Multidimensional stable isotope analysis illuminates resource partitioning in a sub‐Antarctic island bird community

A central theme in community ecology is understanding how similar species co‐exist and how their interactions may evolve in the context of climate change. Most studies of resource partitioning among central place foragers, particularly birds, focus on the offspring‐rearing period, when they are accessible, but breeding success may be determined earlier and little is known about how such species partition resources at the onset of breeding. We used a non‐invasive approach to evaluate resource partitioning in co‐existing females at a sub‐Antarctic island during their pre‐laying periods. Three hypotheses were tested using carbon, nitrogen and oxygen stable isotope ratios measured in shells and membranes of hatched eggs as ecological tracers: 1) resource partitioning by geographic location and trophic level will exist among the 12 bird species and will be enhanced within taxonomic groups; 2) given the absence of strong oxygen gradients in the Southern Ocean we will not detect spatial structuring based on oxygen isotopes, but differences will exist between resident and oceanic species as the former may use meteoric water; 3) capital and income breeder strategies can be differentiated using stable isotopes of egg remains. Two and three dimensional isotopic data showed resource partitioning among species. As predicted, segregation was evident within the four main taxonomic groups: penguins, albatrosses, burrowing petrels and giant petrels. Unexpectedly, oxygen isotopes revealed widespread use of meteoric water among a suite of sub‐Antarctic birds. Stable isotopes allowed us to identify females of most species as income breeders at the onset of breeding, with the exception of the females of the two crested penguin exhibiting a mix of income and capital resources use. Multidimensional isotopic analyses revealed that resource partitioning exists at multiple stages of the annual cycle in ways likely to be important under global change, exhibiting wide potential for ecosystem analysis.     

The influence of landscape,climate and history on spatial genetic patterns in keystone plants (Azorella) on sub‐Antarctic islands

The distribution of genetic variation in species is governed by factors that act differently across spatial scales. To tease apart the contribution of different processes, especially at intermediate spatial scales, it is useful to study simple ecosystems such as those on sub‐Antarctic oceanic islands. In this study, we characterize spatial genetic patterns of two keystone plant species, Azorella selago on sub‐Antarctic Marion Island and Azorella macquariensis on sub‐Antarctic Macquarie Island. Although both islands experience a similar climate and have a similar vegetation structure, they differ significantly in topography and geological history. We genotyped six microsatellites for 1,149 individuals from 123 sites across Marion Island and 372 individuals from 42 sites across Macquarie Island. We tested for spatial patterns in genetic diversity, including correlation with elevation and vegetation type, and clines in different directional bearings. We also examined genetic differentiation within islands, isolation‐by‐distance with and without accounting for direction, and signals of demographic change. Marion Island was found to have a distinct northwest–southeast divide, with lower genetic diversity and more sites with a signal of population expansion in the northwest. We attribute this to asymmetric seed dispersal by the dominant northwesterly winds, and to population persistence in a southwestern refugium during the Last Glacial Maximum. No apparent spatial pattern, but greater genetic diversity and differentiation between sites, was found on Macquarie Island, which may be due to the narrow length of the island in the direction of the dominant winds and longer population persistence permitted by the lack of extensive glaciation on the island. Together, our results clearly illustrate the implications of island shape and geography, and the importance of direction‐dependent drivers, in shaping spatial genetic structure.     

Critical indirect effects of climate change on sub‐Antarctic ecosystem functioning

Sub‐Antarctic islands represent critical breeding habitats for land‐based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub‐Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; “island mass effect”; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long‐term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long‐term decline in enhanced phytoplankton productivity at the islands in response to a climate‐driven shift in the position of the sub‐Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore‐feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of offshore‐feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.     

Molecular and morphometric assessment of the taxonomic status of Ectemnorhinus weevil species (Coleoptera: Curculionidae, Entiminae) from the sub-Antarctic Prince Edward Islands

There are long‐standing controversies on the taxonomic status of Ectemnorhinus weevil species occurring on the sub‐Antarctic Prince Edward Islands. Since the two islands that constitute the Prince Edward Islands archipelago (PEIA), Marion Island (MI) and Prince Edward Island (PEI), differ in terms of alien invasive species such as the introduced house mouse Mus musculus and conservation management strategies, it is important to consider inter‐island dynamics when investigating inter‐specific relationships. Using a combined molecular phylogenetic and morphometric approach, we attempted to resolve the taxonomic status of the PEIA Ectemnorhinus weevil species. A COI gene phylogeny was inferred following the genetic characterization of 52 Ectemnorhinus weevils from both islands, and morphometric assessment using a set of 15 linear, external measurements was used to differentiate between the two currently recognized species, Ectemnorhinus similis and Ectemnorhinus marioni. Analyses revealed the presence of two genetically and morphometrically distinct species on PEI, whilst evidence for a single species, comprising diverse genetically discrete populations was found on MI. Based on these results, the species unique to PEI has been designated Ectemnorhinus kuscheli n. sp. whilst we confirm the synonymy between E. similis and E. marioni, the two species originally described from MI. E. kuscheli appears to be restricted to PEI, whereas E. similis occurs on both MI and PEI.     

Influence of island geography,age and landscape on species composition in different animal groups

Aim To study the importance of ecological and geographical factors in explaining arthropod species composition on islands. Location The Aeolian Islands, a volcanic archipelago in the central Mediterranean, near Sicily. Methods The influence of island area, age, distance to the mainland, distance to the nearest island and land cover categories on species composition of arthropod groups was analysed using canonical correspondence analysis (CCA). The use of multiple animal groups in the same archipelago allowed the development of two complementary approaches based on CCA – a ‘taxon‐focused’ approach and an ‘island‐focused’ approach – to elucidate, respectively, how different taxa respond to the same environmental factors, and which factors are mainly responsible for the composition of the faunas in different locations. Results Island area was an important factor in explaining species composition in Chilopoda, Orthoptera and Tenebrionidae. Distance to the mainland was important mainly for Carabidae. Distance to the closest island was important for many groups. By contrast, island age exerted a significant influence only on the species composition of Orthoptera. Various groups were influenced by a combination of broad‐leaved forest and natural grassland. Main conclusions The example of the arthropods of the Aeolian Islands indicates that the influence of a given island characteristic on species composition varies among groups, although measures of inter‐island isolation were typically more important for taxa than isolation from the mainland source. This suggests that colonization of islands may occur mostly by stepping‐stone dispersal.     

Assessing the conservation value of a human-dominated island landscape: Plant diversity in Hawaii

The conversion of native habitats to pasture and other working lands, unbuilt lands modified by humans for production, is one of the greatest threats to biodiversity. While some human-dominated landscapes on continents support relatively high native biodiversity, this capacity is little studied in oceanic island systems characterized by high endemism and vulnerability to invasion. Using Hawaii as a case study, we assessed the conservation value of working landscapes on an oceanic island by surveying native and non-native plant diversity in mature native forest and in the three dominant land covers/uses to which it has been converted: native, Acacia koa timber plantations, wooded pasture, and open pasture. As expected, native plant diversity (richness and abundance) was significantly higher and non-native abundance significantly lower in mature native forests than any other site type. A. koa plantations and wooded pasture supported four and three times greater, respectively, species richness of native understory plants than open pasture. Also, A. koa plantations and wooded pasture supported similar species communities with about 75% species in common. Conservation and restoration of mature native forest in Hawaii is essential for the protection of native, rare species and limiting the spread of non-native species. A. koa plantations and wooded pasture, however, may help harmonize production and conservation by supporting livelihoods, more biodiversity than open pasture, and some connectivity between native forest remnants important for sustaining landscape-level conservation value into the future.     

Interannual variability in reproductive traits of the Patagonian toothfish Dissostichus eleginoides around the sub‐Antarctic island of South Georgia

Commercial fisheries data, collected as part of an observer programme and covering the period 1997–2014, were utilized in order to define key reproductive traits and spawning dynamics of the Patagonian toothfish Dissostichus eleginoides at South Georgia. Multi‐year spawning site fidelity of D. eleginoides was revealed through the identification of previously unknown spawning hotspots. Timing of female spawning was shown to have shifted later, leading to a shorter spawning duration. A decrease in length and mass of female and male spawning fish and a reduced number of large spawning fish was found, evidence of a change in size structure of spawning D. eleginoides. During the study period fewer later maturity stage females (including spawning stage) were observed in conjunction with increased proportions of early stage female D. eleginoides. The findings are discussed in the context of reproductive success, with consideration of the possible effects such spawning characteristics and behaviours may have on egg and larval survival. This work presents the first long‐term assessment of D. eleginoides spawning dynamics at South Georgia and provides valuable knowledge for both the ecology of the species and for future fisheries management of this commercially important species.     

Phylogeography of North Atlantic intertidal tardigrades: refugia,cryptic speciation and the history of the Mid‐Atlantic Islands

Aim To analyse the phylogeographical history of intertidal tardigrades in the North Atlantic in order to improve our understanding of geographical differentiation in microscopic organisms, and to understand the potential importance of the Mid‐Atlantic Islands as stepping stones between the American and European coasts of the Atlantic Ocean. Location Twenty‐four localities from the Mid‐Atlantic Islands (Greenland, Iceland and the Faroe Islands) and both sides of the North Atlantic Ocean. Methods A mitochondrial marker (cytochrome c oxidase subunit I) was sequenced from individual tardigrades belonging to the genus Echiniscoides. The existence of cryptic species was detected using generalized mixed Yule coalescence analysis; lineage ages were estimated with relaxed clock methods; and the degree of geographical differentiation was analysed with samova analyses, haplotype networks and Mantel tests. Results Echiniscoides hoepneri, previously known only from Greenland, was recovered throughout the Mid‐Atlantic Islands. The Faroe Islands population was isolated from Greenland and Iceland, but overall genetic variation was low. The morphospecies Echiniscoides sigismundi had high genetic variation and consisted of at least two cryptic species. A northern and a southern species were both recovered on both sides of the Atlantic, but only the northern species was found on the Mid‐Atlantic Islands. The northern species showed signs of long‐term isolation between the Western and Eastern Atlantic, despite the potential of the Mid‐Atlantic islands to act as stepping‐stones. There was no sign of long‐term isolation in the southern species. The Mid‐Atlantic individuals of the northern species were of Eastern Atlantic origin, but Greenland and Iceland showed signs of long‐term isolation. The genetic pattern found in the southern species is not clearly geographical, and can probably be best explained by secondary contact between former isolated populations. Main conclusions North Atlantic intertidal tardigrades from the genus Echiniscoides showed strong geographical differentiation, and the Mid‐Atlantic Islands seemed unimportant as stepping stones across the Atlantic. The geographical variation of the northern species of E. sigismundi suggests post‐glacial recolonization from several refugia.     

The ecological impact of oceanic island colonization – a palaeoecological perspective from the Azores

Aim In many cases, human colonization drastically modified the ecosystems of remote oceanic islands before scientists arrived to document the changes. Palaeoecological records before and after human colonization provide insights into the original ecosystems and an assessment of subsequent human impact. We used pollen analysis to compare the impact of 15th century colonization of the Azores with that of natural disturbances such as volcanic eruptions and climate changes. Location Azores archipelago, Atlantic Ocean. Methods Sediment records from three highland sites in the Azores (on the islands of Pico and Flores) were dated radiometrically and analysed palynologically. Pollen taxa were classified as native, endemic or introduced based on comparison with flora lists. Data were statistically zoned and temporal trends identified using detrended correspondence analysis. Results Human colonization of the Azores resulted in rapid, widespread, persistent vegetation changes on a scale unprecedented in the last 2700 years, detectable through the decline of dominant trees, the spread of grasses and fire‐tolerant species, the introduction of exotic plants, evidence for grazing and fire, and changes to soils and moisture availability. During the same period, volcanic eruptions appear to have had more localized impacts on the vegetation, lasting 500–1000 years and favouring endemic taxa. The effect of late Holocene climatic changes on the highland vegetation of the Azores seems to have been minor. Palaeoecological data indicate that at least two plant species went extinct on Pico after human colonization and that some plants regarded as introduced were almost certainly part of the original flora of the islands. Despite a consistent signal of human impact, compositional differences between Juniperus brevifolia communities on Pico and Flores remained after colonization. Main conclusions Human colonization had a greater impact on the pristine vegetation of Pico and Flores than climatic changes and volcanic activity during recent millennia. The similarity between post‐colonization changes on the Azores and other oceanic islands suggests a consistent pattern and scale to historical‐era human impact on otherwise pristine ecosystems. These characteristics could be used to further elaborate biogeographical theory and direct conservation efforts towards species that appear most susceptible to human activity.     

A reconstruction of Palaeo‐Macaronesia,with particular reference to the long‐term biogeography of the Atlantic island laurel forests

Macaronesia is a biogeographical region comprising five Atlantic Oceanic archipelagos: the Azores, Madeira, Selvagen (Savage Islands), Canaries and Cape Verde. It has strong affinities with the Atlantic coast of the Iberian Peninsula and the north‐western fringes of Africa. This paper re‐evaluates the biogeographical history and relationships of Macaronesia in the light of geological evidence, which suggests that large and high islands may have been continuously available in the region for very much longer than is indicated by the maximum surface area of the oldest current island (27 Ma) – possibly for as long as 60 million years. We review this literature, attempting a sequential reconstruction of Palaeo‐Macaronesia from 60 Ma to the present. We consider the implications of these geological dynamics for our understanding of the history of colonization of the present islands of Macaronesia. We also evaluate the role of these archipelagos as stepping stones and as both repositories of palaeo‐endemic forms and crucibles of neo‐endemic radiations of plant and animal groups. Our principal focus is on the laurel forest communities, long considered impoverished relicts of the Palaeotropical Tethyan flora. This account is therefore contextualized by reference to the long‐term climatic and biogeographical history of Southern Europe and North Africa and by consideration of the implications of changes in land–sea configuration, climate and ocean circulation for Macaronesian biogeography. We go on to provide a synthesis of the more recent history of Macaronesian forests, which has involved a process of impoverishment of the native elements of the biota that has accelerated since human conquest of the islands. We comment briefly on these processes and on the contemporary status and varied conservation opportunities and threats facing these forests across the Macaronesian biogeographical region.     

Nebela jiuhuensis nov. sp. (Amoebozoa; Arcellinida; Hyalospheniidae): A New Member of the Nebela saccifera ‐ equicalceus ‐ ansata Group Described from Sphagnum Peatlands in South‐Central China

Hyalospheniids are among the most common and conspicuous testate amoebae in high‐latitude peatlands and forest humus. These testate amoebae were widely studied as bioindicators and are increasingly used as models in microbial biogeography. However, data on their diversity and ecology are still very unevenly distributed geographically: notably, data are lacking for low‐latitude peatlands. We describe here a new species, Nebela jiuhuensis, from peatlands near the Middle Yangtze River reach of south‐central China with characteristic morphology. The test (shell) has hollow horn‐like lateral extensions also found in N. saccifera, N. equicalceus (=N. hippocrepis), and N. ansata, three large species restricted mostly to Sphagnum peatlands of Eastern North America. Mitochondrial cytochrome oxidase (COI) data confirm that N. jiuhuensis is closely related to the morphologically very similar North American species N. saccifera and more distantly to N. ansata within the N. penardiana group. These species are all found in wet mosses growing in poor fens. Earlier reports of morphologically similar specimens found in South Korea peatlands suggest that N. jiuhuensis may be distributed in comparable peatlands in Eastern Asia (China and Korea). The discovery of such a conspicuous new species in Chinese peatlands suggests that many new testate amoebae species are yet to be discovered, including potential regional endemics. Furthermore, human activities (e.g., drainage, agriculture, and pollution) have reduced the known habitat of N. jiuhuensis, which can thus be considered as locally endangered. We, therefore, suggest that this very conspicuous micro‐organism with a probably limited geographical distribution and specific habitat requirement should be considered as a flagship species for microbial biogeography as well as local environmental conservation and management.     

Phylogeography of the pallid kangaroo mouse,Microdipodops pallidus: a sand‐obligate endemic of the Great Basin,western North America

Aim Kangaroo mice, genus Microdipodops Merriam, are endemic to the Great Basin and include two species: M. pallidus Merriam and M. megacephalus Merriam. The pallid kangaroo mouse, M. pallidus, is a sand‐obligate desert rodent. Our principal intent is to identify its current geographical distribution and to formulate a phylogeographical hypothesis for this taxon. In addition, we test for orientation patterns in haplotype sharing for evidence of past episodes of movement and gene flow. Location The Great Basin Desert region of western North America, especially the sandy habitats of the Lahontan Trough and those in south‐central Nevada. Methods Mitochondrial DNA sequence data from portions of three genes (16S ribosomal RNA, cytochrome b, and transfer RNA for glutamic acid) were obtained from 98 individuals of M. pallidus representing 27 general localities sampled throughout its geographical range. Molecular sequence data were analysed using neighbour‐joining, maximum‐parsimony, maximum‐likelihood and Bayesian methods of phylogenetic inference. Directional analysis of phylogeographical patterns, a novel method, was used to examine angular measurements of haplotype sharing between pairs of localities to detect and quantify historical events pertaining to movement patterns and gene flow. Results Collecting activities showed that M. pallidus is a rather rare rodent (mean trapping success was 2.88%), and its distribution has changed little from that determined three‐quarters of a century ago. Two principal phylogroups, distributed as eastern and western moieties, are evident from the phylogenetic analyses (mean sequence divergence for cytochrome b is c. 8%). The western clade shows little phylogenetic structure and seems to represent a large polytomy. In the eastern clade, however, three subgroups are recognized. Nine of the 42 unique composite haplotypes are present at two or more localities and are used for the orientation analyses. Axial data from haplotype sharing between pairwise localities show significant, non‐random angular patterns: a north‐west to south‐east orientation in the western clade, and a north‐east to south‐west directional pattern in the eastern clade. Main conclusions The geographical range of M. pallidus seems to be remarkably stable in historical times and does not show a northward (or elevationally upward) movement trend, as has been reported for some other kinds of organism in response to global climate change. The eastern and western clades are likely to represent morphologically cryptic species. Estimated times of divergence of the principal clades of M. pallidus (4.38 Ma) and between M. pallidus and M. megacephalus (8.1 Ma; data from a related study) indicate that kangaroo mice diverged much earlier than thought previously. The phylogeographical patterns described here may serve as a model for other sand‐obligate members of the Great Basin Desert biota.     

Wing morphology of the active flyer Calliphora vicina (Diptera: Calliphoridae) during its invasion of a sub‐Antarctic archipelago where insect flightlessness is the rule

The cosmopolitan blowfly Calliphora vicina became established in the sub‐Antarctic Kerguelen Islands in the late 1970s, following a warming period that allowed its full development. Although temperature and wind may limit flight activity, the fly invaded the archipelago, reaching sites remote from the introduction point. Most native competitors have converged to flightlessness as a response to stringent environmental conditions and therefore the flight strategy of C. vicina might be either a handicap or a competitive advantage under ongoing climate change. Using geometric morphometrics, we investigated whether the wing had changed over time in C. vicina within the archipelago (1998 vs. 2009) and compared its morphology with that of a continental population from a temperate area (1983 vs. 2009). Wing shape plasticity to temperature was also experimentally investigated. We found no clues of relaxed selection on flight morphology in the range invaded. However, rapid changes of wing shape occurred over time in females from the Kerguelen Islands compared with both males and females of the continental population, despite a shorter time‐lag between samples in the former. The thermal reaction norms for wing shape found for C. vicina from Kerguelen were also different from those of the continental population, but it remains unknown whether this resulted from or preceded the introduction. These combined findings are consistent with a fingerprint of local adaptation in the invasive population. However, the adaptive significance of the changes, in terms of their aerodynamic consequences and the future evolution of C. vicina in the Kerguelen Islands, requires further investigation. From an evolutionary standpoint, sustaining flight capability under the novel sub‐Antarctic conditions might be critical to the invasive success of C. vicina as most competitors are flightless.     

Phylogeography of the chestnut‐tailed antbird (Myrmeciza hemimelaena) clarifies the role of rivers in Amazonian biogeography

Aim We examined patterns of spatial and temporal diversification of the Amazonian endemic chestnut‐tailed antbird, Mymeciza hemimelaena (Thamnophilidae), to evaluate the diversification of a widespread avian taxon across rivers that potentially represent major natural barriers. Location Lowland Amazonia. Methods Sequences of the mitochondrial ND2 and cytochrome b genes were investigated from 65 individuals distributed throughout the entire range of M. hemimelaena, and including the two currently valid subspecies M. h. hemimelaena and M. h. pallens. Based on a combination of phylogeographic tools, molecular dating, and population genetic methods, we reconstructed a spatio‐temporal scenario of diversification of M. hemimelaena in the Amazon. Results The data revealed three genetically divergent and monophyletic groups in M. hemimelaena, which can also be distinguished by a combination of morphological and vocal characters. Two of these clades correspond to the previously described taxa M. h. hemimelaena and M. h. pallens, which are separated by the upper Madeira River, a main Amazonian tributary. The third clade is distributed between the middle reaches of the Madeira River and the much smaller tributaries Jiparaná and Aripuanã, and, although currently treated as M. h. pallens, clearly constitutes an independent evolutionary lineage probably deserving separate species status. Molecular clock and population genetic analyses indicate that diversification in this group occurred throughout the Pleistocene, with demographic fluctuations assumed for M. h. hemimelaena and M. h. pallens. Main conclusions The findings implicate rivers as barriers driving diversification in the M. hemimelaena complex. Levels of mitochondrial DNA divergence and associated morphological and vocal traits support its division into at least three separate species with comparatively small ranges. The existence of a previously unrecognized lineage in the M. hemimelaena complex, and the high degree of population structuring found in M. h. hemimelaena underscore the pervasiveness of cryptic endemism throughout Amazonia and the importance of DNA‐based taxonomic and phylogeographic studies in providing the accurate estimates of diversity that are essential for conservation planning.     

A time‐calibrated phylogenetic approach to assessing the phylogeography,colonization history and phenotypic evolution of snakes in the Japanese Izu Islands

Aim We infer the biogeography and colonization history of a dispersal‐limited terrestrial vertebrate, the Japanese four‐lined ratsnake (Elaphe quadrivirgata), to reveal the number of times mainland populations have invaded the Izu Archipelago of Japan, the mainland sources of these colonists, and the time‐scale of colonization. We compare these results with those of past studies in an attempt to uncover general biogeographical patterns. Moreover, we briefly examine the significance of colonization history when evaluating the evolution of body size and melanism of the Izu Island E. quadrivirgata populations. Location The Izu Islands (Oshima, Toshima, Niijima, Shikine, Kozu, Tadanae and Mikura), a volcanic archipelago off the Pacific coast of central Japan. Methods We obtained DNA sequences of the mitochondrial cytochrome b gene (1117 base pairs) from 373 individual snakes sampled from seven of the Izu Islands and 25 mainland localities. We employed partitioned Bayesian phylogenetic analyses assuming a relaxed molecular clock to estimate phylogenetic relationships among extant haplotypes and to give an explicit temporal scale to the timing of clade divergence, colonization history and tempo of body‐size evolution. Moreover, we employed model‐based biogeographical analysis to calculate the minimum number of times E. quadrivirgata colonized the Izu Islands. Results We found evidence that three separate regions of the Izu Archipelago have been colonized independently from mainland ancestors within the past 0.58–0.20 Ma. The Izu Peninsula plus Oshima and Mikura were both colonized independently from lineages inhabiting eastern mainland Japan. The Toshima, Niijima, Shikine, Kozu and Tadanae populations all derive from a single colonization from western mainland Japan. Oshima has been subject to three or possibly four colonizations. Main conclusions These results support the hypothesis that the extreme body‐size disparity among island populations of this ratsnake evolved in situ. Moreover, the fact that the dwarf, melanistic population inhabiting Oshima descends from multiple mainland colonization events is evidence of an extremely strong natural selection pressure resulting in the rapid evolution of this unique morphology. These results contrast with theoretical predictions that natural selection pressures should play a decreased role on islands close to the mainland and/or subject to frequent or recent immigration.