Phenotypic divergence and reproductive isolation between sympatric forms of Japanese threespine sticklebacks

The threespine stickleback ( Gasterosteus aculeatus ) species complex is well suited for identifying the types of phenotypic divergence and isolating barriers that contribute to reproductive isolation at early stages of speciation. In the present study, we characterize the patterns of genetic and phenotypic divergence as well as the types of isolating barriers that are present between two sympatric pairs of threespine sticklebacks in Hokkaido, Japan. One sympatric pair consists of an anadromous and a resident freshwater form and shows divergence in body size between the forms, despite the lack of genetic differentiation between them. The second sympatric pair consists of two anadromous forms, which originated before the last glacial period and are currently reproductively isolated. These two anadromous forms have diverged in many morphological traits as well as in their reproductive behaviours. Both sexual isolation and hybrid male sterility contribute to reproductive isolation between the anadromous species pair. We discuss the shared and unique aspects of phenotypic divergence and reproductive isolation in the Japanese sympatric pairs compared with postglacial stickleback species pairs. Further studies of these divergent species pairs will provide a deeper understanding of the mechanisms of speciation in sticklebacks.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 671–685.     

Complex trait differentiation between host-populations of the pea aphid Acyrthosiphon pisum (Harris): implications for the evolution of ecological specialisation

Variation in traits affecting preference for, and performance on, new habitats is a key factor in the initiation of ecological specialisation and adaptive speciation. However, habitat and resource use also involves other traits whose influence on ecological and genetic divergence remains poorly understood. In the present study, we investigated the extent of variation of life-history traits among sympatric populations of the pea aphid Acyrthosiphon pisum , which shows several host races that are specialised on various plants of the family Fabaceae plants and is an established model for ecological speciation. First, we assessed the community structure of microbial partners within host populations of the pea aphid. The effect of these microbes on host fitness is uncertain, although there is growing evidence that they may modulate various important adaptive traits of their host such as plant utilisation and resistance against natural enemies. Second, we performed a multivariate analysis on several ecologically relevant features of host populations recorded in the present and previous studies (including microbial composition, colour morph, reproductive mode, and male dispersal phenotype), enabling the identification of correlations between phenotypic traits. We discuss the ecological significance of these associations of traits in relation to the habitat characteristics of pea aphid populations, and their consequences for the evolution of ecological specialisation and sympatric speciation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 718–727.     

Morphological segregation of Icelandic threespine stickleback (Gasterosteus aculeatus L).

Icelandic threespine sticklebacks show parallel sympatric morphological differences related to different substrate habitats in four Icelandic lakes. The level of morphological diversification varies among the lakes, ranging from a population with a wide morphological distribution to a population with clear resource morphs, where morphological diversification was reflected in diet differences. These differences in morphological divergence are closely related to the differences in the ecological surroundings of each population. This appears to be resource polymorphism, which may lead to population differentiation and speciation. Trophically related sexual dimorphism was also common in these sticklebacks, which is possibly the result of sexual selection or habitat segregation by the sexes. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 76 , 247–257.     

Risk of predation as a promoting factor of species divergence in threespine sticklebacks (Gasterosteus aculeatus L.)

Icelandic freshwater systems are geologically young and contain only six species of freshwater fish. As these species colonized Icelandic fresh waters they were presented with a diversity of unique, uncontested habitats and food resources, promoting the evolution of new behaviour strategies crucial to the formation of new morphs and speciation. To determine the likelihood that predation threat could affect the antipredator behaviour and possibly the sympatric divergence of prey populations, we analysed antipredator behaviour of seven groups of Icelandic threespine sticklebacks ( Gasterosteus aculeatus ): two marine groups, one group from a lake without piscine predators, and two polymorphic lake populations, each with two groups occupying unique habitats. Shoaling cohesion, school formation and duration, and vigilance in predator inspection/avoidance behaviour varied greatly among groups. The differences appeared to be related to the risk of predation as well as to opportunities and constraints set by the different habitats. Antipredator behaviour was especially pronounced and differed extensively in two polymorphic forms from the lake Thingvallavatn, where predation risk is very high. By keeping the two morphs separate in their respective habitats, high predation risk may be a contributing factor in promoting the habitat-specific divergence of G. aculeatus seen in the lake. This suggests that in situations where refuge habitats are spatially separated, the risk of predation may contribute to the evolution of separate sympatric forms of small fish such as G. aculeatus .  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 189–203.     

Local adaptation in a plant herbivore interaction depends on the spatial scale

Local adaptation has central importance in the understanding of co-evolution, maintenance of sexual reproduction, and speciation. We investigated local adaptation in the alkaloid-bearing legume Crotalaria pallida and its seed predator, the arctiid moth Utetheisa ornatrix , at different spatial scales. When we studied three populations from south-east Brazil (150 km apart), we did not find evidence of local adaptation, although we did find interpopulational differences in herbivore performance, and a significant interaction between herbivore sex and plant population. These results indicate that both moth and plant populations are differentiated at the regional scale. In a comparison of populations from Brazil and Florida, the herbivore showed local adaptation to its host plant; for both moth populations, the pupae were heavier when the larvae ate the sympatric than the allopatric host population. We discuss the scale dependence of our results and the possible causes for the lack of local adaptation at the regional scale, even in the presence of plant and moth differentiation. The results obtained demonstrate the importance of studying co-evolution and local adaptation at different geographical scales.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 494–502.     

Historical contingency and ecological determinism interact to prime speciation in sticklebacks, Gasterosteus

Historical contingency and determinism are often cast as opposing paradigms under which evolutionary diversification operates. It may be, however, that both factors act together to promote evolutionary divergence, although there are few examples of such interaction in nature. We tested phylogenetic predictions of an explicit historical model of divergence (double invasions of freshwater by marine ancestors) in sympatric species of three-spined sticklebacks (Gasterosteus aculeatus) where determinism has been implicated as an important factor driving evolutionary novelty. Microsatellite DNA variation at six loci revealed relatively low genetic variation in freshwater populations, supporting the hypothesis that they were derived by colonization of freshwater by more diverse marine ancestors. Phylogenetic and genetic distance analyses suggested that pairs of sympatric species have evolved multiple times, further implicating determinism as a factor in speciation. Our data also supported predictions based on the hypothesis that the evolution of sympatric species was contingent upon 'double invasions' of postglacial lakes by ancestral marine sticklebacks. Sympatric sticklebacks, therefore, provide an example of adaptive radiation by determinism contingent upon historical conditions promoting unique ecological interactions, and illustrate how contingency and determinism may interact to generate geographical variation in species diversity     

Evolutionary diversification of opercle shape in Cook Inlet threespine stickleback

We investigated the evolution of a large facial bone, the opercle (OP), in lake populations of the threespine stickleback that were founded by anadromous ancestors, in Cook Inlet, Alaska. Recent studies characterized OP variation among marine and lake populations and mapped a quantitative trait locus with a large influence on OP shape. Using populations from diverse environments and independent evolutionary histories, we examined divergence of OP shape from that of the anadromous ancestor. We report preliminary evidence for divergence between benthic and generalist lake ecotypes, necessitating further investigation. Furthermore, rapid divergence of OP shape has occurred in a lake population that was founded by anadromous stickleback in the 1980s, which is consistent with divergence of other phenotypic traits and with OP diversification in other lake populations. By contrast, there has been limited evolution of OP shape in a second lake population that may have experienced a genetic bottleneck early in its history and lacks genetic variation for OP divergence. Taken together, the results obtained from these two populations are consistent with studies of other stickleback phenotypic traits that implicate ancestral variation in postglacial adaptive radiation of threespine stickleback in fresh water.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 832–844.     

Voice variance may signify ongoing divergence among black-legged kittiwake populations

Acoustic features are important for individual and species recognition. However, while dialectal variations in song characteristics have been described in many songbirds, geographical divergence in vocal features across populations has seldom been studied in birds that are not thought to have song-learning abilities. Here, we document marked differences in the vocal structure of calls of two populations of black-legged kittiwakes ( Rissa tridactyla ), a seabird whose call is considered as not being learned from other individuals. We found that calls vary both within and between populations. Within-population variation may convey individual identity, whereas the marked differences in frequency and temporal parameters observed between the two populations may reveal ongoing divergence among kittiwake populations. Moreover, we were unable to detect any sex signature in adult calls in a Pacific population (Middleton, Alaska), while these were detected in an Atlantic population (Hornøya, Norway), potentially affecting sexual behaviours. Despite the fact that these calls seemed to change over the reproductive season and across years, the individual signature remained fairly stable. Such vocal differences suggest that Pacific and Atlantic populations may be undergoing behavioural divergences that may reveal early stages of speciation, as is suggested by molecular data.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 289–297.     

Parasite burdens differ between sympatric three-spined stickleback species

The ecological theory of adaptive radiation states that differences in ecological circumstances among local populations are the cause of divergence that leads to speciation. The role of parasites in contributing to divergence has seldom been considered, despite their ubiquity and known selective effects. The potential for parasites to contribute to divergence between closely related taxa was examined by quantifying the variation in parasite burdens between sympatric three-spined stickleback species ( Gasterosteus aculeatus complex) in two lakes in coastal British Columbia, Canada. In doing so the relative importance of geographical differences between lakes and trophic or microhabitat differences between species within lakes were evaluated. The entire metazoan parasite burdens of a total of 255 limnetic and benthic sticklebacks in Paxton and Priest lakes were assayed over five time points between spring and autumn. Despite their sympatric distributions, there were large differences in parasite burdens between benthic and limnetic sticklebacks within lakes and these were consistent across both lakes. In particular, limnetics suffered greater burdens of the parasites Schistocephalus solidus and Diplostomum scudderi and benthics had much higher burdens of parasitic glochidia (mollusc larvae). Parasite burdens also differed quantitatively between lakes, but in general such differences were less pronounced than those between the stickleback species. The documented differences in parasite burdens between stickleback species have potential to contribute to divergent selection on life history, immunological and secondary sexual characters that could contribute to reproductive isolation between the species.     

Does sympatry predict life history and morphological diversification in the Mexican livebearing fish Poeciliopsis baenschi?

Understanding why some species coexist and others do not remains one of the fundamental challenges of ecology. Although there is evidence to suggest that closely‐related species are unlikely to occupy the same habitat because of competitive exclusion, there are many cases where closely‐related species do co‐occur. Research comparing sympatric and allopatric populations of co‐occurring species provides a framework for understanding the role of phenotypic diversification in species coexistence. In the present study, we compare phenotypic divergence between sympatric and allopatric populations of the livebearing fish, Poeciliopsis baenschi. We focus on life‐history traits and body shape, comprising two sets of integrated traits likely to diverge in response to varying selective pressures. Given that males and females can express different phenotypic traits, we also test for patterns of divergence among sexes by comparing size at maturity and sexual dimorphism in body shape between males and females in each population type. We take advantage of a natural experiment in western Mexico where, in some locations, P. baenschi co‐occur with a closely‐related species, Poeciliopsis turneri (sympatric populations) and, in other locations, they occur in isolation (allopatric populations). The results obtained in the present study show that sympatric populations of P. baenschi differed significantly in life‐history traits and in body shape compared to their allopatric counterparts. Additionally, males and females showed different responses for size at maturity in sympatric conditions versus allopatric conditions. However, the amount of sexual dimorphism did not differ between sympatric and allopatric populations of P. baenschi. Hence, we conclude that not all traits show similar levels of phenotypic divergence in response to sympatric conditions. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 608–618.     

Intraspecific taxonomy and ecology characterize morphological divergence among cutthroat trout (Oncorhynchus clarkii ssp. Richardson) populations

We compared the proportion of morphological variation accounted for by subspecies categories with the proportion encompassed by ecologically based categories in cutthroat trout ( Oncorhynchus clarkii ssp.), as a means of assessing the relative importance of each approach in identifying intraspecific diversity. We used linear and geometric morphometrics to compare measures of body shape, fin length, and head features between and within subspecies of cutthroat trout. Both categories accounted for a significant proportion of the variation between and within the subspecies; however, the larger proportion was explained by subspecific differences, with the greatest morphological divergence between coastal cutthroat trout ( Oncorhynchus clarkii clarkii ) and interior subspecies. Ecotypic categories within each subspecies also explained significant morphological differences: stream populations had longer fins and deeper, more robust bodies than lake populations. The largest ecotypic differences occurred between stream and lake populations of Yellowstone cutthroat trout ( Oncorhynchus clarkii bouvieri ). Given that many cutthroat trout subspecies are of conservation concern, our study offers a better understanding of intraspecific variation existing within the species, providing precautionary evidence of incipient speciation, and a framework of describing phenotypic diversity that is correlated with ecological conditions.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 266–281.     

Discordance in body size, colour pattern, and advertisement call across genetically distinct populations in a Neotropical anuran (Dendropsophus ebraccatus)

Patterns of intraspecific geographic variation in morphology and behaviour, when examined in a phylogenetic context, can provide insight into the microevolutionary processes driving population divergence and ultimately speciation. In the present study, we quantified behavioural and phenotypic variation among populations from genetically divergent regions in the Central American treefrog, Dendropsophus ebraccatus . Our fine-scale population comparisons demonstrated regional divergence in body size, colour pattern frequencies, and male advertisement call. None of the characters covaried with phylogenetic history or geographic proximity among sampled populations, indicating the importance of highly localized selection pressures and genetic drift in shaping character divergence among isolated regions. The study underscores how multiple phenotypic characters can evolve independently across relatively small spatial scales.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 298–313.     

Molecular phylogeny of icefish Salangidae based on complete mtDNA cytochrome b sequences, with comments on estuarine fish evolution

Phylogenetic relationships among 15 species in the family Salangidae were constructed based on the complete cytochrome b sequence (1141 bp). We confirmed the monophyly of the family Salangidae and defined four primitive lineages within this family: (I) Protosalanx , Neosalanx anderssoni , Neosalanx tangkahkeii , and Neosalanx argentea ; (II) Neosalanx reganius , Neosalanx jordani , Neosalanx oligodontis , and Neosalanx  sp.; (III) Hemisalanx , Salanx , Leucosoma , and Salangichthys ishikawae ; and (IV) Salangichthys microdon . A major finding of our study is the key basal placement of Sg. microdon . According to the tentative estimation, the divergence of the four lineages appears to have been initiated in the early Miocene (21 Mya), with most speciation events occurring 1.05–9.90 Mya. Taxonomic revisions on subfamilial, generic, and specific levels were carried out based on phylogenetic relationships and genetic distance, taking into account some key morphological characters. The speciation mechanism in Salangidae is also discussed, and the evidence shows that geographical isolation, water mass, as well as some ecological factors, may not always play important roles in the speciation of temperate estuarine fish. In the most cases, sympatric salangids are not monophyletic, indicating that their coexistence reflects secondary contact rather than sympatric speciation.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 327–342.     

Variation in stickleback head morphology associated with parasite infection

Parasites can affect host phenotypes, influencing their ecology and evolution. Host morphological changes occurring post-infection might result from pathological by-products of infection, or represent adaptations of hosts or parasites. We investigated the morphology of three-spined sticklebacks, Gasterosteus aculeatus , from a population naturally infected with Schistocephalus solidus , which grows to large sizes in their body cavity. We examined local effects of infection on trunk shape, which are imposed directly by the bulk of the growing parasite, and distant effects on head morphology. We show that trunk shape differed between infection classes, and was affected more severely in fish with heavier total parasite mass. We further show unexpected differences in head morphology. The heads of infected fish were reduced in size and differently shaped to those of non-infected fish, with infected fish having deeper heads. Importantly, both head size and shape were also affected more severely in fish with heavier total parasite mass. This latter result suggests that differences in morphology are caused by post-infection changes. Such changes may be incidental, evolutionarily neutral 'side effects' of infection. However, because head morphology affects foraging ecology, such changes are likely to have fitness consequences for hosts, and may constitute adaptations, either of hosts or of parasites. We discuss our finding in the context of the evolution of phenotypic plasticity, and suggest testable hypotheses examining the proximate mechanisms underlying these morphological effects and their potential evolutionary basis.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 759–768.     

Hierarchical comparative analysis of genetic and genitalic geographical structure: testing patterns of male and female genital evolution in the scarab beetle Phyllophaga hirticula (Coleoptera: Scarabaeidae)

It is generally accepted that genitalia are among the fastest evolving characters in insects and that selection on these structures may increase speciation rates in groups with polygamous mating systems. If selection is causing genitalic divergence between or among populations of a species, one prediction is that geographical structure of genitalic morphology would be in place before genetic structure of a rapidly evolving neutral marker. The current study tests this hypothesis in the geographically widespread scarab beetle Phyllophaga hirticula by evaluating whether standing variation in male and female genitalia is more or less geographically structured than a mitochondrial genetic marker. Geographical structure of mitochondrial (mt)DNA and male and female genitalic shape were analysed using analysis of variance, multivariate analysis of variance, Mantel tests, and tests of spatial autocorrelation. The results show that, although female genitalia are more geographically structured than mtDNA, male genitalia are not. This pattern suggests that selection on female genitalic variation may be causing divergence of these structures among populations.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 135–149.     

Empirical evaluation of phenotype–environment correlation and trait utility with allopatric and sympatric whitefish, Coregonus lavaretus (L.), populations in subarctic lakes

Adaptive phenotypic divergence of sympatric morphs in a single species may have significant evolutionary consequences. In the present study, phenotypic impacts of predator on zooplankton prey populations were compared in two northern Finnish lakes; one with an allopatric whitefish, Coregonus lavaretus (L.), population and the other with three sympatric whitefish populations. First, we examined whether there were phenotypic associations with specific niches in allopatric and sympatric whitefish. Second, trait utility (i.e. number of gillrakers) of allopatric and sympatric whitefish in utilizing a pelagic resource was explored by comparing predator avoidance of prey, prey size in environment, and prey size in predator diet. The allopatric living large sparsely rakered (LSR) whitefish morph, was a generalist using both pelagic and benthic niches. In contrast, sympatric living whitefish morphs were specialized: LSR whitefish was a littoral benthivore, small sparsely rakered whitefish was a profundal benthivore and densely rakered (DR) whitefish was a pelagic planktivore. In the lake with allopatric whitefish, zooplankton prey did not migrate vertically to avoid predation whereas, in the lake with sympatric whitefish, all important prey taxa migrated significantly. Trait utility was observed as significantly smaller size of prey in environment and predator diet in the lake with DR whitefish than in the lake with only LSR whitefish.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 561–572.     

Deep genetic divergences among morphologically similar and parapatric Skistodiaptomus (Copepoda: Calanoida: Diaptomidae) challenge the hypothesis of Pleistocene speciation

We used mitochondrial [cytochrome c oxidase subunit I (CO I ), cytochrome b , and 16S] and nuclear [internal transcribed spacer (ITS) phylogenies of Skistodiaptomus copepods to test hypotheses of Pleistocene divergence and speciation within the genus. Mitochondrial (mt)DNA sequence divergences do not support hypotheses for Pleistocene speciation and instead suggest much more ancient speciation events in the genus. Skistodiaptomus oregonensis and Skistodiaptomus pygmaeus (i.e. two morphologically similar and parapatric species) exhibited uncorrected mtDNA sequence divergences exceeding 20%. Similarly, we identified three divergent clades of Skistodiaptomus pallidus that exhibited mtDNA sequence divergences exceeding 15%, suggesting that even intraspecific divergence within this morphospecies predates the Pleistocene. We found clear evidence of CO I pseudogenes in S. pygmaeus , but their presence did not lead to significant overestimates of sequence divergences for this gene. Substitution saturation and strong purifying selection have most likely led to underestimates of sequence divergences and divergence times among Skistodiaptomus . The widespread phenomenon of morphological stasis among genetically divergent copepod groups indicates that speciation often occurs with little or no morphological change. Instead, morphological evolution may occur idiosyncratically after speciation and create discordant patterns of morphological similarity, shared ancestry and divergence time. Cryptic species complexes are therefore common in copepods, and morphological species concepts underestimate their true species diversity.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 150–165.     

Genetic and morphological divergence reveals local subdivision of perch (Perca fluviatilis L.)

The level of gene flow is an important factor influencing genetic differentiation between populations. Typically, geographic distance is considered to be the major factor limiting dispersal and should thus only influence the degree of genetic divergence at larger spatial scales. However, recent studies have revealed the possibility for small-scale genetic differentiation, suggesting that the spatial scale considered is pivotal for finding patterns of isolation by distance. To address this question, genetic and morphological differentiation were studied at two spatial scales (range 2–13 km and range 300 m to 2 km) in the perch ( Perca fluviatilis L.) from the east coast archipelago of Sweden, using seven microsatellite loci and geometric morphometrics. We found highly significant genetic differentiation between sampled locations at both scales. At the larger spatial scale, the distance per se was not affecting the level of divergence. At the small scale, however, we found subtle patterns of isolation by distance. In addition, we also found morphological divergence between locations, congruent with a spatial separation at a microgeographic scale, most likely due to phenotypic plasticity. The present study highlights the importance of geographical scale and indicates that there might be a disparity between the dispersal capacity of a species and the actual movement of genes. Thus, how we view the environment and possible barriers to dispersal might have great implications for our ability to fully understand the evolution of genetic differentiation, local adaptation, and, in the end, speciation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 746–758.     

Phenotype and gene flow in a marsupial (Antechinus flavipes) in contrasting habitats

Ecological factors are important drivers of phenotypic divergence, which may lead to incipient speciation. A variety of habitats should be preserved to maintain evolutionary potential. We used the marsupial, the yellow-footed antechinus ( Antechinus flavipes ) as a model species for investigating phenotypic differentiation between animals inhabiting two habitat types in south-eastern Australia: flood-plain river red gum and box–ironbark forests. All tested phenotypic characteristics varied between years at the same sites and therefore were not useful for investigating morphological specialization that may lead to speciation. Males generally were significantly heavier when antechinus densities were lower, but exceptions were found, possibly related to food availability. Teat-number variation recently has been shown to be associated with habitat specialization and incipient speciation within Antechinus agilis . We investigated genetic differentiation associated with this trait in A. flavipes . Population genetic analyses of microsatellite genotypes and mitochondrial DNA haplotypes revealed that sympatric 12-, 13- and 14-teat females in Chiltern forest were part of one freely interbreeding population. Our parentage analyses found two cases where 13-teat mothers produced 12-teat daughters. This suggests either plasticity or paternal genetic influence on the offspring's teat-number phenotype. Laboratory matings may be required to resolve the extent to which teat number is heritable in A. flavipes .  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 94 , 303–314.