Molecular correlates of reproductive isolation

Evolution of reproductive isolation as a byproduct of genetic divergence in isolated populations is the dominant (albeit not exclusive) mode of speciation in sexual animals. But little is known about the factors linking speciation to general divergence. Several authors have argued that allopatric speciation should proceed more rapidly if isolated populations also experience divergent selection. Reproductive isolation between allopatric populations is not subject to direct selection; it can accumulate only by random drift or as a fortuitous byproduct of selection on other traits. Here I present a novel analysis of published data, demonstrating that pre- and postmating isolation of Drosophila species are more tightly correlated with allozyme divergence than with silent DNA divergence. Inasmuch as proteins are more subject to the action of natural selection than are silent DNA polymorphisms, this result provides broad support for a model of selection-mediated allopatric speciation.     

Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits

Two genetically distinct habitat races of Drosophila melanogaster coexist in Brazzaville (Congo). One is the typical field type of Afrotropical populations, the other mainly breeds in beer residues in breweries. These two populations differ in their ethanol tolerance, in their allelic frequencies at several enzyme and microsatellite loci and in the composition of their cuticular hydrocarbons. The brewery population is quite similar to European temperate populations with regard to all these traits. Previous investigations of two morphological traits (ovariole number and sternopleural bristle number) failed to detect any difference between the two habitat races. Here we investigated other morphological traits (wing and thorax length, thorax pigmentation and female abdomen pigmentation). The reaction norms of these traits according to growth temperature were compared in the two Afrotropical habitat races and in a French temperate population. As expected, the French population was very different from the field African population: as a general rule, the brewery population (Kronenbourg) was intermediate in several aspects between the other two. We conclude that the strong selective forces that maintain the genetic divergence between the two habitat races also act on morphometrical traits, and the possible selective mechanisms are discussed.     

Pleistocene speciation with and without gene flow in Euphaea damselflies of subtropical and tropical East Asian islands

Climatic oscillations during the Pleistocene period could have had a profound impact on the origin of tropical species by the alternation of allopatric isolation and interpopulation gene flow cycles. However, whether tropical speciation involves strictly allopatric isolation, or proceeds in the face of homogenizing gene flow, is relatively unclear. Here, we investigated geographical modes of speciation in four closely related Euphaea damselfly species endemic to the subtropical and tropical East Asian islands using coalescent analyses of a multilocus data set. The reconstructed phylogenies demonstrated distinct species status for each of the four species and the existence of two sister species pairs, Euphaea formosa/E. yayeyamana and E. decorata/E. ornata. The species divergence time of the sibling Euphaea damselflies dates back to within the last one Mya of the Middle to Lower Pleistocene. The speciation between the populous E. formosa of Taiwan and the less numerous E. yayeyamana of the Yaeyama islands occurred despite significant bidirectional, asymmetric gene flow, which is strongly inconsistent with a strictly allopatric model. In contrast, speciation of the approximately equal-sized populations of E. decorata of the southeast Asian mainland and E. ornata of Hainan is inferred to have involved allopatric divergence without gene flow. Our findings suggest that differential selection of natural or sexual environments is a prominent driver of species divergence in subtropical E. formosa and E. yayeyamana; whereas for tropical E. decorata and E. ornata at lower latitudes, allopatric isolation may well be a pivotal promoter of species formation.     

Evolution of premating reproductive isolation among conspecific populations of the sea rock-pool beetle Ochthebius urbanelliae driven by reinforcing natural selection

How natural selection might be involved in speciation remains a fundamental question in evolutionary biology. When two or more species co-occur in the same areas, natural selection may favor divergence in mating traits. By acting in sympatric but not allopatric populations, natural selection can also affect mate choice within species and ultimately initiate speciation among conspecific populations. Here, we address this potential effect in the sea rock-pool beetles Ochthebius quadricollis and O. urbanelliae. The two species, which inhabit the Mediterranean coasts, co-occurr syntopically in an area along the Italian Tyrrhenian coast and completed reproductive isolation by reinforcement. In this article, through mating trials under laboratory conditions between conspecific populations, we found in O. quadricollis no deviations from random mating. Conversely, in O. urbanelliae, we found a clear pattern of premating isolation between the reinforced populations sympatric with O. quadricollis and those nonreinforced allopatric. This pattern is consistent with the view that natural selection, which completed the reproductive isolation between the two species in sympatry, led incidentally also to partial premating reproductive isolation (I(PSI) estimator from 0.683 to 0.792) between conspecific populations of O. urbanelliae. This case study supports an until recently underappreciated role of natural selection resulting from species interactions in initiating speciation.     

Increased genetic divergence between two closely related fir species in areas of range overlap

Because of introgressive hybridization, closely related species can be more similar to each other in areas of range overlap (parapatry or sympatry) than in areas where they are geographically isolated from each other (allopatry). Here, we report the reverse situation based on nuclear genetic divergence between two fir species, Abies chensiensis and Abies fargesii, in China, at sites where they are parapatric relative to where they are allopatric. We examined genetic divergence across 126 amplified fragment length polymorphism (AFLP) markers in a set of 172 individuals sampled from both allopatric and parapatric populations of the two species. Our analyses demonstrated that AFLP divergence was much greater between the species when comparisons were made between parapatric populations than between allopatric populations. We suggest that selection in parapatry may have largely contributed to this increased divergence.     

Reproductive isolation via divergent genital morphology due to cascade reinforcement in Ohomopterus ground beetles

Secondary contact between incipient species and selection against maladaptive hybridization can drive reinforcement between populations in contact and result in reproductive character displacement (RCD). Resultant divergence in mating traits within a species may generate downstream reproductive isolation between populations with displaced and non-displaced traits, referred to as the cascade reinforcement hypothesis. We examined this hypothesis using three allopatric populations of the ground beetle Carabus maiyasanus with a genital lock-and-key system. This species shows RCD in male and female genital morphologies in populations in contact with the sister species C. iwawakianus. In a reciprocal mating experiment using three allopatric populations with differences in male and female genital sizes, insemination failure increased as the difference in genital size increased. Based on the reproductive isolation index, insemination failure was the major postmating-prezygotic isolation barrier, at least in one population pair with comparable total isolation to those of other species pairs. By contrast, there was only incomplete premating isolation among populations. These results suggest that RCD in genital morphologies drives incipient allopatric speciation, supporting the cascade reinforcement hypothesis. These findings provide insight into the roles of interspecific interactions and subsequent trait diversification in speciation processes.     

Phenotypic divergence but not genetic distance predicts assortative mating among species of a cichlid fish radiation

The hypothesis of ecological divergence giving rise to premating isolation in the face of gene flow is controversial. However, this may be an important mechanism to explain the rapid multiplication of species during adaptive radiation following the colonization of a new environment when geographical barriers to gene flow are largely absent but underutilized niche space is abundant. Using cichlid fish, we tested the prediction of ecological speciation that the strength of premating isolation among species is predicted by phenotypic rather than genetic distance. We conducted mate choice experiments between three closely related, sympatric species of a recent radiation in Lake Mweru (Zambia/DRC) that differ in habitat use and phenotype, and a distantly related population from Lake Bangweulu that resembles one of the species in Lake Mweru. We found significant assortative mating among all closely related, sympatric species that differed phenotypically, but none between the distantly related allopatric populations of more similar phenotype. Phenotypic distance between species was a good predictor of the strength of premating isolation, suggesting that assortative mating can evolve rapidly in association with ecological divergence during adaptive radiation. Our data also reveals that distantly related allopatric populations that have not diverged phenotypically, may hybridize when coming into secondary contact, e.g. upon river capture because of diversion of drainage systems.     

Asymmetric Response of Costa Rican White-Breasted Wood-Wrens (Henicorhina leucosticta) to Vocalizations from Allopatric Populations

Divergence in song between allopatric populations can contribute to premating reproductive isolation in territorial birds. Song divergence is typically measured by quantifying divergence in vocal traits using audio recordings, but field playback experiments provide a more direct way to behaviorally measure song divergence between allopatric populations. The White-breasted Wood-Wren (Henicorhina leucosticta; hereafter “WBWW”) is an abundant Neotropical species with four mitochondrial clades (in Central America, the Darién, the Chocó and the Amazon) that are deeply divergent (~5–16% sequence divergence). We assessed the possibility that the WBWW as currently defined may represent multiple biological species by conducting both statistical analysis of vocal characters and field playback experiments within three clades (Central America, Chocó and Amazon). Our analysis of vocal traits revealed that Central American songs overlapped in acoustic space with Chocó songs, indicating vocal similarity between these two populations, but that Central American songs were largely divergent from Amazonian songs. Playback experiments in the Caribbean lowlands of Costa Rica revealed that Central American WBWWs typically responded aggressively to songs from the Chocó population but did not respond to playback of songs from the Amazonian population, echoing the results of the vocal trait analysis. This marked difference in behavioral response demonstrates that the songs of Central American and Amazonian WBWWs (but not Central American and Chocó WBWWs) have diverged sufficiently that Central American WBWWs no longer recognize song from Amazonian WBWWs as a signal to elicit territorial defense. This suggests that significant premating reproductive isolation has evolved between these two populations, at least from the perspective of the Central American population, and is consistent with the possibility that Central American and Amazonian populations represent distinct biological species. We conclude by advocating for the further use of field playback experiments to assess premating reproductive isolation (and species limits) between allopatric songbird populations, a situation where behavioral systematics can answer questions that phylogenetic systematics cannot.     

Vicariance divergence and gene flow among islet populations of an endemic lizard

Allopatry and allopatric speciation can arise through two different mechanisms: vicariance or colonization through dispersal. Distinguishing between these different allopatric mechanisms is difficult and one of the major challenges in biogeographical research. Here, we address whether allopatric isolation in an endemic island lizard is the result of vicariance or dispersal. We estimated the amount and direction of gene flow during the divergence of isolated islet populations and subspecies of the endemic Skyros wall lizard Podarcis gaigeae, a phenotypically variable species that inhabits a major island and small islets in the Greek archipelago. We applied isolation-with-migration models to estimate population divergence times, population sizes and gene flow between islet-mainland population pairs. Divergence times were significantly correlated with independently estimated geological divergence times. This correlation strongly supports a vicariance scenario where islet populations have sequentially become isolated from the major island. We did not find evidence for significant gene flow within P. g. gaigeae. However, gene-flow estimates from the islet to the mainland populations were positively affected by islet area and negatively by distance between the islet and mainland. We also found evidence for gene flow from one subspecies (P. g. weigandi) into another (P. g. gaigeae), but not in the other direction. Ongoing gene flow between the subspecies suggests that even in this geographically allopatric scenario with the sea posing a strong barrier to dispersal, divergence with some gene flow is still feasible.     

Multiple speciation events in an arthropod with divergent evolution in sexual morphology

Sexual selection can facilitate divergent evolution of traits related to mating and consequently promote speciation. Theoretically, independent operation of sexual selection in different populations can lead to divergence of sexual traits among populations and result in allopatric speciation. Here, we show that divergent evolution in sexual morphology affecting mating compatibility (body size and genital morphologies) and speciation have occurred in a lineage of millipedes, the Parafontaria tonominea species complex. In this millipede group, male and female body and genital sizes exhibit marked, correlated divergence among populations, and the diverged morphologies result in mechanical reproductive isolation between sympatric species. The morphological divergence occurred among populations independently and without any correlation with climatic variables, although matching between sexes has been maintained, suggesting that morphological divergence was not a by-product of climatic adaptation. The diverged populations underwent restricted dispersal and secondary contact without hybridization. The extent of morphological difference between sympatric species is variable, as is diversity among allopatric populations; consequently, the species complex appears to contain many species. This millipede case suggests that sexual selection does contribute to species richness via morphological diversification when a lineage of organisms consists of highly divided populations owing to limited dispersal.     

Divergent host-plant use promotes reproductive isolation among cynipid gall wasp populations

Ecological speciation occurs when reproductive isolation evolves as a consequence of divergent natural selection among environments. A direct prediction of this process is that ecologically divergent pairs of populations will exhibit greater reproductive isolation than ecologically similar pairs of populations. By comparing allopatric populations of the cynipid gall wasp Belonocnema treatae infesting Quercus virginiana and Quercus geminata, we tested the role that divergent host use plays in generating ecological divergence and sexual isolation. We found differences in body size and gall structure associated with divergent host use, but no difference in neutral genetic divergence between populations on the same or different host plant. We observed significant assortative mating between populations from alternative host plants but not between allopatric populations on the same host plant. Thus, we provide evidence that divergent host use promotes speciation among gall wasp populations.     

Host-related genetic differentiation in the anther smut fungus Microbotryum violaceum in sympatric, parapatric and allopatric populations of two host species Silene latifolia and S. dioica

We investigated genetic diversity in West European populations of the fungal pathogen Microbotryum violaceum in sympatric, parapatric and allopatric populations of the host species Silene latifolia and S. dioica, using four polymorphic microsatellite loci. In allopatric host populations, the fungus was highly differentiated by host species, exhibiting high values of F(ST) and R(ST), and revealed clear and distinct host races. In sympatric and parapatric populations we found significant population differentiation as well, except for one sympatric population in which the two host species grew truly intermingled. The mean number of alleles per locus for isolates from each of the host species was significantly higher in sympatric/parapatric than in allopatric populations. This suggests that either gene flow between host races in sympatry, or in case of less neutral loci, selection in a more heterogeneous host environment can increase the level of genetic variation in each of the demes. The observed pattern of host-related genetic differentiation among these geographically spread populations suggest a long-term divergence between these host races. In sympatric host populations, both host races presumably come in secondary contact, and host-specific alleles are exchanged depending on the amount of fungal gene flow.     

The evolution of host preference in allopatric vs. parapatric populations of Timema cristinae walking-sticks

Divergent habitat preferences can contribute to speciation, as has been observed for host-plant preferences in phytophagous insects. Geographic variation in host preference can provide insight into the causes of preference evolution. For example, selection against maladaptive host-switching occurs only when multiple hosts are available in the local environment and can result in greater divergence in regions with multiple vs. a single host. Conversely, costs of finding a suitable host can select for preference even in populations using a single host. Some populations of Timema cristinae occur in regions with only one host-plant species present (in allopatry, surrounded by unsuitable hosts) whereas others occur in regions with two host-plant species adjacent to one another (in parapatry). Here, we use host choice and reciprocal-rearing experiments to document genetic divergence in host preference among 33 populations of T. cristinae. Populations feeding on Ceanothus exhibited a stronger preference for Ceanothus than did populations feeding on Adenostoma. Both allopatric and parapatric pairs of populations using the different hosts exhibited divergent host preferences, but the degree of divergence tended to be greater between allopatric pairs. Thus, gene flow between parapatric populations apparently constrains divergence. Host preferences led to levels of premating isolation between populations using alternate hosts that were comparable in magnitude to previously documented premating isolation caused by natural and sexual selection against migrants between hosts. Our findings demonstrate how gene flow and different forms of selection interact to determine the magnitude of reproductive isolation observed in nature.     

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.     

Microgeographic evolution of snail shell shape and predator behavior

Genetic divergence in geographically isolated populations is a prerequisite for allopatric speciation, one of the most common modes of speciation. In ecologically equivalent populations existing within a small, environmentally homogeneous area, an important role for environmentally neutral divergence is often found or inferred. We studied a species complex of conspicuously shaped Opisthostoma land snails on scattered limestone outcrops within a small area of lowland rainforest in Borneo. We used shell morphometrics, mitochondrial and nuclear DNA sequences, and marks of predation to study the factors involved in allopatric divergence. We found that a striking geographic divergence exists in shell morphology, which is partly associated with neutral genetic divergence. We also found geographic differentiation in the behavior of the snails' invertebrate predator and evidence of an evolutionary interaction between aspects of shell shape and predator behavior. Our study shows that adaptation to biotic aspects of the environment may play a more important role in allopatric speciation than previously suspected, even on a geographically very small scale.     

Pleistocene divergence of Dinaric Drusus endemics (Trichoptera, Limnephilidae) in multiple microrefugia within the Balkan Peninsula

The Balkan Peninsula is one of three major European refugial areas. It has high biodiversity and endemism, but data on the age and origin of its fauna, especially endemics, are limited. Mitochondrial sequence data (COI and 16S genes) were used to study the population structure and phylogeography of the caddisfly Drusus croaticus and the phylogeny and divergence of seven other Drusus species, mostly range-restricted endemics of the Dinaric region of the Balkan Peninsula. The divergence of D. croaticus populations in Croatia and allopatric Drusus species in Bosnia dated to the Pleistocene, showing the importance of this time period for the origin and diversification of Balkan endemic taxa. The divergence of more distantly related species dated to the Late Miocene/Early Pliocene. Population genetic and phylogeographic analysis of 115 individuals from 11 populations of D. croaticus revealed a high level of genetic differentiation and absence of gene flow between populations separated by more than 10 km. The existence of allopatrically fragmented lineages in D. croaticus and the endemic Bosnian species is most likely the result of long-term isolation in multiple microrefugia, probably due to the specific habitat requirements and life-history traits of Drusinae coupled with the topographic complexity and historical changes in geomorphology of the region. Overall, these findings shed light on the processes generating the high genetic complexity of this refugial region that parallels the 'refugia within refugia' pattern widely reported from the Iberian refugium.     

Existence of a pattern of reproductive character displacement in Homobasidiomycota but not in Ascomycota

Generally, stronger reproductive isolation is expected between sympatric than between allopatric sibling species. Such reproductive character displacement should predominantly affect premating reproductive isolation and can be due to several mechanisms, including population extinction, fusion of insufficiently isolated incipient species and reinforcement of reproductive isolation in response to low hybrid fitness. Experimental data on several taxa have confirmed these theoretical expectations on reproductive character displacement, but they are restricted to animals and a few plants. Using results reported in the literature on crossing experiments in fungi, we compared the degree and the nature of reproductive isolation between allopatric and sympatric species pairs. In accordance with theoretical expectations, we found a pattern of enhanced premating isolation among sympatric sibling species in Homobasidiomycota. By contrast, we did not find evidence for reproductive character displacement in Ascomycota at similar genetic distances. Both allopatric and sympatric species of Ascomycota had similarly low levels of reproductive isolation, being mostly post-zygotic. This suggests that some phylogeny-dependent life-history trait may strongly influence the evolution of reproductive isolation between closely related species. A significant correlation was found between degree of reproductive isolation and genetic divergence among allopatric species of Homobasidiomycota, but not among sympatric ones or among Ascomycota species.     

Adaptive gamete-recognition divergence in a hybridizing Mytilus population

Gamete-recognition proteins often evolve rapidly, but it is not known if their divergence occurs within species and corresponds with the evolution of reproductive isolation, or if divergence typically accumulates between already isolated lineages. We examined the evolution of a candidate gamete-recognition protein in several sympatric and allopatric populations of Mytilus blue mussels, species that hybridize in nature. Within a single species, Mytilus galloprovincialis, we found adaptive divergence of Lysin-M7, a sperm acrosomal protein that dissolves the egg vitelline envelope during fertilization. Mytilus galloprovincialis Lysin-M7 alleles group into two distinct clades (termed G and G(D)), and individual alleles in these clades are separated from each other by at least three and up to eleven amino-acid substitutions. Maximum-likelihood estimates of selective pressure (dN/dS =omega) implicate selection in the divergence between M. galloprovincialis Lysin-M7 clades, and within the G(D) clade. Exact tests of population differentiation indicate that the relative frequency of G and G(D) Lysin-M7 alleles differs significantly among M. galloprovincialis populations. Compared with allopatric Mediterranean samples, Lysin-M7 alleles in the G(D) clade are found at elevated frequency in samples from the East Atlantic and California, areas of secondary contact and hybridization between Mytilus species, and Australia, an area of unknown species composition. Adaptive divergence between the alleles most common in allopatry and those found at elevated frequency in samples from sympatry suggests that selection pressures acting in hybridizing populations, likely following Pleistocene secondary contact with M. edulis in the East Atlantic, drove the divergence of Lysin-M7 in M. galloprovincialis.     

Testing alternative mechanisms of evolutionary divergence in an African rain forest passerine bird

Abstract Models of speciation in African rain forests have stressed either the role of isolation or ecological gradients. Here we contrast patterns of morphological and genetic divergence in parapatric and allopatric populations of the Little Greenbul, Andropadus virens, within different and similar habitats. We sampled 263 individuals from 18 sites and four different habitat types in Upper and Lower Guinea. We show that despite relatively high rates of gene flow among populations, A. virens has undergone significant morphological divergence across the savanna-forest ecotone and mountain-forest boundaries. These data support a central component of the divergence-with-gene-flow model of speciation by suggesting that despite large amounts of gene flow, selection is sufficiently intense to cause morphological divergence. Despite evidence of isolation based on neutral genetic markers, we find little evidence of morphological divergence in fitness-related traits between hypothesized refugial areas. Although genetic evidence suggests populations in Upper and Lower Guinea have been isolated for over 2 million years, morphological divergence appears to be driven more by habitat differences than geographic isolation and suggests that selection in parapatry may be more important than geographic isolation in causing adaptive divergence in morphology.     

Reproductive isolation in natural populations of Drosophila melanogaster from Brazzaville (Congo)

The aim of this work is to analyze the homogamy previously detected between two natural populations of Drosophila melanogaster from Brazzaville. It is shown that mating isolation was still maintained under laboratory conditions 10 years after the populations samples were trapped. Isolation seemed to be due mainly to pre-mating isolation and we checked for any suggestion of post-mating mortality of hybrids. Pre-mating isolation was not symmetrical, and significant ² values were found in 3/4 possible 3-way mating choice experiments. The only exception involved a male from the countryside and two females (one from each population) for which no significant mating preference was detected. Mortality of hybrids was intermediate between those of the parental strains showing a clear maternal effect and the existence of partial dominance. Major differences in the cuticular hydrocarbons were also found and they could account for the isolation. These findings in populations from African breweries indicate that they are closely related to European ones, suggesting that this phenomenon is not a case of sympatric speciation, but probably attributable to the reintroduction of an allopatric population.