Physiological change in an insular lizard population confirms the reversed island syndrome

Unpredictable environmental conditions and highly fluctuating population densities are believed to have produced a ‘reversed island syndrome’ (RIS) in an insular population of the Wall lizard on Licosa Islet, Italy. Several of the physiological, behavioural, and life‐history changes that constitute the RIS could result from positive selection on increased activity of melanocortins. For example, increased levels of α‐melanocyte‐stimulating hormone (MSH) should lead to increased investment in reproduction and increased immunocompetence in the island population. We tested the crucial assumption of this idea that plasma levels of α‐MSH in Licosa Islet lizards are elevated compared to those of the mainland relatives. We also tested for differences in reproductive effort between populations, by measuring plasma levels of 5‐α‐dihydrotestosterone in males and clutch mass in females. In addition, we assessed ectoparasite load as an indicator for the lizards’ resistance to environmental stress. In agreement with the RIS, we found that insular lizards exhibit higher α‐MSH levels, allocate more energy to reproduction, and have a reduced ectoparasite load compared to the nearest mainland population. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Genetic variation and structure of house sparrow populations: is there an island effect?

Population genetic structure and intrapopulation levels of genetic variation have important implications for population dynamics and evolutionary processes. Habitat fragmentation is one of the major threats to biodiversity. It leads to smaller population sizes and reduced gene flow between populations and will thus also affect genetic structure. We use a natural system of island and mainland populations of house sparrows along the coast of Norway to characterize the different population genetic properties of fragmented populations. We genotyped 636 individuals distributed across 14 populations at 15 microsatellite loci. The level of genetic differentiation was estimated using F‐statistics and specially designed Mantel tests were conducted to study the influence of population type (i.e. mainland or island) and geographic distance on the genetic population structure. Furthermore, the effects of population type, population size and latitude on the level of genetic variation within populations were examined. Our results suggest that genetic processes on islands and mainland differed in two important ways. First, the intrapopulation level of genetic variation tended to be lower and the occurrence of population bottlenecks more frequent on islands than the mainland. Second, although the general level of genetic differentiation was low to moderate, it was higher between island populations than between mainland populations. However, differentiation increased in mainland populations somewhat faster with geographical distance. These results suggest that population bottleneck events and genetic drift have been more important in shaping the genetic composition of island populations compared with populations on the mainland. Such knowledge is relevant for a better understanding of evolutionary processes and conservation of threatened populations.     

Convergence in reduced body size,head size,and blood glucose in three island reptiles

Many oceanic islands harbor diverse species that differ markedly from their mainland relatives with respect to morphology, behavior, and physiology. A particularly common morphological change exhibited by a wide range of species on islands worldwide involves either a reduction in body size, termed island dwarfism, or an increase in body size, termed island gigantism. While numerous instances of dwarfism and gigantism have been well documented, documentation of other morphological changes on islands remains limited. Furthermore, we lack a basic understanding of the physiological mechanisms that underlie these changes, and whether they are convergent. A major hypothesis for the repeated evolution of dwarfism posits selection for smaller, more efficient body sizes in the context of low resource availability. Under this hypothesis, we would expect the physiological mechanisms known to be downregulated in model organisms exhibiting small body sizes due to dietary restriction or artificial selection would also be downregulated in wild species exhibiting dwarfism on islands. We measured body size, relative head size, and circulating blood glucose in three species of reptiles—two snakes and one lizard—in the California Channel Islands relative to mainland populations. Collating data from 6 years of study, we found that relative to mainland population the island populations had smaller body size (i.e., island dwarfism), smaller head sizes relative to body size, and lower levels of blood glucose, although with some variation by sex and year. These findings suggest that the island populations of these three species have independently evolved convergent physiological changes (lower glucose set point) corresponding to convergent changes in morphology that are consistent with a scenario of reduced resource availability and/or changes in prey size on the islands. This provides a powerful system to further investigate ecological, physiological, and genetic variables to elucidate the mechanisms underlying convergent changes in life history on islands.     

Genetic and phylogenetic consequences of island biogeography

Abstract.— Island biogeography theory predicts that the number of species on an island should increase with island size and decrease with island distance to the mainland. These predictions are generally well supported in comparative and experimental studies. These ecological, equilibrium predictions arise as a result of colonization and extinction processes. Because colonization and extinction are also important processes in evolution, we develop methods to test evolutionary predictions of island biogeography. We derive a population genetic model of island biogeography that incorporates island colonization, migration of individuals from the mainland, and extinction of island populations. The model provides a means of estimating the rates of migration and extinction from population genetic data. This model predicts that within an island population the distribution of genetic divergences with respect to the mainland source population should be bimodal, with much of the divergence dating to the colonization event. Across islands, this model predicts that populations on large islands should be on average more genetically divergent from mainland source populations than those on small islands. Likewise, populations on distant islands should be more divergent than those on close islands. Published observations of a larger proportion of endemic species on large and distant islands support these predictions.     

ALLOZYME DIFFERENTIATION AND GENETIC STRUCTURE IN ISLAND AND MAINLAND JAPANESE POPULATIONS OF CAMPANULA PUNCTATA (CAMPANULACEAE)

Allozyme variation was investigated in 17 Japanese populations of Campanula punctata, ten from the Izu Islands and seven in the mainland (Honshu). The data indicate that there are two groups, a mainland group and an island one, and that the systematically problematic Oshima Island (northernmost Izu island) populations are closely related to those of the other islands. Nei's genetic identity values among islands and among mainland populations were 0.95 and 0.97, respectively, while the value between island and mainland populations was 0.84, suggesting that the island populations are an independent species. Total genetic variation was nearly the same among island and mainland populations. However, the apportionment of variation within and among populations was considerably different; 14% of gene diversity exists among mainland populations, while 31% of the diversity exists among island populations. Mean outcrossing rates of self-incompatible mainland and Oshima populations are 0.62–0.79, supporting xenogamy; those in self-compatible island populations are 0.37–0.57 in the northern islands, indicating a mixed mating system, and 0.16–0.25 in southern ones, indicating dominant inbreeding. Total genetic diversity in each island population decreased with distance from the mainland. Genetic and geological data suggest that the ancestors of insular populations were founded on northern islands in a relatively ancient period and that they dispersed progressively to the southern ones. Chromosome number (2n = 34) and isozyme numbers indicate gene duplications in this species, which suggests it is an ancient polyploid.     

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color-pattern variation in Lake Erie island water snake (Nerodia sipedon) populations, rates of gene flow among island and mainland populations were estimated from patterns of allozymic variation detected using electrophoresis. Rates of gene flow were high with Nm, the number of migrants per generation, averaging 25.5 among island sites, 9.2 between the Ontario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark-recapture work and of past population size extrapolated from the extent of shoreline habitat, values of m between island and mainland populations ranged from 0.0008–0.01. Synthesis of estimates of the rate of gene flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color-pattern variation in island populations results from a balance between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as drift may have been important in the initial stages of differentiation between island and mainland populations.     

Evolutionary history of Scinax treefrogs on land‐bridge islands in south‐eastern Brazil

Aim We investigated how Pleistocene refugia and recent (c. 12,000 years ago) sea level incursions shaped genetic differentiation in mainland and island populations of the Scinax perpusillus treefrog group. Location Brazilian Atlantic Forest, São Paulo state, south‐eastern Brazil. Methods Using mitochondrial and microsatellite loci, we examined population structure and genetic diversity in three species from the S. perpusillus group, sampled from three land‐bridge islands and five mainland populations, in order to understand the roles of Pleistocene forest fragmentation and sea level incursions on genetic differentiation. We calculated metrics of relatedness and genetic diversity to assess whether island populations exhibit signatures of genetic drift and isolation. Two of the three island populations in this study have previously been described as new species based on a combination of distinct morphological and behavioural characters, thus we used the molecular datasets to determine whether phenotypic change is consistent with genetic differentiation. Results Our analyses recovered three distinct lineages or demes composed of northern mainland São Paulo populations, southern mainland São Paulo populations, and one divergent island population. The two remaining island populations clustered with samples from adjacent mainland populations. Estimates of allelic richness were significantly lower, and estimates of relatedness were significantly higher, in island populations relative to their mainland counterparts. Main conclusions Fine‐scale genetic structure across mainland populations indicates the possible existence of local refugia within São Paulo state, underscoring the small geographic scale at which populations diverge in this species‐rich region of the Atlantic Coastal Forest. Variation in genetic signatures across the three islands indicates that the populations experienced different demographic processes after marine incursions fragmented the distribution of the S. perpusillus group. Genetic signatures of inbreeding and drift in some island populations indicate that small population sizes, coupled with strong ecological selection, may be important evolutionary forces driving speciation on land‐bridge islands.     

Testing the island effect in adaptive radiation: rates and patterns of morphological diversification in Caribbean and mainland Anolis lizards

Many of the classic examples of adaptive radiation, including Caribbean Anolis lizards, are found on islands. However, Anolis also exhibits substantial species richness and ecomorphological disparity on mainland Central and South America. We compared patterns and rates of morphological evolution to investigate whether, in fact, island Anolis are exceptionally diverse relative to their mainland counterparts. Quite the contrary, we found that rates and extent of diversification were comparable--Anolis adaptive radiation is not an island phenomenon. However, mainland and Caribbean anoles occupy different parts of morphological space; in independent colonizations of both island and mainland habitats, island anoles have evolved shorter limbs and better-developed toe pads. These patterns suggest that the two areas are on different evolutionary trajectories. The ecological causes of these differences are unknown, but may relate to differences in predation or competition among mainland and island communities.     

Dwarfism in insular sloths: biogeography,selection, and evolutionary rate

The islands of Bocas del Toro, Panama, were sequentially separated from the adjacent mainland by rising sea levels during the past 10,000 years. Three-toed sloths (Bradypus) from five islands are smaller than their mainland counterparts, and the insular populations themselves vary in mean body size. We first examine relationships between body size and physical characteristics of the islands, testing hypotheses regarding optimal body size, evolutionary equilibria, and the presence of dispersal in this system. To do so, we conduct linear regressions of body size onto island area, distance from the mainland, and island age. Second, we retroactively calculate two measures of the evolutionary rate of change in body size (haldanes and darwins) and the standardized linear selection differential, or selection intensity (i). We also test the observed morphological changes against models of evolution by genetic drift. The results indicate that mean body size decreases linearly with island age, explaining up to 97% of the variation among population means. Neither island area nor distance from the mainland is significant in multiple regressions that include island age. Thus, we find no evidence for differential optimal body size among islands, or for dispersal in the system. In contrast, the dependence of body size on island age suggests uniform directional selection for small body size in the insular populations. Although genetic drift cannot be discounted as the cause for this evolution in body size, the probability is small given the consistent direction of evolution (repeated dwarfism). The insular sloths show a sustained rate of evolution similar to those measured in haldanes over tens of generations, appearing to unite micro- and macroevolutionary time scales. Furthermore, the magnitude and rate of this example of rapid differentiation fall within predictions of theoretical models from population genetics. However, the linearity of the relationship between body size and island age is not predicted, suggesting that either more factors are involved than those considered here, or that theoretical advances are necessary to explain constant evolutionary rates over long time spans in new selective environments.     

Enhanced colour polymorphisms in island populations of the land snail Euhadra peliomphala

Variations in visible genetic polymorphisms are assumed to decrease in populations on small islands because of intense founder effects, genetic drift and inbreeding. However, we have found evidence of a marked enhancement of colour polymorphisms within populations on small oceanic islands that were colonized from the mainland. The source populations on the mainland of the land snail Euhadra peliomphala in four oceanic islands were estimated by phylogenetic analysis of mitochondrial DNA sequences. Diversity of shell colour was higher in the island populations than in the source populations on the mainland. In addition, the shell colour morphs differed not only among populations from different islands but also between the island populations and the source populations on the mainland. By contrast, no mtDNA variations were found in any of the island populations, even though the source populations possessed high mtDNA diversity. Thus, components of colour morphs changed in the island populations after their colonization, and colour polymorphisms are enhanced in these islands despite the loss of genetic variation. The above findings suggest that ecological mechanisms such as morphological release owing to a release from competition may overcome the tendency toward reduced genetic variation in islands to enhance the colour polymorphism.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 417–425.     

Size evolution in island lizards

Aim  The island rule, small animal gigantism and large animal dwarfism on islands, is a topic of much recent debate. While size evolution of insular lizards has been widely studied, whether or not they follow the island rule has never been investigated. I examined whether lizards show patterns consistent with the island rule.
Location  Islands worldwide.
Methods  I used literature data on the sizes of island–mainland population pairs in 59 species of lizards, spanning the entire size range of the group, and tested whether small insular lizards are larger than their mainland conspecifics and large insular lizards are smaller. I examined the influence of island area, island isolation, and dietary preferences on lizard size evolution.
Results  Using mean snout–vent length as an index of body size, I found that small lizards on islands become smaller than their mainland conspecifics, while large ones become larger still, opposite to predictions of the island rule. This was especially strong in carnivorous lizards; omnivorous and herbivorous species showed a pattern consistent with the island rule but this result was not statistically significant. No trends consistent with the island rule were found when maximum snout–vent length was used. Island area had, at best, a weak effect on body size. Using maximum snout–vent length as an index of body size resulted in most lizard populations appearing to be dwarfed on islands, but no such pattern was revealed when mean snout–vent length was used as a size index.
Main conclusions  I suggest that lizard body size is mostly influenced by resource availability, with large size allowing some lizard populations to exploit resources that are unavailable on the mainland. Lizards do not follow the island rule. Maximum snout–vent length may be biased by sampling effort, which should be taken into account when one uses this size index.     

Genetic and morphometric differentiation among island populations of two Norops lizards (Reptilia: Sauria: Polychrotidae) on independently colonized islands of the Islas de Bahia (Honduras)

Aim Anole lizards (Reptilia: Sauria: Polychrotidae) display remarkable morphological and genetic differentiation between island populations. Morphological differences between islands are probably due to both adaptive (e.g. differential resource exploitation and intra‐ or interspecific competition) and non‐adaptive differentiation in allopatry. Anoles are well known for their extreme diversity and rapid adaptive speciation on islands. The main aim of this study was to use tests of morphological and genetic differentiation to investigate the population structure and colonization history of islands of the Islas de Bahia, off the coast of Honduras. Location Five populations of Norops bicaorum and Norops lemurinus were sampled, four from islands of the Islas de Bahia and one from the mainland of Honduras. Methods Body size and weight differentiation were measured in order to test for significant differences between sexes and populations. In addition, individuals were genotyped using the amplified fragment length polymorphism technique. Bayesian model‐based and assignment/exclusion methods were used to study genetic differentiation between island and mainland populations and to test colonization hypotheses. Results Assignment tests suggested migration from the mainland to the Cayos Cochinos, and from there independently to both Utila and Roatán, whereas migration between Utila and Roatán was lacking. Migration from the mainland to Utila was inferred, but was much less frequent. Morphologically, individuals from Utila appeared to be significantly different in comparison with all other localities. Significant differentiation between males of Roatán and the mainland was found in body size, whereas no significant difference was detected between the mainland and the Cayos Cochinos. Main conclusions Significant genetic and morphological differentiation was found among populations. A stepping‐stone model for colonization, in combination with an independent migration to Utila and Roatán, was suggested by assignment tests and was compatible with the observed morphological differentiation.     

Conservation genetics of insular Podarcis lizards using partial cytochrome b sequences

Sequence data derived from a 306 bp fragment of mitochondrial cytochrome b and molecular variance estimates were used to investigate the genetic population structure of the endangered and endemic lizard Podarcis atrata of the Columbretes archipelago (Mediterranean, Spain). Our results show a very high and significant among-population genetic differentiation. F _ST values and phylogenetic analyses confirm the evolutionary distinctiveness of P. atrata populations, suggesting that the populations of these islands deserve special protection measures. The populations of the two islands Columbrete Grande and Mancolibre are less differentiated than those of Foradada and Lobo, and seem to have retained mainland haplotypes. This situation needs further attention as the origin of the mainland haplotypes is still unclear. If they are a result of recent introductions from mainland specimens, then they may represent a threat to the endemic lizards of the Columbretes islands.     

Allozyme variation at a PGI locus in differently aged populations of Moehringia trinervia (Caryophyllaceae) in a successional area

We studied genetic effects of the colonisation process during primary succession by analysing allozyme variation at a PGI locus in differently aged populations of Moehringia trinervia , which is a selfing annual with low dispersal ability. The populations studied come from islands and shores created in the 1880s by a drop in the water table of a Swedish lake and from old parts of a large island and of the mainland. The population age is known from five vegetation analyses over a century. We have also analysed the genetic composition of M. trinervia derived from seeds in the soil. Mainland populations had a higher genetic diversity than island populations that were little differentiated and differed genetically from the mainland populations. There was no temporal trend in the distribution of genetic variation on the new islands. The presence of alleles in the extant populations was associated with the proportion of that allele in the seed bank, indicating a main recruitment from the seed bank and not by repeated immigrations. We suggest that some of the new islands were colonised by a few early founders from the mainland. Later colonisation has occurred between adjacent islands, which preserves the founder effect and could explain the uniform, low genetic variation in the island populations.     

Associations between physical isolation and geographical variation within three species of Neotropical birds

We studied effects of physical isolation on geographical variation in mtDNA RFLP polymorphisms and a suite of morphological characters within three species of neotropical forest birds; the crimson-backed tanager Ramphocelus dimidiatus, the blue-gray tanager Thraupis episcopus, and the streaked saltator Saltator albicollis. Variation among populations within continuous habitat on the Isthmus of Panama was compared with that among island populations isolated for about 10000 years. Putative barriers to dispersal were influential, but apparent isolation effects varied by species, geographical scale, and whether molecular or morphological traits were being assessed. We found no geographical structuring among the contiguous, mainland sampling sites. Migration rates among the islands appeared sufficient to maintain homogeneity in mtDNA haplotype frequencies. In contrast, variation in external morphology among islands was significant within two of three species. For all species, we found significant variation in genetic and morphological traits between the island (collectively) and mainland populations. Interspecific variation in the effects of isolation was likely related to differential vagility. These data generally corroborate other studies reporting relatively great geographical structuring within tropical birds over short distances. Behaviourally based traits - low vagility and high ‘sensitivity’ to geographical barriers - may underlie extensive diversification within neotropical forest birds, but more extensive ecological and phylogeographic information are needed on a diverse sample of species.     

Island/mainland body size differences in Australian varanid lizards

Island varanids seem to be an exception to the rule that territorial vertebrate taxa often become gigantic relative to mainland relatives when on islands, whereas non-territorial species become dwarfed (Case 1978). However, no systematic island/mainland studies have examined the empirical size trends in this group of carnivorous lizards. We perform such an analysis for the Australian region and critically evaluate various selective agents that might be responsible for size changes in several island populations. Insular gigantism occurs at least four times among the island populations examined. The magnitude of size change is positively correlated to prey abundance on the islands (as indirectly measured through a condition index of the lizards, essentially a measure of how fat they arc) and the size of prey: islands with large prey have large varanids and vice versa. Since the island population with the largest size change, the Reevesby Varanus rosenbergi, was introduced less than 100 years ago, these size changes can be quite rapid. This might indicate that selective coefficients are strong; however, we can not exclude the possibility that these size differences have no genetic component and simply reflect environmental differences in growth rate and shifts in age structure between island and mainland locations.     

Evolutionary and biogeographical implications of variation in skull morphology of raccoon dogs (Nyctereutes procyonoides,Mammalia: Carnivora)

Morphological characteristics reflect geographical variation resulting from adaptation to varying environmental conditions. Carnivore species distributed over a wide geographical range generally have highly polymorphic morphological variation. The raccoon dog (Nyctereutes procyonoides) has a longitudinal distribution restricted to East Asia and the northern Indochina Peninsula. Its unique geographical range makes it an appropriate model to examine how morphological differences are influenced by geography. To demonstrate morphological evolution of Russian, Chinese, Korean and Japanese raccoon dogs predicted by geographical differences, we tested the island rule and Bergmann's rule. We compared craniodental variation among populations and examined morphological implications for intraspecific taxonomic status. Insular raccoon dogs possessed substantially smaller body size than those from the mainland. Moreover, different island effects among Japanese islands were demonstrated by markedly larger occipital condyle breath in the Hokkaido population. Larger skull size in Russian and Hokkaido raccoon dogs could be explained by Bergmann's rule. Based on previous chromosomal and molecular studies and results of our morphological analyses, we suggest Japanese raccoon dogs are a distinct species from the mainland N. procyonoides.     

Large mainland populations of South Island robins retain greater genetic diversity than offshore island refuges

For conservation purposes islands are considered safe refuges for many species, particularly in regions where introduced predators form a major threat to the native fauna, but island populations are also known to possess low levels of genetic diversity. The New Zealand archipelago provides an ideal system to compare genetic diversity of large mainland populations where introduced predators are common, to that of smaller offshore islands, which serve as predator-free refuges. We assessed microsatellite variation in South Island robins (Petroica australis australis), and compared large mainland, small mainland, natural island and translocated island populations. Large mainland populations exhibited more polymorphic loci and higher number of alleles than small mainland and natural island populations. Genetic variation did not differ between natural and translocated island populations, even though one of the translocated populations was established with five individuals. Hatching failure was recorded in a subset of the populations and found to be significantly higher in translocated populations than in a large mainland population. Significant population differentiation was largely based on heterogeneity in allele frequencies (including fixation of alleles), as few unique alleles were observed. This study shows that large mainland populations retain higher levels of genetic diversity than natural and translocated island populations. It highlights the importance of protecting these mainland populations and using them as a source for new translocations. In the future, these populations may become extremely valuable for species conservation if existing island populations become adversely affected by low levels of genetic variation and do not persist.     

The colonization of Scottish islands by the common shrew, Sorex araneus (Eulipotyphla: Soricidae)

In this paper we apply molecular methods to study the colonization of islands off the west coast of Scotland by the common shrew ( Sorex araneus L.), and current gene flow. We collected 497 individuals from 13 islands of the Inner Hebrides and Clyde Island groups and six mainland regions. Individuals were genotyped at eight microsatellite loci, and the mitochondrial cytochrome b sequence (1140 base pairs) was obtained for five individuals from each island/mainland region. Based on these molecular data, island colonization apparently proceeded directly from the mainland, except for Islay, for which Jura was the most likely source population. Raasay may also have been colonized by island hopping. Most island populations are genetically very distinct from the mainland populations, suggesting long periods of isolation. Two exceptions to this are the islands of Skye and Seil, which are geographically and genetically close to the mainland, suggesting in each case that there has been long-term gene flow between these islands and the mainland. We consider possible methods of island colonization, including human-mediated movement, swimming, and land and ice bridges.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 797–808.     

Genetic variation and island biogreography: Microsatellite and mitochondrial DNA variation in island populations of the Australian bush rat, Rattus fuscipes greyii

To understand the impact of various factors on the maintenance of genetic variation in natural populations, we need to focus on situations where at least some of these factors are removed or controlled. In this study, we used highly variable, presumably neutral, microsatellite and mtDNA markers to assess the nature of genetic variation in 14 island and two mainland populations of the Australian bush rat, where there is no migration between islands. Thus we are controlling for selection and gene flow. Both marker sets revealed low levels of diversity within the small island populations and extreme differentiation between populations. For six microsatellite loci, all of the small island populations had less genetic variation than the mainland populations; reduction in allelic diversity was more pronounced than loss of heterozygosity. Kangaroo Island, the large island population, had similar levels of diversity to the mainland populations. A 442 base pair (bp) section of the mtDNA control region was screened for variation by outgroup heteroduplex analysis/temperature gradient gel electrophoresis (OHA/TGGE). Only three of the 13 small island populations showed haplotypic diversity: Gambier (2), Waldegrave (2), and Eyere (3). The level of haplotypic diversity in the small island populations was similar to that on the mainland, most likely reflecting a recent population bottleneck on the mainland. In contrast, Kangaroo Island had 9 mtDNA haplotypes. The dominant factor influencing genetic diversity on the islands was island size. No correlation was detected between genetic diversity and the time since isolation or distance form the mainland. The combination of genetic drift within and complete isolation among the small island populations has resulted in rapid and extreme population divergence. Population pair-wise comparisons of allele frequency distributions showed significant differences for all populations for all loci (F _st = 0.11–0.84, R _st = 0.07–0.99). For the mtDNA control region, 92.6% of variation was apportioned between populations; only the Pearson islands shared a haplotype. Mantel tests of pair-wise genetic distance with pair-wise geographic distance showed no significant geographical clustering of haplotypes. However, population substructuring was detected within populations where sampling was conducted over a broader geographical range, as indicated by departures from Hardy-Weinberg equilibrium. Thus substructuring in the ancestral population cannot be ruled out. The dominant evolutionary forces on the islands, after the initial founder event, are stochastic population processes such as genetic drift and mutation. This revised version was published online in July 2006 with corrections to the Cover Date.