Morphology and taxonomic status of the grass snake, Natrix natrix (L.) (Reptilia, Squamata, Colubridae) on the island of Gotland, Sweden

The isolated population of grass snakes, Natrix natrix (L.), on the island of Gotland in die Baltic Sea is described and illustrated. Comparisons are made with the surrounding mainland grass snake. On the basis of morphological and ecological divergence, the conclusion is drawn that the Gotland population represents a new subspecies.     

Microevolution and taxonomy of European reptiles with particular reference to the grass snake Natrix natrix and the wall lizards Podarcis sicula and P. melisellensis

The grass snake Natrix natrix and the wall lizards Podarcis sicula and P. melisellensis are used as examples to compare the procedure and achievements of the conventional approach to naming subspecies with the use of multivariate morphometries to investigate racial differentiation.
The conventional procedure, which has changed little over the last 50 years, fails to take into account the appropriate evolutionary facts or refer to any abstracted levels of divergence necessary for subspecific recognition. Consequently, the patterns of population differentiation are obscured by the recognition of a large number of rather meaningless subspecies. There is a tendency to section clines into artificial categories and arbitrarily delimit subspecies by physiographic features.
On the other hand, the use of multivariate morphometries reveals the patterns of population differentiation which can be related to geological events and patterns of differentiation in other species and species groups. The nature of 'hybrid' zones and population differentiation enables the relative importance of evolutionary forces such as gene flow, selection and genetic drift to be discussed and provides evidence concerning speciation mechanisms. These techniques also contribute to the discussion regarding the nature of species and provide abstracted and operational criteria for taxonomic decisions.
The difference between the results of multivariate analysis and the conventional approach cannot be explained solely on the basis of choice of characters. Some of the advantages and disadvantages of using multivariate morphometries, as opposed to other modern techniques, for investigating racial affinities are discussed.     

Quantitative handling of characters useful in snake systematics with particular reference to intraspecific variation in the Ringed Snake Natrix natrix (L.)

This paper is based on the numerous characters used in a biometric study of the intraspecific variation of Natrix natrix (L.). These characters are drawn from the internal morphology, scalation, colour pattern, body proportions, dentition and dermal sense organs. The criteria involved in selecting these characters for taxonomic work are considered.
The alternative methods for quantifying the above features are discussed and so are the theoretical problems involved in coding the continuous characters.
The value of the characters in studying the geographic variation, ontogenetic variation and sexual dimorphism in N. natrix is indicated.
The use of a wider range of characters than just the scalation and colour pattern is urged when diagnosing the lower taxonomic levels of ophidians. The neglected internal morphology may well be able to supply characters to this end.     

The development of the trigeminal jaw adductor musculature in the grass snake Natrix natrix

The ontogenetic development of the jaw adductor musculature in Natrix natrix (L.) is described in detail and related to patterns of ossification in the skull. A comparison with the development of the jaw adductors in the lizard Podarcis sicula reveals some interesting differences of dynamics in the developing head. The problem of the establishment of topological relations of similarity (homology) in developing systems is discussed.
The homologies of the external jaw adductor compartments in lizards and snakes are revised. Early ontogenetic divergence explains the reversal of fibre direction in the anterior portion of the external adductor of snakes, and renders the homologization of that fibre bundle with part of the external adductor in lizards impossible.     

Haematology of the grass snake Natrix natrix natrix L.

1. Haematological parameters and cytomorphological picture of circulating blood of the grass snake Natrix natrix natrix L. were studied. Mean annual values of the haemoglobin concentration, haematocrit level, red cell counts, erythrocytes indices, erythrocyte sedimentation rate, thrombocyte and leucocyte counts, per cent composition of white blood cells and size of morphotic elements were determined for a population sample (N = 154) of the species, considering the sex of snakes. 2. Parameters of red blood cell system (RCC, Hb, Hct) were statistically significantly higher in males, remaining indices showed no statistically significant differences. 3. A strict positive correlation was found between RCC, Hb and Hct in the studied sample. 4. Cytomorphology of blood is presented in microphotographs. 5. In the discussion haematological data for the grass snake and other snakes are compared.     

Complex patterns of genetic and phenotypic divergence in an island bird and the consequences for delimiting conservation units

Substantial phenotypic and genetic variation is often found below the species level and this may be useful in quantifying biodiversity and predicting future diversification. However, relatively few studies have tested whether different aspects of intraspecific variation show congruent patterns across populations. Here, we quantify several aspects of divergence between 13 insular populations of an island endemic bird, the Vanuatu white-eye ( Zosterops flavifrons ). The components of divergence studied are mitochondrial DNA (mtDNA), nuclear DNA microsatellites and morphology. These different aspects of divergence present subtly different scenarios. For instance, an mtDNA phylogenetic tree reveals a potential cryptic species on the most southerly island in Vanuatu and considerable divergence between at least two other major phylogroups. Microsatellite loci suggest that population genetic divergence between insular populations, both between and within phylogroups, is substantial, a result that is consistent with a low level of interisland gene flow. Finally, most populations were found to be strongly morphologically divergent, but no single population was morphologically diagnosable from all others. Taken together, our results show that, although many measures of divergence are concordant in this system, the number of divergent units identified varies widely depending on the characters considered and approach used. A continuum of divergence and a degree of discordance between different characters are both to be expected under simple models of evolution, but they present problems in terms of delimiting conservation units.     

Nuclear gene genealogies reveal historical,demographic and selective factors associated with speciation in field crickets

Population genetics theory predicts that genetic drift should eliminate shared polymorphism, leading to monophyly or exclusivity of populations, when the elapsed time between lineage-splitting events is large relative to effective population size. We examined patterns of nucleotide variation in introns at four nuclear loci to relate processes affecting the history of genes to patterns of divergence among natural populations and species. Ancestral polymorphisms were shared among three recognized species, Gryllus firmus, G. pennsylvanicus, and G. ovisopis, and genealogical patterns suggest that successive speciation events occurred recently and rapidly relative to effective population size. High levels of shared polymorphism among these morphologically, behaviorally, and ecologically distinct species indicate that only a small fraction of the genome needs to become differentiated for speciation to occur. Among the four nuclear gene loci there was a 10-fold range in nucleotide diversity, and patterns of polymorphism and divergence suggest that natural selection has acted to maintain or eliminate variation at some loci. While nuclear gene genealogies may have limited applications in phylogeography or other approaches dependent on population monophyly, they provide important insights into the historical, demographic, and selective forces that shape speciation.     

Non-gradual variation in colour morphs of the strawberry poison frog Dendrobates pumilio: genetic and geographical isolation suggest a role for selection in maintaining polymorphism

The relative roles that geographical isolation and selection play in driving population divergence remain one of the central questions in evolutionary biology. We approached this question by investigating genetic and morphological variation among populations of the strawberry poison frog, Dendrobates pumilio, in the Bocas del Toro archipelago, Panama. We found significant population genetic structure and isolation by distance based on amplified fragment length polymorphism markers. Snout vent length (SVL), coloration and the extent and size of dorsal black spots showed large variation among the studied populations. Differences in SVL correlated with genetic distance, whereas black spot patterns and other coloration parameters did not. Indeed, the latter characters were observed to be dramatically different between contiguous populations located on the same island. These results imply that neutral divergence among populations may account for the genetic patterns based on amplified fragment length polymorphism markers and SVL. However, selective pressures need to be invoked in order to explain the extraordinary variation in spot size and coverage, and coloration. We discuss the possibility that the observed variation in colour morphs is a consequence of a combination of local variation in both natural selection on an aposematic signal towards visual predators and sexual selection generated by colour morph-specific mate preferences.     

MICROGEOGRAPHIC PATTERNS OF GENETIC AND MORPHOLOGICAL VARIATION IN SAVANNAH SPARROWS (PASSERCULUS SANDWICHENSIS)

Surveys of genetic population structure are often limited to large geographic scales because geographically close populations are indistinguishable. Genetic uniformity across adjacent demes can be interpreted as evidence for cohesion (panmixia) or recent divergence. However, poor genetic resolution at microgeographic scales can also arise from the use of overly conservative (slowly evolving) markers. This study examines the ability of hypervariable, minisatellite loci to discriminate among geographically close populations of Savannah sparrows (Passerculus sandwichensis) and to track morphological differentiation at a microgeographic scale (interregional distance < 55 km). Savannah sparrows breeding at five island and two mainland sites in the Bay of Fundy, New Brunswick, Canada, show concordant patterns of variation in external morphology (seven characters) and multilocus DNA fingerprinting profiles (S_xy): island sparrows are phenotypically larger and genetically more similar to each other than they are to mainland sparrows. This pattern of variation is consistent with both adaptive (natural selection) and nonadaptive (genetic drift) mechanisms of population divergence. Based on minisatellite diversity, the effective size of both island and mainland populations is 37, an estimate substantially lower than census population sizes. These data are discordant with observations of sparrow vagility and abundance and suggest a closer examination of microgeographic patterns in avian systems.     

Contrasting genetic responses to population fragmentation in a coevolving fig and fig wasp across a mainland–island archipelago

Interacting species of pollinator–host systems, especially the obligate ones, are sensitive to habitat fragmentation, due to the nature of mutual dependence. Comparative studies of genetic structure can provide insights into how habitat fragmentation contributes to patterns of genetic divergence among populations of the interacting species. In this study, we used microsatellites to analyse genetic variation in Chinese populations of a typical mutualistic system – Ficus pumila and its obligate pollinator Wiebesia sp. 1 – in a naturally fragmented landscape. The plants and wasps showed discordant patterns of genetic variation and geographical divergence. There was no significant positive relationship in genetic diversity between the two species. Significant isolation‐by‐distance (IBD) patterns occurred across the populations of F. pumila and Wiebesia sp. 1 as whole, and IBD also occurred among island populations of the wasps, but not the plants. However, there was no significant positive relationship in genetic differentiation between them. The pollinator populations had significantly lower genetic variation in small habitat patches than in larger patches, and three island pollinator populations showed evidence of a recent bottleneck event. No effects of patch size or genetic bottlenecks were evident in the plant populations. Collectively, the results indicate that, in more fragmented habitats, the pollinators, but not the plants, have experienced reduced genetic variation. The contrasting patterns have multiple potential causes, including differences in longevity and hence number of generations experiencing fragmentation; different dispersal patterns, with the host's genes dispersed as seeds as well as a result of pollen dispersal via the pollinator; asymmetrical responses to fluctuations in partner populations; and co‐existence of a rare second pollinating wasp on some islands. These results indicate that strongly interdependent species may respond in markedly different ways to habitat fragmentation.     

Constraints on geographic variation in fiddler crabs (Ocypodidae: Uca) from the western Atlantic

A key question in evolutionary biology is how intraspecific variation biases the evolution of a population and its divergence from other populations. Such constraints potentially limit the extent to which populations respond to selection, but may endure long enough to have macroevolutionary consequences. Previous studies have focused on the association between covariation patterns and divergence among isolated populations. Few have focused on geographic variation among semi‐connected populations, however, even though this may be indicative of early selective pressures that could lead to long‐term divergence and speciation. Here, we test whether covariation in the shape of the carapace of fiddler crabs (genus Uca Leach, 1814) is important for structuring geographic variation. We find that morphological divergence among populations is associated with evolvability in the direction of divergence in only a few species. The shape of the ancestral covariation matrix in these species differs from other species in having notably more variation concentrated along fewer directions (i.e. higher eccentricity). For most species, there is some evidence that covariation has constrained the range of directions into which populations have diverged but not the degree of divergence. These results suggest that even though fiddler crab populations have diverged morphologically in directions predicted by covariation, constraints on the extent to which divergence has occurred may only be manifested in species where variation patterns are eccentric enough to limit populations’ ability to respond effectively in many directions.     

Microevolution in island forms: the roles of drift and directional selection in morphological divergence of a passerine bird

Abstract.— Theory predicts that in small isolated populations random genetic drift can lead to phenotypic divergence; however this prediction has rarely been tested quantitatively in natural populations. Here we utilize natural repeated island colonization events by members of the avian species complex, Zosterops lateralis , to assess whether or not genetic drift alone is an adequate explanation for the observed patterns of microevolutionary divergence in morphology. Morphological and molecular genetic characteristics of island and mainland populations are compared to test three predictions of drift theory: (1) that the pattern of morphological change is idiosyncratic to each island; (2) that there is concordance between morphological and neutral genetic shifts across island populations; and (3) for populations whose time of colonization is known, that the rate of morphological change is sufficiently slow to be accounted for solely by genetic drift. Our results are not consistent with these predictions. First, the direction of size shifts was consistently towards larger size, suggesting the action of a nonrandom process. Second, patterns of morphological divergence among recently colonized populations showed little concordance with divergence in neutral genetic characters. Third, rate tests of morphological change showed that effective population sizes were not small enough for random processes alone to account for the magnitude of microevolutionary change. Altogether, these three lines of evidence suggest that drift alone is not an adequate explanation of morphological differentiation in recently colonized island Zosterops and therefore we suggest that the observed microevolutionary changes are largely a result of directional natural selection.     

The relative roles of cultural drift and acoustic adaptation in shaping syllable repertoires of island bird populations change with time since colonization

In birds, song divergence often precedes and facilitates divergence of other traits. We assessed the relative roles of cultural drift, innovation, and acoustic adaptation in divergence of island bird dialects, using silvereyes (Zosterops lateralis). In recently colonized populations, syllable diversity was not significantly lower than source populations, shared syllables between populations decreased with increasing number of founder events, and dialect variation displayed contributions from both habitat features and drift. The breadth of multivariate space occupied by recently colonized Z. l. lateralis populations was comparable to evolutionarily old forms that have diverged over thousands to hundreds of thousands of years. In evolutionarily old subspecies, syllable diversity was comparable to the mainland and the amount of variation in syllable composition explained by habitat features increased by two‐ to threefold compared to recently colonized populations. Together these results suggest that cultural drift influences syllable repertoires in recently colonized populations, but innovation likely counters syllable loss from colonization. In evolutionarily older populations, the influence of acoustic adaptation increases, possibly favoring a high diversity of syllables. These results suggest that the relative importance of cultural drift and acoustic adaptation changes with time since colonization in island bird populations, highlighting the value of considering multiple mechanisms and timescale of divergence when investigating island song divergence.     

Adaptive divergence despite strong genetic drift: genomic analysis of the evolutionary mechanisms causing genetic differentiation in the island fox (Urocyon littoralis)

The evolutionary mechanisms generating the tremendous biodiversity of islands have long fascinated evolutionary biologists. Genetic drift and divergent selection are predicted to be strong on islands and both could drive population divergence and speciation. Alternatively, strong genetic drift may preclude adaptation. We conducted a genomic analysis to test the roles of genetic drift and divergent selection in causing genetic differentiation among populations of the island fox (Urocyon littoralis). This species consists of six subspecies, each of which occupies a different California Channel Island. Analysis of 5293 SNP loci generated using Restriction‐site Associated DNA (RAD) sequencing found support for genetic drift as the dominant evolutionary mechanism driving population divergence among island fox populations. In particular, populations had exceptionally low genetic variation, small N_e (range = 2.1–89.7; median = 19.4), and significant genetic signatures of bottlenecks. Moreover, islands with the lowest genetic variation (and, by inference, the strongest historical genetic drift) were most genetically differentiated from mainland grey foxes, and vice versa, indicating genetic drift drives genome‐wide divergence. Nonetheless, outlier tests identified 3.6–6.6% of loci as high F_ST outliers, suggesting that despite strong genetic drift, divergent selection contributes to population divergence. Patterns of similarity among populations based on high F_ST outliers mirrored patterns based on morphology, providing additional evidence that outliers reflect adaptive divergence. Extremely low genetic variation and small N_e in some island fox populations, particularly on San Nicolas Island, suggest that they may be vulnerable to fixation of deleterious alleles, decreased fitness and reduced adaptive potential.     

Microsatellite analysis of genetic divergence among populations of giant Galápagos tortoises

Giant Galápagos tortoises represent an interesting model for the study of patterns of genetic divergence and adaptive differentiation related to island colonization events. Recent mitochondrial DNA work elucidated the evolutionary history of the species and helped to clarify aspects of nomenclature. We used 10 microsatellite loci to assess levels of genetic divergence among and within island populations. In particular, we described the genetic structure of tortoises on the island of Isabela, where discrimination of different taxa is still subject of debate. Individual island populations were all genetically distinct. The island of Santa Cruz harboured two distinct populations. On Isabela, populations of Volcan Wolf, Darwin and Alcedo were significantly different from each other. On the other hand, Volcan Wolf showed allelic similarity with the island of Santiago. On Southern Isabela, lower genetic divergence was found between Northeast Sierra Negra and Volcan Alcedo, while patterns of gene flow were recorded among tortoises of Cerro Azul and Southeast Sierra Negra. These tortoises have endured heavy exploitation during the last three centuries and recently attracted much concern due to the current number of stochastic and deterministic threats to extant populations. Our study complements previous investigation based on mtDNA diversity and provides further information that may help devising tortoise management plans.     

TESTING MODELS OF MIGRATION AND ISOLATION AMONG POPULATIONS OF CHINOOK SALMON (ONCORHYNCHUS TSCHAWYTSCHA)

The chinook salmon (Oncorhynchus tschawytscha) is a behaviorally, morphologically, and ecologically variable species distributed over a large geographic range. Although previous genetic surveys have revealed considerable genetic differences among populations with different life history types and from different major river drainages, it is not clear to what degree these genetically distinct populations are connected by low levels of gene flow. The work described in this paper addresses this question by surveying DNA restriction site variation at six nuclear genes from nine populations encompassing most of the species's North American range, and then attempting to fit the patterns of variation observed at these genes to five different evolutionary models using computer simulations of the coalescent process. Two commonly used constant population size models, one hypothesizing no migration among populations and one hypothesizing equal rates of migration among populations, were found to be statistically inconsistent with the observed patterns of variation. The other three models, which involved either recent divergence among populations coupled with large changes in populations size, unequal migration rates among populations, or selection, were all found to be consistent with the observed patterns of variation. Assuming selective neutrality, these results suggest that either the populations have all descended from a common ancestral population within the last ~50,000 years and have all suffered large declines in effective population size since that time, or that they have a more ancient divergence time but are connected by low levels of gene flow. These conclusions rest on the assumption of selective neutrality. With the methods employed, it was not possible to simultaneously test hypotheses of both selective neutrality and population structure.     

The influence of gene flow and drift on genetic and phenotypic divergence in two species of Zosterops in Vanuatu

Colonization of an archipelago sets the stage for adaptive radiation. However, some archipelagos are home to spectacular radiations, while others have much lower levels of diversification. The amount of gene flow among allopatric populations is one factor proposed to contribute to this variation. In island colonizing birds, selection for reduced dispersal ability is predicted to produce changing patterns of regional population genetic structure as gene flow-dominated systems give way to drift-mediated divergence. If this transition is important in facilitating phenotypic divergence, levels of genetic and phenotypic divergence should be associated. We consider population genetic structure and phenotypic divergence among two co-distributed, congeneric (Genus: Zosterops) bird species inhabiting the Vanuatu archipelago. The more recent colonist, Z. lateralis, exhibits genetic patterns consistent with a strong influence of distance-mediated gene flow. However, complex patterns of asymmetrical gene flow indicate variation in dispersal ability or inclination among populations. The endemic species, Z. flavifrons, shows only a partial transition towards a drift-mediated system, despite a long evolutionary history on the archipelago. We find no strong evidence that gene flow constrains phenotypic divergence in either species, suggesting that levels of inter-island gene flow do not explain the absence of a radiation across this archipelago.     

Recent radiation of endemic Caribbean Drosophila of the dunni subgroup inferred from multilocus DNA sequence variation

Studies of island endemism provide a unique opportunity to elucidate fundamental mechanisms of speciation. Here we examine intra- and interspecific DNA sequence variation at four unlinked genetic loci among populations of the Drosophila dunni subgroup to provide a detailed genealogical portrait of the process of speciation among these island endemic species. Our data indicate two major rounds of diversification that have shaped the D. dunni subgroup. The first occurred 1.6-2.6 million years ago and separated three major lineages, one in Puerto Rico and the Virgin Islands, a second in the northern Lesser Antilles and Barbados, and a third in St. Vincent and Grenada. A second round of diversification occurred in the last 96,000 years in the northern Lesser Antilles and Barbados. The four distinct species that resulted from this recent round of diversification maintain relatively high amounts of genetic variation, similar to that of a closely related mainland species, and share extensive ancestral polymorphism. These data suggest a minimal role for population bottlenecks linked to founder events in the history of the D. dunni subgroup. Further, the recent divergence of these island populations highlights the extremely rapid development of reproductive isolation and distinct patterns of abdominal pigmentation that has occurred in these species.     

The influence of history and contemporary stream hydrology on the evolution of genetic diversity within species: an examination of microsatellite DNA variation in bull trout,Salvelinus confluentus (Pisces: Salmonidae)

An understanding of the relative roles of historical and contemporary factors in structuring genetic variation is a fundamental, but understudied aspect of geographic variation. We examined geographic variation in microsatellite DNA allele frequencies in bull trout (Salvelinus confluentus, Salmonidae) to test hypotheses concerning the relative roles of postglacial dispersal (historical) and current landscape features (contemporary) in structuring genetic variability and population differentiation. Bull trout exhibit relatively low intrapopulation microsatellite variation (average of 1.9 alleles per locus, average He = 0.24), but high levels of interpopulation divergence (F(ST) = 0.39). We found evidence of historical influences on microsatellite variation in the form of a decrease in the number of alleles and heterozygosities in populations on the periphery of the range relative to populations closer to putative glacial refugia. In addition, one region of British Columbia that was colonized later during deglaciation and by more indirect watershed connections showed less developed and more variable patterns of isolation by distance than a similar region colonized earlier and more directly from refugia. Current spatial and drainage interconnectedness among sites and the presence of migration barriers (falls and cascades) within individual streams were found to be important contemporary factors influencing historical patterns of genetic variability and interpopulation divergence. Our work illustrates the limited utility of equilibrium models to delineate population structure and patterns of genetic diversity in recently founded populations or those inhabiting highly heterogeneous environments, and it highlights the need for approaches incorporating a landscape context for population divergence. Substantial microsatellite DNA divergence among bull trout populations may also signal divergence in traits important to population persistence in specific environments.     

Ancient differentiation in the single-island avian radiation of endemic Hispaniolan chat-tanagers (Aves: Calyptophilus)

The simple geographic structure of island systems often makes them tractable for studies of the patterns and processes of biological diversification. The Calyptophilus chat-tanagers of Hispaniola are of general evolutionary interest because their multiple lineages might have arisen on a single island, of conservation concern because several isolated populations are nearly extinct, and taxonomically ambiguous because they have been variously lumped or split into one to four species. To explore the context of diversification of the seven extant Calyptophilus populations, we conducted a multilocus coalescent analysis based on sequences of mitochondrial ND2 and three nuclear intron loci. We then compared patterns of phylogeographic genetic variation with the morphological differences that distinguish these populations. Mitochondrial haplotypes formed two reciprocally monophyletic groups separated by a large magnitude of nucleotide divergence. Intron structure largely paralleled the geographic grouping pattern of the mitochondrial DNA (mtDNA), but these groups were only reciprocally monophyletic at one of the three introns. Also, the magnitude of between-group divergence was much lower in the introns than mtDNA genealogies. Multilocus coalescent analyses inferred a nonzero divergence time between these two major geographic groups, but suggested that they have experienced a low level of gene flow. All four markers showed substantial allele sharing within each of the two groups, demonstrating that many now separated montane populations do not have long histories of isolation. Considered in concert, our multilocus phylogeographic reconstructions support the recognition of two species within the Calyptophilus complex, and raise the possibility that these taxa differentiated prior to the fusion of the two palaeo-islands that form present-day Hispaniola.