island size and degree of skeletal variant evolution and between island distance from the mainland (or nearest island) and degree of cranial and external character evolution. Thus, microevolution of rodents is greater on smaller and more remote islands.
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... Ecomorphological studies centered on mammal species have widely pondered cranial variations, both in shape and size, as a proxy for assessing the influences of environmental factors on structuring of phenotypic variation (Carnivora: Bubadué et al., 2016;Schiaffini et al., 2013;Schiaffini, Segura & Prevosti, 2019;Migliorini, Fornel & Kasper, 2020;Artiodactyla: Hendges, Bubadué & Cáceres, 2016;Chiroptera: Marchan-Rivadeneira et al., 2012;Ariosa-Olea & Mancina, 2018;Cingulata: Feijó, Patterson & Cordeiro-Estrela, 2020;Primates: Cardini, Jansson & Elton, 2007;Cardini & Elton, 2008;Cáceres et al., 2014;Didelphimorphia: López-Fuster et al., 2000;Magnus, Machado & Cáceres, 2017;Bubadué et al., 2021). The skull is a key structure that holds most sensory and some food processing organs, whose intraspecific variation frequently mirrors the influence of the clinal or steep environmental gradients (see Pergams & Ashley, 2001;Pergams & Lawler, 2009;Samuels, 2009;Grieco & Rizk, 2010). Rodent cranial variation has been broadly employed as a proxy to study the ecogeographical association and potential drivers of phenotypic variability in landscapes with natural (e.g., Bacigalupe, Iriarte-Díaz & Bozinovic, 2002;Monteiro, Duarte & dos Reis, 2003;Cordero & Epps, 2012;Alvarado-Serrano, Luna & Knowles, 2013;Camargo et al., 2019) or human modified (e.g., Martínez et al., 2014;Yalkovskaya et al., 2016;Caccavo et al., 2021; also see references in Coda et al., 2021) environmental configurations. ...
The relationship between phenotypic variation and landscape heterogeneity has been extensively studied to understand how the environment influences patterns of morphological variation and differentiation of populations. Several studies had partially addressed intraspecific variation in the sigmodontine rodent Abrothrix olivacea, focusing on the characterization of physiological aspects and cranial variation. However, these had been conducted based on geographically restricted populational samples, and in most cases, the aspects characterized were not explicitly contextualized with the environmental configurations in which the populations occurred. Here, the cranial variation of A. olivacea was characterized by recording twenty cranial measurements in 235 individuals from 64 localities in Argentina and Chile, which widely cover the geographic and environmental distribution of this species. The morphological variation was analyzed and ecogeographically contextualized using multivariate statistical analyses, which also included climatic and ecological variation at the localities where the individuals were sampled. Results indicate that the cranial variation of this species is mostly clustered in localized patterns associated to the types of environments, and that the levels of cranial differentiation are higher among the populations from arid and treeless zones. Additionally, the ecogeographical association of cranial size variation indicate that this species does not follow Bergmann’s rule and that island populations exhibit larger cranial sizes compared to their continental counterparts distributed at the same latitudes. These results suggest that cranial differentiation among the populations of this species is not homogeneous throughout its geographic distribution, and that the patterns of morphological differentiation are also not completely consistent with the patterns of genetic structuring that have been described recently. Finally, the analyses performed to ponder morphological differentiation among populations suggest that the contribution of genetic drift in the formation of these patterns can be ruled out among Patagonian populations, and that the selective effect imposed by the environment could better explain them.
... Bradshaw and Holzapfel, 2006;Scheiner, 1993;Valladares et al., 2014;Via and Lande, 1985). Although sigmodontine rodents show high speciation rates, especially in tropical lowlands where P. simplex usually inhabits (Parada et al., 2015), microevolutionary changes in such short time periods (e.g. the last 5000 years) are mainly observed in islands in the case of rodents (Cucchi et al., 2014;Pergams and Ashley, 2001;Van der Geer, 2020). Some cases of local adaptation and phenotypic plasticity in inland areas have been observed in several rodent species at short time scales (Bittner et al., 2021;Menéndez et al., 2022) so that it cannot be ruled out that there may have been populations of P. simplex with different climatic preferences. ...
Pseudoryzomys simplex (Winge 1887) is a small to medium sized terrestrial oryzomyine rodent (Cricetidae, Sigmodontinae) widely distributed in humid environments of tropical and subtropical lowlands of South America with a marked rainy season. The occurrence of this species in the south-central Pampean region between 4700 and 200 ka cal BP was originally associated with some warm climatic episodes. A later hypothesis points out that P. simplex was a typical element of the Humid Pampa subregion during this period, whose distribution was retracted to the north due to the strong anthropic modifications in the area. To corroborate these hypotheses, the niche of occurrence of P. simplex was calculated to analyse the preferred climatic conditions of contemporaneous and fossil record of the species since 4700 ka cal BP. The derived climatic niche of P. simplex shows a marked preference for areas with seasonality precipitation patterns and tropical/subtropical conditions. Geographical estimations and climatic niches derived revealed a moderate overlapping between contemporaneous and Pampean late-Holocene projection, which in no case includes the localities where fossil data were found. Although a better comprehension of the ecological requirements of the species is necessary to discard a shift in its realized climatic niche, it is probable that P. simplex is able to maintain stable populations under similar conditions to the ones represented by fossil occurrences. The distribution of the species in the Humid Pampa subregion is probably affected by non-climatic factors today which are reducing its contemporaneous realized climatic niche and distorting the derived climatic niches obtained. These results support that this species cannot be considered a reliable palaeoclimatic indicator of more Brazilian conditions in the Humid Pampean subregion during the past.
... Pergams ve Ashley [54], rodentlerin fenotip karakterlerindeki farklıkların; genetik etki, popülasyonlarındaki gen transferleri, doğal seleksiyon, çevresel, coğrafi, cinsiyet vb. gibi etkilere de bağlı olabileceğini bildirmiştir. ...
In this study, 10 tree squirrels (Sciurus anomalus) and 9 ground squirrels (Spermophilius xanthoprymnus) was used. For geometric morfometrics analysis, 15 homologous landmarks in the dorsal view and 16 homologous landmarks in the ventral view of the skull of both species were used. The differences in the dorsal face of the skull of both species were much more evident compared to the ventral face of the skull. The relative warp analysis based on geometric morphometrics in dorsal and ventral view of the skull of the tree and ground squirrels was found as Rw1 % 65.50, Rw2 % 12.35 and Rw1 % 53.33, Rw2 % 12.71 respectively. The gathering into groups on the axis of both species were clearly demonstrated the geometric, anatomical, and morphological differences. The differences in the dorsal view of the skull, especially in the level of the end point of the proc. zygomaticus and the incisura supraorbitale of the os frontale in TPS analysis were quite evident. On the other hand, the most differences in ventral view of the skull of both species was the level of the posterior point of the os palatinum, the middle of the upper teeth, posterior edge of the for. magnum, and the tuber faciale.
... Insular mammals show elevated rates of morphological evolution (Stock 1935;Roth 1992;Pergams and Ashley 2001;Millien 2006). Those traits that consistently change in response to island colonization provide clues into the process of adaptation. ...
Some of the most compelling examples of morphological evolution come from island populations. Alterations in the size and shape of the mandible have been repeatedly observed in murid rodents following island colonization. Despite this pattern and the significance of the mandible for dietary adaptation, the genetic basis of island-mainland divergence in mandibular form remains uninvestigated. To fill this gap, we examined mandibular morphology in 609 F2s from a cross between Gough Island mice, the largest wild house mice on record, and mice from a mainland reference strain (WSB). Univariate genetic mapping identifies three quantitative trait loci (QTL) for relative length of the temporalis lever arm and two distinct QTL for relative condyle length, two traits expected to affect mandibular function that differ between Gough Island mice and WSB mice. Multivariate genetic mapping of coordinates from geometric morphometric analyses identifies 27 QTL contributing to overall mandibular shape. QTL show a complex mixture of modest, additive effects dispersed throughout the mandible, with landmarks including the coronoid process and the base of the ascending ramus frequently modulated by QTL. Additive effects of most shape QTL do not align with island-mainland divergence, suggesting that directional selection played a limited role in the evolution of mandibular shape. In contrast, Gough Island mouse alleles at QTL for centroid size and QTL for jaw length increase these measures, suggesting selection led to larger mandibles, perhaps as a correlated response to the evolution of larger bodies.
... As rearticulated by Heaney and Lomolino in the 1980s, the island rule describes a "graded trend from gigantism in small species to dwarfism in large species" (Lomolino, 1985). Besides, rapid evolution of rodents is greater on smaller and more remote islands (Benítez-López et al., 2021;Pergams & Ashley, 2001). Reduced predation (Steen et al., 1990), reduced interspecific competition (Millien, 2004) and increased intraspecific competition (Adler & Levins, 1994) have been proposed to explain the island rule. ...
Aim
To compare the rapid shifts in body size of mainland and island populations of a native rodent and examine the mechanisms underlying these changes.
Location
Thousand Island Lake, China, which was created in 1959 when the Xin'anjiang Dam was constructed for generating hydroelectricity.
Taxon
The Chinese white-bellied rat, Niviventer confucianus.
Methods
Field surveys were conducted from 2015 to 2018 to collect data on body size of the rodents from a set of islands and nearby mainland sites. We constructed multiple linear models to examine the relationships between body size (length and mass) of rodents and biological variables (predators, competitors and food availability). We also conducted structural equation modelling (SEM) by constructing models via confirmatory path analysis.
Results
All island populations of N. confucianus had significantly larger body size (both body mass and body length) than their mainland counterparts. Moreover, populations on small and more isolated islands had larger body size than their relatives on big islands. The relative absence of predators (large-bodied mammals, snakes and raptors) on islands was most strongly associated with shifts in the body size of rodents. The documented changes occurred after only a half-century of fragmentation.
Main conclusions
The observed rapid body enlargement of rodents after habitat fragmentation is consistent with a release from predation pressure. SEM indicated that island area, rather than island isolation, had positive effects on the abundance of predators, interspecific competitors and food resources, which then had an indirect impact on body size of the rodents. In this study, we report a remarkably rapid case of mammal morphological shifts in a small mammal in response to habitat fragmentation. Given the omnipresence of dams and other anthropogenic disturbances, our findings suggest that a wave of rapid phenotypic shifts in terrestrial vertebrates is taking place in the Anthropocene.
... Commensalism with humans has allowed mice and rats to invade islands around the world, exposing them to a wide range of new selective regimes on recent timescales. Skeletal phenotypes from island murids show some of the strongest Downloaded from https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msaa255/5918475 by guest on 17 November 2020 evidence for rapid phenotypic change (Berry et al. 1978;Pergams and Ashley 2001;Millien 2006;Boell and Tautz 2011). Murid rodents usually evolve larger body sizes on islands (Adler and Levins 1994;Meiri et al. 2008), providing multiple compelling examples of the broader pattern of unusual size evolution in island populations, known as the island rule (Foster 1964;Van Valen 1973;Lomolino 1985). ...
A key challenge in understanding how organisms adapt to their environments is to identify the mutations and genes that make it possible. By comparing patterns of sequence variation to neutral predictions across genomes, the targets of positive selection can be located. We applied this logic to house mice that invaded Gough Island, an unusual population that shows phenotypic and ecological hallmarks of selection. We used massively parallel short-read sequencing to survey the genomes of 14 Gough Island mice. We computed a set of summary statistics to capture diverse aspects of variation across these genome sequences, used approximate Bayesian computation to reconstruct a null demographic model, and then applied machine learning to estimate the posterior probability of positive selection in each region of the genome. Using a conservative threshold, 1,463 5kb windows show strong evidence for positive selection in Gough Island mice but not in a mainland reference population of German mice. Disproportionate shares of these selection windows contain genes that harbor derived nonsynonymous mutations with large frequency differences. Over-represented gene ontologies in selection windows emphasize neurological themes. Inspection of genomic regions harboring many selection windows with high posterior probabilities pointed to genes with known effects on exploratory behavior and body size as potential targets. Some genes in these regions contain candidate adaptive variants, including missense mutations and/or putative regulatory mutations. Our results provide a genomic portrait of adaptation to island conditions and position Gough Island mice as a powerful system for understanding the genetic component of natural selection.
... There are published data on rapid (on the ecological time scale, < 100 years or < 30 generations) morphological changes in rodent species under conditions of island isolation (Pergams and Ashley, 2001), whose rates are on the order of 10 -3 h per generation. On the basis of the data of B. Kurtén on the evolution of M1 size in the lineage of cave bears, Lynch (1990, 730) estimated the average rate of evolution at 4.8 × 10 -5 (7.5 × 10 -6 -8.6 × 10 -5 ) per generation (generally accepted as 10 years) or in the range of approximately 0.0075-0.086 ...
... There are published data on rapid (on the ecological time scale, < 100 years or < 30 generations) morphological changes in rodent species under conditions of island isolation (Pergams and Ashley, 2001), whose rates are on the order of 10 -3 h per generation. On the basis of the data of B. Kurtén on the evolution of M1 size in the lineage of cave bears, Lynch (1990, 730) estimated the average rate of evolution at 4.8 × 10 -5 (7.5 × 10 -6 -8.6 × 10 -5 ) per generation (generally accepted as 10 years) or in the range of approximately 0.0075-0.086 ...
The variability of cheek teeth over time was studied with the use of univariate and multivatiate statistical analyses for 850 tooth specimens of the Kudaro cave bear (Ursus kudarensis) collected in the Kudaro 1 and Kudaro 3 caves in the Caucasus. The material examined included two chronosubspecies: U. k. prae-kudarensis (middle part of the Middle Pleistocene, nearly 350000 years ago) and U. k. kudarensis (Late Pleis-tocene, nearly 120000-40000 years ago), as well as a sample of intermediate stratigraphic position referred to as a "transitional form" (the end of the Middle Pleistocene, nearly 250000 years ago). Our study detected no single pathway of evolutionary change in the cheek teeth of U. kudarensis, instead showing specific modifications for the premolars, molars, and even individual teeth (e.g., m3) in the context of high individual variability. In size, the teeth of U. k. praekudarensis occupied an intermediate position between the more robust teeth of the "transitional form" and the smaller teeth of U. k. kudarensis. By the range of morphometric characters (size and crown proportions), the "transitional form" was more closely related to U. k. kudarensis than to U. k. praekudarensis. At the same time, the parameters of allometric variation of the "transitional form" either related it to U. k. praekudarensis or were of the original nature. The calculated rate of evolution of the metric characters was found to be dissimilar not only for different teeth but also for different measurements of the same tooth crown, and these modifications themselves were oppositely directed. A comparison with the main evolutionary trends in European cave bears revealed the unique, independent character of evolution of the cheek teeth in the Kudaro cave bear, which is expressed to different degrees for the upper and lower rows of teeth.
Island rule describes the graded trend of gigantism in small-bodied species to dwarfism in large-bodied species inhabiting islands, but causal explanations remain unresolved. We used geometric morphometrics to quantify cranial morphology of 544 meadow vole Microtus pennsylvanicus samples across 11 island and 3 mainland populations from the Outer Lands of New England (Atlantic) and the Alexander Archipelago of Alaska (Pacific). We compared the thermoregulation and endurance (TRE) and ecological release (ER) hypotheses using all-subsets linear models employing residual randomization permutation procedures (rrpp), and Akaike information criterion (AIC) for model selection. We decoupled direct and indirect effects of island variables on size using path analysis. We evaluated shape with principal components analysis (PCA) and Procrustes ANOVA on Procrustes shape coordinates, then assessed the impact of static allometry and TRE and ER variables on shape. Six Atlantic island populations exhibit significant signals of gigantism with the largest voles occurring on the smallest islands lacking predators. ER explains 63% of cranial size differences. Island area has a significant total effect on size by influencing the number of mammalian predators, resulting in a 0.011 increase in unit centroid size for a 100 km² decrease in island area. This corresponds to a predicted 0.9% change in size for every 100 km². Given static allometry, cranial shape does not respond to insularity independent of size. These results suggest that island rule is a latent evolutionary process whose manifestation depends on nuanced biogeographic and ecological contexts that have important conservation and taxonomic implications.
Isolation due to habitat fragmentation can lead to morphological and functional variation between populations, with the effect being well documented in rodents. Here, we investigated whether such morphological variation could be identified between British populations of the Eurasian red squirrel (Sciurus vulgaris). This species was once widespread across Great Britain, but suffered a severe population decline across the 20th century, leaving a highly fragmented distribution. The aim was to test for morphological and biomechanical variation of the mandible between the remaining British red squirrel populations, and between British and continental European red squirrels. Linear and geometric morphometric methods were used to analyse shape in a sample of over 250 red squirrel hemi-mandibles from across Britain plus a sample from Germany representing the central European subspecies. Procrustes ANOVA identified significant shape variation between populations, with particularly distinct differences being noted between red squirrels from Germany and several British red squirrel populations, which may reflect their evolutionary history. Linear biomechanical measurements showed that the red squirrels from Formby and Jersey had a significantly lower mechanical advantage of the temporalis muscle than other British populations, suggesting they were less efficient at gnawing. This functional difference may be related to many factors, such as founder effect, potential inbreeding and/or past supplemental feeding with less mechanically resistant food items.
Patton, J. L., S. Y. Yang, and P. Myers 1975. Genetic and morphologic divergence among introduced rat populations (Rattus rattus) of the Galápagos Archipelago, Ecuador. Syst. Zool. 24:296–310.—The roof rat (Rattus rattus Linnaeus) was initially introduced into the Galápagos Archipelago before Darwin's visit in 1835. The species is now known
from seven of the 16 major islands and exhibits a wide range in both human and non-human associated habitat usages. Morphological
distinctiveness of island populations was first noted by Heller in 1904, consisting of overall size and shape as well as pelage
color differences. Analyses involving allozyme frequencies at 37 genetic loci, epigenetic cranial characters, and multivariate
treatments of mensural characters confirm and extend these observations. The level of concordance between each analysis is
extremely high; each delineates the same three groupings of islands based on overall similarity: (1) Isla Santiago-Bartolome;
(2) Isla Floreana-Isabela-Pinzón-San Cristóbal; and (3) Isla Santa Cruz-Baltra. An hypothesis of multiple origins best accounts
for the similarity relationships between islands as each of the groupings fits a known separate period of human use activity.
The initial introduction was most likely at Santiago in the late 1600's; the most recent on Baltra-Santa Cruz during World
War II. Gene flow between the different island groups, past and present, is considered slight, but continual introduction
of immigrants from outside source populations, particularly to Baltra, Santa Cruz, and San Cristóbal, is highly probable.
Genie variability as demonstrated by allozyme analysis is quite low (mean heterozygosity = 2.85 per cent) when compared to
other rodent species. The pattern of variability, however, suggests that this low level does not result from a severe founder
effect, but that it is a general characteristic of the species as a whole.
Analyses of craniodental measurement data from 15 wild-collected population samples of the Neotropical muroid rodent genus Zygodontomys reveal consistent patterns of relative variability and correlation that suggest a common latent structure. Eigenanalysis of each sample covariance matrix of logarithms yields a first principal component that accounts for a large fraction of the total variance. Variances of subsequent sample principal components are much smaller, and the results of bootstrap resampling together with asymptotic statistics suggest that characteristic roots of the covariance matrix after the first are seldom distinct. The coefficients of normalized first principal components are strikingly similar from sample to sample: inner products of these vectors reveal an average between-sample correlation of 0.989, and the mean angle of divergence is only about eight degrees. Since first principal component coefficients identify the same contrasts among variables as comparisons of relative variability and correlation, we conclude that a single factor accounts for most of the common latent determination of these sample dispersions. Analyses of variance based on toothwear (a coarse index of age) and sex in the wild-collected samples, and on known age and sex in a captive-bred population, reveal that specimen scores on sample first principal components are age- and sex-dependent; residual sample dispersion, however, is essentially unaffected by age, sex, or age x sex interaction. The sample first principal component therefore reflects the covariance among measured dimensions induced by general growth, and its coefficients are interpretable as exponents of postnatal growth allometry. Path-analytic models that incorporate prior knowledge of the equivalent allometric effects of general growth within these samples can be used to decompose the between-sample variance by factors corresponding to other ontogenetic mechanisms of form change. The genetic or environmental determinants of differences in sample mean phenotypes induced by such mechanisms, however, can be demonstrated only by experiment.
Mitochondrial DNA (mtDNA) from 131 deer mice, Peromyscus maniculatus, collected on the eight California Channel Islands and from seven southern California mainland locations, was isolated and analyzed for restriction endonuclease fragment polymorphisms. A total of 26 mtDNA genotypes were distinguishable among the deer mice sampled. All of the island samples had mtDNA restriction-fragment patterns not found among the mainland samples. Distributions of specific restriction-fragment patterns provide evidence for at least four separate colonization events to the Channel Islands. The estimated percentage of sequence divergence between all mtDNA's in this study was less than 1%, suggesting that colonization of the islands occurred fairly recently, probably within the last 500,000 years. Levels of mtDNA heterogeneity were much lower within island populations than within mainland populations.
A survey of electrophoretic variation in proteins encoded by 24 gene loci revealed differences in allele frequencies (principally at ADH, ES-1, and GOT-1) between two karyotypically characterized species of deer mice (Peromyscus maniculatus and P. oreas) from Vancouver Island and smaller islands in the Queen Charlotte Strait and Strait of Georgia. The large number of alleles common to the two taxa and the absence of fixed allelic differences are consistent with a hypothesis of recent common ancestry. In addition, random variance in allele frequencies among small-island populations of both species is indicative of interspecific divergence accelerated by stochastic processes. Nevertheless, genetic consequences of geographic isolation (low genic variability and high interpopulational differentiation) are more pronounced in P. oreas than in P. maniculatus, and this is attributed to differences in potential for gene flow. Patterns of geographic variation evident in morphological analyses, summarized in the recognition of P. maniculatus austerus and three subspecies of P. oreas (P. o. oreas, P. o. interdictus, and P. o. isolatus) from Pacific regions of British Columbia and Washington, may stem from genetic factors related to peripheral isolation of P. oreas from P. maniculatus and interspecific differences in dispersal abilities.
We report on eight years of data for a population of deer mice (Peromyscus maniculatus) on Santa Barbara Island, California which reaches exceptionally high densities and fluctuates markedly in an apparent three- to four-year cycle. The cyclic increase follows winters with high rainfall, and the decline may be similarly associated with low rainfall winters. The peak and early decline is marked by nearly complete cessation of breeding, along with heavy predation by barn owls (Tyto alba), whose numbers track those of the deer mice. This island population of Peromyscus differs from the well-established pattern for the species on the mainland, where populations occur in low to moderate numbers and are relatively stable from year to year. The pattern seen on the island is instead similar to that of cyclic microtines. We compare Microtus and the Santa Barbara Island deer mice and discuss parallels in the possible causes of the respective cycles.
Deer mice, Peromyscus maniculatus, collected over 90 years from three California Channel Islands, were examined for evidence of morphological change. Rapid morphological change has occurred in the endemic subspecies from Santa Barbara (P. m. elusus), Anacapa (P. m. anacapae), and Santa Cruz Island (P. m. santacruzae). Data were divided into two temporal classes, 1897-1941 and 1955-1988. Of the 16 morphological characters measured, between five and 10 measures changed significantly (P ≤ 0.05) with temporal class in each subspecies, and multivariate test statistics were significant (P ≤ 0.05) for all three subspecies. For each subspecies, depth of braincase, total length, tail length, and hind foot length became smaller over time, except depth of braincase, which became larger in P. m. elusus. The rates of change dramatically exceed those estimated from paleontological records and are even higher than those reported in some experimental selection studies. Temporal change in two characters exceeds differentiation between subspecies. Although changing, each subspecies remained well differentiated, and incorporation of temporal change allowed correct classification of most specimens. Unlike nearly all previous reports of rapid evolution, the changes in these populations were not associated with a founder events or recent introductions. This study demonstrates that rapid phenotypic change can occur in long-established natural populations and temporal stability of morphological characters in such populations, even over short evolutionary time periods, cannot be assumed.
There have been mice (Mus musculus) on the Welsh island of Skokholm for only about 70 years, yet they are very distinct from their mainland neighbors and relatives both in skeletal characters and overall size. Divergence of the same order was found between the mice on the similar Scottish island of the May and their nearest neighbors. There was no sign of convergence between the Skokholm and May populations. Skulls of the extinct M. m. muralis from St. Kilda showed less evidence of divergence from the pattern of skeletal variation that characterizes all mice classified from the British mainland (which populations are relatively uniform over a wide area). It is concluded that the peculiar features of these island races stem from the chance characteristics of their founder members.