Adjacent Mainland Populations Research Articles

How did the toad get over the sea to Skye? Tracing the colonisation of Scottish inshore islands by common toads (Bufo bufo)

Processes of island colonisation have long been of interest to biologists. Colonisation events themselves are rarely observed, but the processes involved may be inferred using genetic approaches. We investigated possible means of island colonisation by common toads (Bufo bufo) in western Scotland (the Isle of Skye and five neighbouring small islands), using evidence derived from nuclear microsatellites and mitochondrial (mt) DNA. Levels of microsatellite allelic richness for populations on Skye were high and comparable to adjacent mainland populations, but lower for populations on small islands. Pairwise measures of genetic distances between populations and a clustering algorithm were both suggestive of frequent gene flow between Skye and the mainland. For small islands the levels of genetic differentiation were higher, implying stronger isolation and no evidence for inbreeding. The distribution of mtDNA haplotypes broadly mirrored the genetic structure revealed by microsatellites. Reconciled with existing palaeoclimatological evidence, since the last glaciation, our findings rule out the possibility that the B. bufo populations stem from glacial refugia, or that recent anthropogenic transfer of toads is responsible for their current distribution. The most parsimonious explanation of our data is that the studied inshore islands have been repeatedly colonised via rafting from the mainland or neighbouring islands. This may give us insights into the processes likely to take place when ice sheets retreat poleward as a result of climate change. It also has implications for the colonisation of both native and invasive non-native species, and hence the biosecurity of island refugia.

Genetic differentiation predicts body size divergence between island and mainland populations of common wall lizards (Podarcis muralis)

AbstractSmall-bodied vertebrates sometimes evolve gigantism on islands, but there is a lack of consistent association with ecological factors or island characteristics. One possible reason is that, even if the ecological conditions are right, body size might fail to diverge on islands that were isolated recently or if there is gene flow between islands and the mainland. We studied body size, ventral colour polymorphism and genetic structure across nine islands and adjacent mainland populations of common wall lizards (Podarcis muralis) off the western coast of France. Population genetic data suggested that island populations might have maintained gene flow after their geographical isolation from the mainland. Island lizards were larger and heavier than mainland lizards on average, but the extent of gigantism varied substantially between islands. Island size and distance from the mainland were poor predictors of body size, but lizards from populations that were highly genetically differentiated from the mainland were larger than lizards from less differentiated populations. Colour morphs that were rare on the mainland tended to be more common on islands. We propose that genetic isolation or bottlenecks promote body size evolution in island lizards, which makes it challenging to identify ecological causes of island gigantism without complementary genetic information.

Genetic differentiation between Apis cerana cerana populations from Damen Island and adjacent mainland in China

Geographic isolation interrupted gene flow between populations leading to population differentiation during the long evolutionary period. In this paper, 33 colonies from Damen Island and 100 colonies from adjacent mainland populations, Juxi and Chixi, were analyzed with both mitochondrial tRNAleu-COII sequences and five microsatellite loci. The results showed that Apis cerana cerana population from Damen Island significantly differentiated from its adjacent mainland populations. In addition, Damen Island population showed a lower level of genetic diversity in terms of the number of mitochondrial haplotypes while both island and mainland populations showed a low level of genetic diversity with mutilocus analysis. The divergent small island population A.c. cerana might probably have suffered inbreeding and genetic drift as well as limited gene flow across the strait. Our data provides useful information for management and preservation for the Damen Island population.

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

AbstractAim 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.

A phylogenetic analysis of Neotoma varia (Rodentia: Cricetidae), a rediscovered, endemic, and threatened rodent from Datil Island, Sonora, Mexico

The systematics of the rediscovered and threatened rodent, Neotoma varia, from Datil Island in the Gulf of California, was assessed using sequences from the mitochondrial cytochrome b gene (Cyt b) regarding specimens of N. albigula from Tiburon Island and populations on the mainland off Datil Island. Neotoma varia was originally described as a species and subsequently considered a subspecies, relegated to subspecific status based on morphologic characters and few specimens; no genetic analyses have been published. Bayesian inference, maximum-parsimony, maximumlikelihood, and distance optimality criteria based on 828-bp of the Cyt b gene from individuals representing 11 populations, converged on essentially identical tree topologies, consistent with the inclusion of N. varia within N. albigula. The population of Datil Island is related to specimens from Tiburon Island and the adjacent mainland populations showing low levels of genetic differentiation with other subspecies of N. albigula (0.2–1.4%). Previous morphologic analyses indicated inconstancy in characters regarding the holotype; however, N. varia is morphologically different in the oclusal view of the upper molars. Under these conditions, we consider N. varia as a subspecies of N. albigula. N. a. varia has a very specific habitat and is present only on a very small part of the island; in spite of low divergence regarding other N. albigula subspecies, N. a. varia possesses a genetic identity and needs to be considered as a critically endangered population.

Island populations have high conservation value for northern Australia's top marsupial predator ahead of a threatening process

AbstractThe rarity of top predators has been identified as a contributing factor in the collapse of entire ecological systems. Elucidating the ecology, evolutionary potential and population response of species before a threatening process requires a combination of measures of phenotypic variation, demography and genetic diversity using an array of markers. Here we use such information to determine the population structure and its conservation implications for a keystone marsupial carnivore in Western Australia, the northern quoll Dasyurus hallucatus, ahead of the spread of the exotic and poisonous cane toad Chaunus marinus from the east. The western populations occur in two discrete geographic regions on the mainland, the Kimberley and Pilbara, separated by the arid Great Sandy Desert, and on several continental islands. Both mitochondrial DNA sequences and 11 nuclear microsatellite loci reveal clear differentiation of the Kimberley and Pilbara regions. Offshore islands with permanent sea‐channels from the mainland show reduced diversity and were genetically distinct indicating genetic isolation, whereas those separated from the mainland by very shallow channels were more similar to the adjacent mainland populations, suggesting on‐going gene flow. There was little indication that any population within the distribution of the species had experienced recent declines but there was marked variation in sexual dimorphism suggesting diversity in demographic performance. These populations in Western Australia differ from those remaining to the east in Queensland and the Northern Territory, in both genetic structure and demographic parameters and represent the last intact populations in Australia that have not experienced major declines concomitant with the spread of the cane toad. Consequently, the genetic and demographic flexibility recorded by this study provides guidance for the conservation and management of a major marsupial predator before and during its potential decline associated with an invasive, threatening prey species.

Insular pest control within a metapopulation context

Introduced pests threaten many species and their control is generally beneficial for conservation, particularly on islands where complete eradication is possible. Unfortunately on ‘nearshore’ islands neighbouring source populations exist and unaided reinvasion is likely. Pest control programmes at these sites thus require a metapopulation context to adequately manage movements between source and sink populations. We investigated the ecology of introduced ship rats ( Rattus rattus) on a nearshore island, and gene flow with adjacent mainland populations, in order to understand the metapopulation dynamics and relative levels of pest control required within the landscape. We sampled the entire population by trapping ( n = 30), achieving eradication, and found a low rat density (3.2 ha −1) indicative of a sink population. Seed and other plant material constituted the major dietary component of rats. Despite its proximity to mainland source populations, the island population was genetically distinct with reduced allelic diversity caused by a recent reinvasion founder effect. Genetic analyses also detected recent migrants between the populations. In contrast, two mainland populations separated by a similar distance displayed complete genetic mixing. The small water gap therefore provides a sufficient barrier to lower the migration rate to the island and delay reinvasion, which nonetheless eventually happens. In order to maintain nearshore islands pest-free, conservation management will require a metapopulation approach simultaneously focusing on both island and source population pest control.

The role of vicariance vs. dispersal in shaping genetic patterns in ocellated lizard species in the western Mediterranean

AbstractThe schism between North Africa and Southern Europe caused by the opening of the Strait of Gibraltar and the consequent refilling of the Mediterranean basin at the end of Messinian salinity crisis (MSC), 5.33 million years ago, has been advocated as the main event shaping biogeographical patterns in the western Mediterranean as exemplified by the distribution of species and subspecies and genetic variation within the ocellated lizard group. To reassess the role of the MSC, partial sequences of three mitochondrial DNA genes (cytochrome b, 12S and 16S ribosomal RNA) and two nuclear genes (β‐fibrinogen and C‐mos) from species of the ocellated lizard group were analysed. Three alternative hypotheses were tested: that divergence was initiated (i) by post‐MSC vicariance as the basin filled, (ii) when separate populations established either side of the strait by pre‐MSC overseas dispersal, and (iii) by post‐MSC overseas dispersal. The pattern and level of divergence detected clearly refute the post‐MSC vicariance hypothesis, and support a model of divergence initiated by earlier overseas dispersal. Indeed, our best estimate is that the basal Euro‐African divergence predates the MSC event by several million years. The estimated divergence times among the populations in former Miocene Mediterranean islands, the current Betic and Rifian mountains, from adjacent mainland populations suggest overseas dispersal for the former and overland dispersal, or perhaps vicariance, for the latter. These results suggest that the MSC may have played a much less important role in shaping the current western Mediterranean biogeographical patterns than might have been anticipated from the dramatic nature of the episode.

Population genetic structure of Anopheles gambiae mosquitoes on Lake Victoria islands, west Kenya

BackgroundUnderstanding the genetic structure of island Anopheles gambiae populations is important for the current tactics in mosquito control and for the proposed strategy using genetically-modified mosquitoes (GMM). Genetically-isolated mosquito populations on islands are a potential site for testing GMM. The objective of this study was to determine the genetic structure of A. gambiae populations on the islands in Lake Victoria, western Kenya.MethodsThe genetic diversity and the population genetic structures of 13 A. gambiae populations from five islands on Lake Victoria and six villages from the surrounding mainland area in the Suba District were examined using six microsatellite markers. The distance range of sampling sites varied between 2.5 and 35.1 km.ResultsA similar level of genetic diversity between island mosquito populations and adjacent mainland populations was found. The average number of alleles per locus was 7.3 for the island populations and 6.8 for the mainland populations. The average observed heterozygosity was 0.32 and 0.28 for the island and mainland populations, respectively. A low but statistically significant genetic structure was detected among the island populations (FST = 0.019) and between the island and mainland populations (FST = 0.003). A total of 12 private alleles were found, and nine of them were from the island populations.ConclusionA level of genetic differentiation between the island and mainland populations was found. Large extent of gene flow between the island and mainland mosquito populations may result from wind- or human-assisted dispersal. Should the islands on Lake Victoria be used as a trial site for the release program of GMM, mosquito dispersal between the islands and between the island and the mainland should be vigorously monitored.

The Thevenard Island mouse: historic and conservation implications from mitochondrial DNA sequence-variation

The level of mitochondrial differentiation between Thevenard Island and mainland populations of the short-tailed mice Leggadina lakedownensis was determined using DNA sequencing of the Control Region. Using temperature gradient gel electrophoresis, outgroup heteroduplex analysis detected eight haplotypes. These were sequenced for 362 basepairs. Our results show that the Thevenard Island Short-tailed Mouse is indeed L. lakedownensis, and is most closely related to L. lakedownensis in the Pilbara in Western Australia. Together, Thevenard Island and adjacent mainland populations are sufficiently divergent from those in northern Australia as to be recognized as two clearly distinct mitochondrial DNA lineages. Conservation and taxonomic implications arising from a phylogeny of haplotypes suggest that two Management Units exist within L. lakedownensis ? a northern unit that includes individuals from the Kimberley (Western Australia) to Kakadu National Park (Northern Territory), and a western unit comprising individuals from Thevenard Island and the Pilbara (Western Australia). These conservation units should be managed as separate subspecies of L. lakedownensis, and a high conservation priority should be given to the Thevenard Island population because it provides an important refugium for L. lakedownensis not just in the Pilbara, but in Australia.

Song Similarity in an Isolated Population of Fox Sparrows (Passerella iliaca)

The structure of song in island populations of birds has been the subject of a number of studies (see review in Miller 1982). Two different hypotheses predict how song might evolve in such populations. First, island populations may have more complex songs because they are subject to weaker selective pressures for species-specific signals than are mainland populations (Miller 1982). We refer to this as the song complexity hypothesis. Second, founder effects may constrain song structure (Nottebohm 1969, Thielcke 1973, Mundinger 1975, Mirsky 1976, Baptista and Johnson 1982, Lynch and Baker 1986); a population bottleneck could lead to reduced song variation because young birds are exposed to fewer kinds of songs and song elements (Baker and Jenkins 1987). We refer to this as the founder effect hypothesis. These two hypotheses are not necessarily incompatible, as a population could possess complex songs that are highly stereotyped within each bird and vary little among birds, or could possess simple songs that vary greatly from male to male. Furthermore, in a small population, periodic bottlenecks after low population levels, such as might occur after a season of high mortality, could function to re-establish song similarity. As a contribution to our understanding of song characteristics in island populations, we present data on the song structure of an insular population of Fox Sparrows (Passerella iliaca) in Nova Scotia, Canada. Fox Sparrows reach the southeastern limit of their breeding range in Nova Scotia, and breeding birds are largely restricted to scattered small offshore islands along the coast, where relatively few other passerine species breed. These populations are therefore isolated by the location of islands and the paucity of adjacent mainland populations. Fox Sparrows are largely migratory, but probably exhibit high fidelity to the island (see discussion).

Morphological Variation and Fluctuating Asymmetry Among Insular Populations of the Sleepy Lizard, Trachydosaurus-Rugosus Gray (Squamata, Scincidae)

Morphological characteristics of island populations of the sleepy lizard, Trachydosaurus rugosus, were compared with those of three adjacent mainland populations in South Australia. An analysis of 21 morphological characters revealed greater divergence among the island populations than among those on the mainland. However, in contrast with the findings of an earlier study using allozyme electrophoresis, morphological relationships among populations showed little congruence with the relationships expected from their geographic proximity. Two measures of developmental stability (fluctuating asymmetry and percentage gross abnormalities) varied significantly among the populations. High levels of developmental instability were observed in three of the island populations. It is not possible to rule out environmental causes of the increased developmental instability but it appears more likely that it is caused by genetic drift and inbreeding resulting from small population size. These results suggest that developmental stability may be more useful in monitoring genetic changes in wildlife populations than the conventional method of allozyme electrophoresis.

Genetic-Variation Among Insular Populations of the Sleepy Lizard, Trachydosaurus-Rugosus Gray (Squamata, Scincidae)

Seven island populations of the sleepy lizard, Trachydosaurus rugosus, in South Australia were studied to establish the genetic effects of isolation. These effects were assessed by comparing genetic characteristics (using allozyme electrophoresis) of the island populations with those of three adjacent mainland populations. Heterozygosity levels did not vary significantly among the populations although the island populations exhibited reduced allelic diversity. Alleles that were rare on the mainland were not present in the island populations. Genetic divergence among the island populations was much greater than among populations on the mainland, reinforcing the notion that evolutionary forces, probably genetic drift, were greatest among the insular populations. This study demonstrates that the intra-specific component of variation can be significant, and that the importance of this component will increase with the fragmentation and isolation of populations. This finding serves to emphasis the importance of considering the population as the unit of conservation.

Interactive effects of temperature and geography on emigration behavior of Drosophila melanogaster: climatic and island factors.

These studies were desgned to detect geographic variation in emigration behavior, at various temperatures, of newly collected wild strains ofDrosophila melanogaster. Natural populations from various geographic regions showed three basic emigration response patterns to temperature: linear, threshold, and optimum-temperature response types. The emigration activity of northern mainland populations increased in the range of 15–20°C, whereas the activity of comparable southern populations increased linearly with increasing temperatures. The northern island populations showed the optimum-temperature response type, and the comparable southern island populations showed all three emigration patterns. In most cases emigration activity on islands was generally reduced compared with that on the adjacent mainland. The northern island populations, however, showed a higher emigration activity at 25°C than the adjacent mainland populations. Here the different sensitivities to temperature seemed to be related to differences in both climatic conditions and insular conditions.

Biogeography of Intraspecific Morphological Variation in the Bush Crickets Decticus verrucivorus (L.) and D. albifrons (F.) (Orthoptera: Tettigoniidae)

Decticus verrucivorus and D. albifrons are two large bush crickets that have been collected extensively across Europe, Asia and North Africa since the 1840s. D. verrucivorus is a northerly species compared to albifrons. From a large number of specimens deposited in European museums, it was possible to speculate, from the morphometrics, on the gene flow and relative degree of isolation of some of the populations. D. verrucivorus shows conservative morpho- logy across NW Europe (including Britain) suggesting only recent separation of the component populations across the area. In SE Europe and Italy, the moun- tain ranges and plains have caused considerable fragmentation of the species into several distinct morphological forms. Certain characteristic forms have arisen in different areas possibly arising out of similar selection pressures in diverse areas. D. albifrons morphology is less variable than that of verrucivorus. There seems to have been recent gene flow between Italy, Sicily and N. Africa. However, albifrons on certain islands (e.g. Lipari and Canaries) seems to have been isolated for some time from adjacent mainland populations. Correlations of morphology with latitude or longitude showed that with increasing eastward continentality the femur, tegmina and ovipositor of verrucivorus were longer. Body size and ovipositor length of this species increased to the south. The only significant trend in the more conservative morphology of albifrons was an increase in ovipositor length to the east.

Effect of Prey Density on Reproduction, Foraging and Other Activities in the Adder, Vipera berus

AbstractReproduction, foraging and other activities of an island population of adder, Vipera berus, were documented in 1974 after a high density of rodents in 1973. The study was repeated in 1975 after one year of diminished food resources. In spring 1974 the weight status (body mass/length) of all categories of island adders was significantly higher than in adjacent mainland populations. After one year of food scarcity mean body mass had decreased significantly in all categories of adders. At the same time mean body length in males, (but not in females) increased significantly. Survival after one year of low food resources was markedly higher in non-reproductive females than in reproductive ones, pointing to a high cost of reproduction at low food density. In 1975, spring sloughing in males was delayed and prolonged, and reproduction failed completely. The foraging period was prolonged and included spring. Most adders then left their normal spring habitats close to the hibernation area and moved to foraging habitats. Predator evasion distance was much shorter than in well-nourished adders.