Allopatric Species Pairs Research Articles

Comparative analysis reveals assortative mate preferences in darters independent of sympatry and sex.

A preference for mating with conspecifics over heterospecifics is fundamental to the maintenance of species diversity in sexually reproducing organisms. This type of positive assortative preference results in sexual isolation, and a reduction in gene flow between species due to differences in mate choice. The proximate and ultimate causes of sexual isolation therefore constitute active areas of research in evolutionary biology. Sexual isolation is often stronger between closely related sympatric species as compared to allopatric species because of processes such as reinforcement. In addition, traditional theories of sexual selection suggest that because reproduction is more costly to females, they should be the choosier sex and play a more central role in sexual isolation. We conducted a comparative analysis of assortative mate preferences in males and females of sympatric and allopatric species pairs of darters (fish genus Etheostoma). We performed a meta-analysis of 17 studies, encompassing 21 species, in which assortative preference was measured when fish were (in most cases) allowed only visual information. As expected, we found stronger preferences for conspecifics over heterospecifics across studies and species. However, we did not find an effect of sympatry or sex on the strength of preference for conspecifics, but rather remarkable variation across species. We offer several testable hypotheses to explain the variation we observed in the strength of assortative preference.

The role of divergent host use and geography in the evolution of habitat isolation and sexual isolation among sister species of Belonocnema gall wasps.

Ecology and geography can play important roles in the evolution of reproductive isolation across the speciation continuum, but few studies address both at the later stages of speciation. This notable gap in knowledge arises from the fact that traditional ecological speciation studies have predominantly focused on the role of ecology in initiating the speciation process, while many studies exploring the effect of geography (e.g., reinforcement) concentrate on species pairs that lack divergent ecological characteristics. We simultaneously examine the strength of habitat isolation and sexual isolation among three closely related species of Belonocnema gall-forming wasps on two species of live oaks, Quercus virginiana and Q. geminata, that experience divergent selection from their host plants and variable rates of migration due to their geographic context. We find that the strength of both habitat isolation and sexual isolation is lowest among allopatric species pairs with the same host plant association, followed by allopatric species with different host plant associations, and highest between sympatric species with different host-plant associations. This pattern suggests that divergent selection due to different host use interacts with geography in the evolution of habitat isolation and sexual isolation during the later stages of speciation of Belonocnema wasps.

Convergence in sympatric swallowtail butterflies reveals ecological interactions as a key driver of worldwide trait diversification

Ecological interactions can promote phenotypic diversification in sympatric species. While competition can enhance trait divergence, other ecological interactions may promote convergence in sympatric species. Within butterflies, evolutionary convergences in wing color patterns have been reported between distantly related species, especially in females of palatable species, where mimetic color patterns are promoted by predator communities shared with defended species living in sympatry. Wing color patterns are also often involved in species recognition in butterflies, and divergence in this trait has been reported in closely related species living in sympatry as a result of reproductive character displacement. Here, we investigate the effect of sympatry between species on the convergence vs. divergence of their wing color patterns in relation to phylogenetic distance, focusing on the iconic swallowtail butterflies (family Papilionidae). We developed an unsupervised machine learning-based method to estimate phenotypic distances among wing color patterns of 337 species, enabling us to finely quantify morphological diversity at the global scale among species and allowing us to compute pairwise phenotypic distances between sympatric and allopatric species pairs. We found phenotypic convergence in sympatry, stronger among distantly related species, while divergence was weaker and restricted to closely related males. The convergence was stronger among females than males, suggesting that differential selective pressures acting on the two sexes drove sexual dimorphism. Our results highlight the significant effect of ecological interactions driven by predation pressures on trait diversification in Papilionidae and provide evidence for the interaction between phylogenetic proximity and ecological interactions in sympatry, acting on macroevolutionary patterns of phenotypic diversification.

Genomics reveals broad hybridization in deeply divergent Palearctic grass and water snakes (Natrix spp.)

Understanding speciation is one of the cornerstones of biological diversity research. Currently, speciation is often understood as a continuous process of divergence that continues until genetic or other incompatibilities minimize or prevent interbreeding. The Palearctic snake genus Natrix is an ideal group to study speciation, as it comprises taxa representing distinct stages of the speciation process, ranging from widely interbreeding parapatric taxa through parapatric species with very limited gene flow in narrow hybrid zones to widely sympatric species. To understand the evolution of reproductive isolation through time, we have sequenced the genomes of all five species within this genus and two additional subspecies. We used both long-read and short-read methods to sequence and de-novo-assemble two high-quality genomes (Natrix h. helvetica, Natrix n. natrix) to their 1.7 Gb length with a contig N50 of 4.6 Mbp and 1.5 Mbp, respectively, and used these as references to assemble the remaining short-read-based genomes. Our phylogenomic analyses yielded a well-supported dated phylogeny and evidence for a surprisingly complex history of interspecific gene flow, including between widely sympatric species. Furthermore, evidence for gene flow was also found for currently allopatric species pairs. Genetic exchange among these well-defined, distinct, and several million-year-old reptile species emphasizes that speciation and maintenance of species distinctness can occur despite continued genetic exchange.

Limited dispersal and local adaptation promote allopatric speciation in a biodiversity hotspot.

Recently diverged or diverging populations can offer unobstructed insights into early barriers to gene flow during the initial stages of speciation. The current study utilised a novel insect system (order Mantophasmatodea) to shed light on the early drivers of speciation. The members of this group have limited dispersal abilities, small allopatric distributions and strong habitat associations in the Cape Floristic Region biodiversity hotspot in South Africa. Sister taxa from the diverse family Austrophasmatidae were chosen as focal species (Karoophasma biedouwense, K.botterkloofense). Population genetics and Generalized Dissimilarity Modelling (GDM) were used to characterise spatial patterns of genetic variation and evaluate the contribution of environmental factors to population divergence and speciation. Extensive sampling confirmed the suspected allopatry of these taxa. However, hybrids were identified in a narrow region occurring between the species' distributions. Strong population structure was found over short geographic distances; particularly in K.biedouwense in which geographic distance accounted for 32% of genetic variation over a scale of 50km (r=.56, p<.001). GDM explained 42%-78% of the deviance in observed genetic dissimilarities. Geographic distance was consistently indicated to be important for between species and within population differentiation, suggesting that limited dispersal ability may be an important neutral driver of divergence. Temperature, altitude, precipitation and vegetation were also indicated as important factors, suggesting the possible role of adaptation to local environmental conditions for species divergence. The discovery of the hybrid-zone, and the multiple allopatric species pairs in Austrophasmatidae support the idea that this could be a promising group to further our understanding of speciation modes.

Sympatry drives colour and song evolution in wood-warblers (Parulidae).

Closely related species often exhibit similarities in appearance and behaviour, yet when related species exist in sympatry, signals may diverge to enhance species recognition. Prior comparative studies provided mixed support for this hypothesis, but the relationship between sympatry and signal divergence is likely nonlinear. Constraints on signal diversity may limit signal divergence, especially when large numbers of species are sympatric. We tested the effect of sympatric overlap on plumage colour and song divergence in wood-warblers (Parulidae), a speciose group with diverse visual and vocal signals. We also tested how number of sympatric species influences signal divergence. Allopatric species pairs had overall greater plumage and song divergence compared to sympatric species pairs. However, among sympatric species pairs, plumage divergence positively related to the degree of sympatric overlap in males and females, while male song bandwidth and syllable rate divergence negatively related to sympatric overlap. In addition, as the number of species in sympatry increased, average signal divergence among sympatric species decreased, which is likely due to constraints on warbler perceptual space and signal diversity. Our findings reveal that sympatry influences signal evolution in warblers, though not always as predicted, and that number of sympatric species can limit sympatry's influence on signal evolution.

Description and comparison of Philippine hornbill (Bucerotidae) vocalizations.

The role of vocalisation for the Philippine hornbills' ecology and speciation and their implication in understanding speciation is not well understood. We described and compared recorded calls of seven hornbill taxa in captivity namely Mindanao Wrinkled hornbill (Rhabdotorrhinus leucocephalus), Rufous-headed hornbill (Rhabdotorrhinus waldeni), Luzon Rufous hornbill (Buceros hydrocorax hydrocorax), Samar Rufous hornbill (Buceros hydrocorax semigaleatus), Mindanao Rufous hornbill (Buceros hydrocorax mindanensis), Mindanao Tarictic hornbill (Penelopides affinis), Samar Tarictic hornbill (Penelopides samarensis), Visayan Tarictic hornbill (Penelopides panini) and Luzon Tarictic hornbill (Penelopides manillae), as well as comparison with the non-native Papuan hornbill (Rhyticeros plicatus). Vocalisation analysis included call duration, minimum frequency, maximum frequency, bandwidth and peak frequency. For each species in the sample, the mean and standard deviation were used to calculate the Cohen’s d statistic by using an effect size calculator. Results showed that the effect size for minimum frequency was small for P. panini vs. P. samarensis and B. hydrocorax vs. B. h. mindanensis. However, bandwidth, duration, minimum frequency, maximum frequency and peak frequency have large effect sizes for the rest of the allopatric species pairs. Hornbills' conspicuous resonating calls are sufficiently quantifiable for bioacoustic analysis and may provide new insights for their taxonomic review.

The evolution of floral signals in relation to range overlap in a clade of California Jewelflowers (Streptanthus s.l.).

Because of their function as reproductive signals in plants, floral traits experience distinct selective pressures related to their role in speciation, reinforcement, and prolonged coexistence with close relatives. However, few studies have investigated whether population-level processes translate into detectable signatures at the macroevolutionary scale. Here, we ask whether patterns of floral trait evolution and range overlap across a clade of California Jewelflowers reflect processes hypothesized to shape floral signal differentiation at the population level. We found a pattern of divergence in floral scent composition across the clade such that close relatives had highly disparate floral scents given their age. Accounting for range overlap with close relatives explained additional variation in floral scent over time, with sympatric species pairs having diverged more than allopatric species pairs given their age. However, three other floral traits (flower size, scent complexity and flower color) did not fit these patterns, failing to deviate from a null Brownian motion model of evolution. Together, our results suggest that selection for divergence among close relatives in the composition of floral scents may play a key, sustained role in mediating speciation and coexistence dynamics across this group, and that signatures of these dynamics may persist at the macroevolutionary scale.

Factors contributing to the accumulation of reproductive isolation: A mixed model approach.

The analysis of large datasets describing reproductive isolation between species has been extremely influential in the study of speciation. However, the statistical methods currently used for these data limit the ability to make direct inferences about the factors predicting the evolution of reproductive isolation. As a result, our understanding of iconic patterns and rules of speciation rely on indirect analyses that have clear statistical limitations. Phylogenetic mixed models are commonly used in ecology and evolution, but have not been applied to studies of reproductive isolation. Here I describe a flexible framework using phylogenetic mixed models to analyze data collected at different evolutionary scales, to test both categorical and continuous predictor variables, and to test the effect of multiple predictors on rates and patterns of reproductive isolation simultaneously. I demonstrate the utility of this framework by re‐analyzing four classic datasets, from both animals and plants, and evaluating several hypotheses that could not be tested in the original studies: In the Drosophila and Bufonidae datasets, I found support for more rapid accumulation of reproductive isolation in sympatric species pairs compared to allopatric species pairs. Using Silene and Nolana, I found no evidence supporting the hypothesis that floral differentiation elevates postzygotic reproductive isolation. The faster accumulation of postzygotic isolation in sympatry is likely the result of species coexistence determined by the level of postzygotic isolation between species. In addition, floral trait divergence does not appear to translate into pleiotropic effects on postzygotic reproductive isolation. Overall, these methods can allow researchers to test new hypotheses using a single statistical method, while remedying the statistical limitations of several previous methods.

Gene flow, ancient polymorphism, and ecological adaptation shape the genomic landscape of divergence among Darwin's finches.

Genomic comparisons of closely related species have identified “islands” of locally elevated sequence divergence. Genomic islands may contain functional variants involved in local adaptation or reproductive isolation and may therefore play an important role in the speciation process. However, genomic islands can also arise through evolutionary processes unrelated to speciation, and examination of their properties can illuminate how new species evolve. Here, we performed scans for regions of high relative divergence (FST) in 12 species pairs of Darwin's finches at different genetic distances. In each pair, we identify genomic islands that are, on average, elevated in both relative divergence (FST) and absolute divergence (dXY). This signal indicates that haplotypes within these genomic regions became isolated from each other earlier than the rest of the genome. Interestingly, similar numbers of genomic islands of elevated dXY are observed in sympatric and allopatric species pairs, suggesting that recent gene flow is not a major factor in their formation. We find that two of the most pronounced genomic islands contain the ALX1 and HMGA2 loci, which are associated with variation in beak shape and size, respectively, suggesting that they are involved in ecological adaptation. A subset of genomic island regions, including these loci, appears to represent anciently diverged haplotypes that evolved early during the radiation of Darwin's finches. Comparative genomics data indicate that these loci, and genomic islands in general, have exceptionally low recombination rates, which may play a role in their establishment.

Divergence at the edges: peripatric isolation in the montane spiny throated reed frog complex.

BackgroundPeripatric speciation and peripheral isolation have uncertain importance in species accumulation, and are largely overshadowed by assumed dominance of allopatric modes of speciation. Understanding the role of different speciation mechanisms within biodiversity hotspots is central to understanding the generation of biological diversity. Here, we use a phylogeographic analysis of the spiny-throated reed frogs and examine sister pairings with unbalanced current distributional ranges for characteristics of peripatric speciation. We further investigate whether forest/grassland mosaic adapted species are more likely created through peripatric speciation due to instability of this habitat type.ResultsWe reconstructed a multi-locus molecular phylogeny of spiny-throated reed frogs which we then combined with comparative morphologic data to delimit species and analyze historical demographic change; identifying three new species. Three potential peripatric speciation events were identified along with one case of allopatric speciation. Peripatric speciation is supported through uneven potential and realized distributions and uneven population size estimates based on field collections. An associated climate shift was observed in most potentially peripatric splits. Morphological variation was highest in sexually dimorphic traits such as body size and gular shape, but this variation was not limited to peripatric species pairs as hypothesized. The potentially allopatric species pair showed no niche shifts and equivalent effective population sizes, ruling out peripatry in that speciation event. Two major ecological niche shifts were recovered within this radiation, possibly as adaptations to occupy areas of grassland that became more prevalent in the last 5 million years. Restricted and fluctuating grassland mosaics within forests might promote peripatric speciation in the Eastern Arc Biodiversity Hotspot (EABH).ConclusionsIn our case study, peripatric speciation appears to be an important driver of diversity within the EABH biodiversity hotspot, implying it could be a significant speciation mechanism in highly fragmented ecosystems. Extensive peripatric speciation in this montane archipelago may explain the abundance of discrete lineages within the limited area of the EABH, as inferred in remote island archipelagos. Future phylogenetic studies incorporating demographic and spatial analyses will clarify the role of peripatric speciation in creating biodiversity hotspots.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0384-3) contains supplementary material, which is available to authorized users.

Obligatory duetting behaviour in theChrysoperla carnea‐group of cryptic species (Neuroptera:Chrysopidae): its role in shaping evolutionary history

An unusual system of communication has evolved in green lacewings of the Chrysoperla carnea-group, triggering rapid proliferation of numerous cryptic species across all of the Northern Hemisphere and large portions of Africa. The system is based on sexually monomorphic, substrate-borne vibrational signals, produced by abdominal oscillation. These low-frequency signals are exchanged between courting individuals in a precise duetting format. The song of each of the more than 20 described species exhibits a unique acoustical phenotype that reproductively isolates the taxon from all other species with which it might come into contact. Here, we review what is known about duetting behaviour in the carnea-group, emphasizing the dominant role that duetting has played in the evolution, ecology, and speciation of the complex. Included are descriptions and discussions of song diversity and its impact on reproductive isolation among species, the genetic basis of interspecific song differences, partitioning of acoustic space among sympatric species, parallel song evolution in allopatric species pairs, and modes of speciation within the complex. We also emphasize the importance of correctly identifying by song all species of the carnea-group that are to be used either in experimental studies or programs of biological control, while acknowledging the continuing relevance of morphology to carnea-group systematics.

Genital divergence in sympatric sister snails

A pattern of greater divergence in mating traits between sister-species pairs with overlapping ranges than between allopatric species pairs is expected if reinforcement commonly contributes to speciation. Few large-scale comparative analyses have addressed this prediction, especially for genital form. Here, we show that penial morphology follows the predicted pattern in 40 robustly identified sister-species pairs in the marine gastropod subfamily Littorininae. Further work is needed to exclude other processes that may contribute to genital divergence between sympatric species, but the clear pattern we observe strongly suggests a role for genital form in reproductive isolation in this large clade.

Patterns and processes in the evolutionary history of parrotfishes (Family Labridae)

Phylogenetic reconstruction of the evolutionary relationships among 61 of the 70 species of the parrotfish genera Chlorurus and Scarus (Family Labridae) based on mitochondrial and nuclear gene sequences retrieved 15 well-supported clades with mid Pliocene/Pleistocene diversification. Twenty-two reciprocally monophyletic sister-species pairs were identified: 64% were allopatric, and the remainder were sympatric. Age of divergence was similar for allopatric and sympatric species pairs. Sympatric sister pairs displayed greater divergence in morphology, ecology, and sexually dimorphic colour patterns than did allopatric pairs, suggesting that both genetic drift in allopatric species pairs and ecologically adaptive divergence between members of sympatric pairs have played a role in diversification. Basal species typically have small geographical ranges and are restricted to geographically and ecologically peripheral reef habitats. We found little evidence that a single dominant process has driven diversification, nor did we detect a pattern of discrete, sequential stages of diversification in relation to habitat, ecology, and reproductive biology. The evolution of Chlorurus and Scarus has been complex, involving a number of speciation processes.

Relationship between genetic distances and postzygotic reproductive isolation in diploid Fragaria (Rosaceae)

Studies of reproductive isolation are important for understanding speciation and species delimitation. We studied seven components of reproductive isolation at different taxonomic and geographical levels and their relationship with genetic distance in diploid Fragaria. Isolation was only evident between different taxa at later stages (post F1 survivorship), and not at the earlier ones of fruit and seed set, F1 seed germination, and survivorship. Within Fragaria vesca, isolation at the later stages was positively correlated with genetic distance, although this was not found at interspecific levels. The lack of isolation between F. vesca and the other species at the early stages provide the chance for hybrid formation, although high levels of infertility expressed among the offspring can lead to potentially opposing evolutionary outcomes. Within F. vesca, there was evidence of unexpected isolation promoting evolutionary diversification and incipient speciation. Reproductive isolation values between sympatric and allopatric species pairs were similar and indicate a lack of reinforcement in Fragaria. This, combined with the lack of correlation between genetic distance and isolation at interspecific levels, shows that genetic distance is not always proportionally correlated with the degree of isolation in plants and suggests that evolutionary processes may be playing differently in plants compared to animals. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 510–526.

PERIPATRIC SPECIATION DRIVES DIVERSIFICATION AND DISTRIBUTIONAL PATTERN OF REEF HERMIT CRABS (DECAPODA: DIOGENIDAE:CALCINUS)

The diversity on coral reefs has long captivated observers. We examine the mechanisms of speciation, role of ecology in speciation, and patterns of species distribution in a typical reef-associated clade-the diverse and colorful Calcinus hermit crabs-to address the origin of tropical marine diversity. We sequenced COI, 16S, and H3 gene regions for approximately 90% of 56 putative species, including nine undescribed, "cryptic" taxa, and mapped their distributions. Speciation in Calcinus is largely peripatric at remote locations. Allopatric species pairs are younger than sympatric ones, and molecular clock analyses suggest that >2 million years are needed for secondary sympatry. Substantial niche conservatism is evident within clades, as well as a few major ecological shifts between sister species. Color patterns follow species boundaries and evolve rapidly, suggesting a role in species recognition. Most species prefer and several are restricted to oceanic areas, suggesting great dispersal abilities and giving rise to an ocean-centric diversity pattern. Calcinus diversity patterns are atypical in that the diversity peaks in the west-central oceanic Pacific rather than in the Indo-Malayan "diversity center." Calcinus speciation patterns do not match well-worn models put forth to explain the origin of Indo-West Pacific diversity, but underscore the complexity of marine diversification.

Existence of a pattern of reproductive character displacement inHomobasidiomycotabut not inAscomycota

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.

Threatened fishes of the world: Alosa alabamae (Jordan and Evermann, 1896) (Clupeidae)

Common names: Alabama shad, Gulf shad. Conservation status: US National Marine Fisheries Service Species of Concern, AFS: Vulnerable; IUCN: Endangered. Identification: Upper jaw median notch, protruding lower jaw, one row tongue teeth, 41–48 rakers on lower anterior gill arch; no dorsal filament. Silver with black spots on lower jaw; greenish-blue dorsally. Females 36–46 cm, males 9–42 cm SL. Distribution: Euryhaline, anadromous, schooling; spawns in medium to large rivers from the Mississippi River drainage to the Suwannee River, Florida (Mettee and O’Neil 2003); extirpated from at least 8 of the 14 native states (Mettee and O’Neil 2003). Abundance: Once commerically harvested, now rare (Mettee and O’Neil 2003); largest remaining population occurs in the Apalachicola River, Florida (Barkuloo et al. 1993). Habitat and ecology: Juveniles are found in rivers for their first 6–8 months, eat fishes and invertebrates. Diurnally, small juveniles use sandbars, then switch to open channels and steep bank habitat; they select cooler temperatures (Mickle 2006). Little is known of their ecology in marine environments. Juveniles enter the Gulf of Mexico from late summer to early winter. Spawning starts primarily at age 2 and live 6 years (Mettee and O’Neil 2003). Reproduction: Broadcast spawn spring/early summer at 18–238C over coarse sand and gravel in moderate currents; no foraging during spawning (Mills 1972). Arrival time varies by sex and age (Mette and O’Neil 2003), return to sea after spawning (Barkuloo et al. 1993). Some homing and genetic differences among drainages (Bowen 2005). Threats: Dams block spawning access and alter hydrology and substrates (Adams et al. 2000; Mettee and O’Neil 2003); poor water quality, siltation, altered habitat and thermal regimes, dredging, and perhaps bycatch in marine fisheries. Conservation recommendations: Implement effective fish passage; restore hydrologic regimes. Research into the marine phase including migrations, feeding and bycatch; and spawning, rearing, and other habitat needs. Remarks: Allopatric species pair with A. sapidissima. D. W. Meadows (&) Office of Protected Resources F/PR3, National Marine Fisheries Service, 1315 East West Highway, Silver Spring, MD 20910, USA e-mail: Dwayne.Meadows@noaa.gov

The genetics of hybrid male sterility between the allopatric species pair Drosophila persimilis and D. pseudoobscura bogotana: dominant sterility alleles in collinear autosomal regions.

F(1) hybrid male sterility is thought to result from interactions between loci on the X chromosome and dominant-acting loci on the autosomes. While X-linked loci that contribute to hybrid male sterility have been precisely localized in many animal taxa, their dominant autosomal interactors have been more difficult to localize precisely and/or have been shown to be of relatively smaller effect. Here, we identified and mapped at least four dominant autosomal factors contributing to hybrid male sterility in the allopatric species pair Drosophila persimilis and D. pseudoobscura bogotana. Using these results, we tested predictions of reduced recombination models of speciation. Consistent with these models, three of the four QTL associated with hybrid male sterility occur in collinear (uninverted) regions of these genomes. Furthermore, these QTL do not contribute significantly to hybrid male sterility in crosses between the sympatric species D. persimilis and D. pseudoobscura pseudoobscura. The autosomal loci identified in this study provide the basis for introgression mapping and, ultimately, for molecular cloning of interacting genes that contribute to F(1) hybrid sterility.

Premating, Not Postmating, Barriers Drive Genetic Dynamics in Experimental Hybrid Populations of the Endangered Sonoran Topminnow

The timing and pattern of reproductive barrier formation in allopatric populations has received much less attention than the accumulation of reproductive barriers in sympatry. The theory of allopatric speciation suggests that reproductive barriers evolve simply as by-products of overall genetic divergence. However, observations of enhanced premating barriers in allopatric populations suggest that sexual selection driven by intraspecific competition for mates may enhance species-specific signals and accelerate the speciation process. In a previous series of laboratory trials, we examined the strength of premating and postmating barriers in an allopatric species pair of the endangered Sonoran topminnow, Poeciliopsis occidentalis and P. sonoriensis. Behavioral observations provided evidence of asymmetrical assortative mating, while reduced brood sizes and male-biased F(1) sex ratios suggest postmating incompatibilities. Here we examine the combined effects of premating and postmating barriers on the genetic makeup of mixed populations, using cytonuclear genotype frequencies of first- and second-generation offspring. Observed genotype frequencies strongly reflect the directional assortative mating observed in behavioral trials, illustrating how isolating barriers that act earlier in the reproductive cycle will have a greater effect on total reproductive isolation and may be more important to speciation than subsequent postmating reproductive barriers.