Divergent outcomes of reinforcement speciation: the relative importance of assortative mating and migration modification

Most studies of reinforcement speciation focus on the evolution of assortative mating, but R. A. Fisher argued that migration modification is likely to be a common alternative mechanism. Despite previous models showing that assortative mating and migration modification may both be involved in reinforcement, no one has determined their relative evolutionary importance. This is surprising because understanding the biological conditions favoring these mechanisms may explain why certain pairs of species exhibit abutting, nonoverlapping geographical ranges with habitat fidelity while other pairs coexist in sympatry with sexual isolation. In this article, we explicitly model the evolution of both mechanisms simultaneously. First, we explore how these mechanisms differ in their evolutionary dynamics. Second, we ask how they affect each other's evolution and whether the interaction alters their relative importance in reinforcement. Our results reveal that assortative mating may evolve faster and under a broader range of biological conditions than migration modification. However, direct evolutionary interactions favor migration modification when populations experience strong divergent selection. Depending on the nature of postmating isolation, these mechanisms may either interfere with each other's evolution or coevolve in the same system. These results illustrate the importance of studying multiple mechanisms of speciation simultaneously in future speciation models.

Species delimitation in native South American fire ants

The taxonomy of fire ants has been plagued by difficulties in recognizing species on the basis of morphological characters. We surveyed allozyme markers and sequences of the mtDNA COI gene in several closely related nominal species from two areas of sympatry in the native ranges to learn whether the morphology-based delimitation of these species is supported by genetic data. We found that Solenopsis invicta and Solenopsis richteri, pest species whose distinctiveness has been debated, appear to be fully reproductively isolated at both study sites. This isolation contrasts with the extensive hybridization occurring between them in the USA, where both have been introduced. We also found strong genetic differentiation consistent with barriers to gene flow between Solenopsis quinquecuspis and the other two species. However, several lines of evidence suggest that nuclear and mitochondrial genes of S. invicta and S. richteri are introgressing into S. quinquecuspis. The latter apparently is a recently derived member of the clade that includes all three species, suggesting that there has been insufficient time for its full development of intrinsic isolating mechanisms. Finally, our discovery of genetically distinct populations within both S. invicta and S. richteri suggests the presence of previously unrecognized (cryptic) species. Their existence, together with the difficulties in developing diagnostic morphological characters for described species, imply that the group is actively radiating species and that morphological divergence generally does not keep pace with the development of reproductive isolation and neutral genetic divergence in this process.

Interspecies mating in sympatric species of Syngnathus pipefish

Hybridization is thought to be an important source of novel genetic variation, and interspecific hybridization may increase the adaptive potential of wild populations. While hybridization has not been previously reported in syngnathid fishes (seahorses and pipefish), the sympatric occurrence of closely related species at high densities increases the probability of interspecies mating in this group. Southern California is home to five species of Syngnathus pipefish, and these species frequently co-occur in near-shore eelgrass beds along the California coast. Recent work has identified exceptionally high levels of genetic diversity in southern populations of Syngnathus leptorhynchus, a widespread species which ranges from Mexico to Alaska. Microsatellite genotyping and mitochondrial sequence data are used here to study the population genetics of S. leptorhynchus and S. auliscus at a site in San Diego Bay where they are found to co-occur at high densities. While no adult hybrids were detected in the study population, analysis of male broods indicates that interspecies mating is occurring between the two species. The lack of premating isolating mechanisms between these two relatives suggests that hybridization may be common in sympatric species of Syngnathus.

Evidence for a One-Allele Assortative Mating Locus

Theoretical models have shown that speciation with gene flow can occur readily via a "one-allele mechanism," where the spread of the same allele within both of two diverging species reduces their subsequent hybridization. Here we present direct genetic evidence for such an allele in Drosophila pseudoobscura. Alleles conferring high or low assortative mating in D. pseudoobscura produce the same effects when inserted into D. persimilis. This observation suggests that the type of genetic variation that is most conducive to controversial modes of speciation with gene flow, such as reinforcement or sympatric speciation, is present in nature.

Reinforcement drives rapid allopatric speciation

Allopatric speciation results from geographic isolation between populations. In the absence of gene flow, reproductive isolation arises gradually and incidentally as a result of mutation, genetic drift and the indirect effects of natural selection driving local adaptation. In contrast, speciation by reinforcement is driven directly by natural selection against maladaptive hybridization. This gives individuals that choose the traits of their own lineage greater fitness, potentially leading to rapid speciation between the lineages. Reinforcing natural selection on a population of one of the lineages in a mosaic contact zone could also result in divergence of the population from the allopatric range of its own lineage outside the zone. Here we test this with molecular data, experimental crosses, field measurements and mate choice experiments in a mosaic contact zone between two lineages of a rainforest frog. We show that reinforcing natural selection has resulted in significant premating isolation of a population in the contact zone not only from the other lineage but also, incidentally, from the closely related main range of its own lineage. Thus we show the potential for reinforcement to drive rapid allopatric speciation.

Adaptation to environmental stress: a rare or frequent driver of speciation?

Recent results of evolutionary genomics and other research programmes indicate an important role for environment-dependent selection in speciation, but the conceptual frameworks of speciation genetics and environmental stress physiology have not been fully integrated. Only a small number of model systems have been established for cross-disciplinary studies of this type in animals and plants. In these taxa (e.g. Drosophila and Arabidopsis/Arabis), studies of the mechanistic basis of various stress responses are increasingly combined with attempts to understand their evolutionary consequences. Our understanding of the role of environmental stress in speciation would benefit from studies of a larger variety of taxa. We pinpoint areas for future study and predict that in many taxa 'broad' hybrid zones maintained by ecological selection will be valuable venues for addressing the link between environmental stress, adaptation, and speciation.

Reinforcement of pre-zygotic isolation and karyotype evolution in Agrodiaetus butterflies

The reinforcement model of evolution argues that natural selection enhances pre-zygotic isolation between divergent populations or species by selecting against unfit hybrids or costly interspecific matings. Reinforcement is distinguished from other models that consider the formation of reproductive isolation to be a by-product of divergent evolution. Although theory has shown that reinforcement is a possible mechanism that can lead to speciation, empirical evidence has been sufficiently scarce to raise doubts about the importance of reinforcement in nature. Agrodiaetus butterflies (Lepidoptera: Lycaenidae) exhibit unusual variability in chromosome number. Whereas their genitalia and other morphological characteristics are largely uniform, different species vary considerably in male wing colour, and provide a model system to study the role of reinforcement in speciation. Using comparative phylogenetic methods, we show that the sympatric distribution of 15 relatively young sister taxa of Agrodiaetus strongly correlates with differences in male wing colour, and that this pattern is most likely the result of reinforcement. We find little evidence supporting sympatric speciation: rather, in Agrodiaetus, karyotypic changes accumulate gradually in allopatry, prompting reinforcement when karyotypically divergent races come into contact.

Reinforcement of reproductive isolation between adjacent populations in the Park Grass Experiment

It has been debated, ever since Charles Darwin and Alfred Russell Wallace disagreed about the matter, whether natural selection plays a role in reinforcing reproductive isolation during the earliest stages of speciation. Recent theory suggests that it can do so, but until now the empirical evidence has conspicuously lacked a case in which reinforcement has actually been observed to split a population. We show that this has occurred at least once in populations of the grass Anthoxanthum odoratum growing in the Park Grass Experiment where flowering time has shifted at the boundaries between plots. As a consequence, gene flow via pollen has been severely limited and adjacent populations that had a common origin at the start of the experiment in 1856 have now diverged at neutral marker loci.

The genetic basis of reproductive isolation: insights from Drosophila

Recent studies of the genetics of speciation in Drosophila have focused on two problems: (i) identifying and characterizing the genes that cause reproductive isolation, and (ii) determining the evolutionary forces that drove the divergence of these "speciation genes." Here, I review this work. I conclude that speciation genes correspond to ordinary loci having normal functions within species. These genes fall into several functional classes, although a role in transcriptional regulation could prove particularly common. More important, speciation genes are typically very rapidly evolving, and this divergence is often driven by positive Darwinian selection. Finally, I review recent work in Drosophila pseudoobscura on the possible role of meiotic drive in the evolution of the genes that cause postzygotic isolation.

Sexual Isolation Between Sympatric and Allopatric Populations of Drosophila pseudoobscura and D. persimilis*

According to reinforcement theory, sexual isolation between species in sympatry is strengthened by natural selection against maladaptive hybrids. Drosophila pseudoobscura and D. persimilis from four locations where these species are sympatric, and from three locations where only D. pseudoobscura has been found, were utilized in studies of sexual isolation. Multiple-choice observation chambers were used to record matings between sympatric and allopatric strains of the two species. There was a wide variation in sexual isolation between the two species in the four localities we studied. The average isolation index for sympatric strains of the species was not significantly different from the average index for allopatric strains. There were no meaningful differences between the isolation indices in sympatric and allopatric strains of the species. The failure to find a relationship is likely the result of gene flow among populations within the two species.

The genetics of speciation by reinforcement

Reinforcement occurs when natural selection strengthens behavioral discrimination to prevent costly interspecies matings, such as when matings produce sterile hybrids. This evolutionary process can complete speciation, thereby providing a direct link between Darwin's theory of natural selection and the origin of new species. Here, by examining a case of speciation by reinforcement in Drosophila, we present the first high-resolution genetic study of variation within species for female mating discrimination that is enhanced by natural selection. We show that reinforced mating discrimination is inherited as a dominant trait, exhibits variability within species, and may be influenced by a known set of candidate genes involved in olfaction. Our results show that the genetics of reinforced mating discrimination is different from the genetics of mating discrimination between species, suggesting that overall mating discrimination might be a composite phenomenon, which in Drosophila could involve both auditory and olfactory cues. Examining the genetics of reinforcement provides a unique opportunity for both understanding the origin of new species in the face of gene flow and identifying the genetic basis of adaptive female species preferences, two major gaps in our understanding of speciation.

Mating discrimination between two species of Drosophila is more pronounced where they hybridize and genes related to odor appear responsible for this "reinforcement," thus providing insights into the genetics of speciation

SEXUAL CONFLICT AND PROTEIN POLYMORPHISM

Sexual conflict, where male and female reproductive interests differ, is probably widespread and often mediated by male or sperm proteins and female or egg proteins that bind to each other during mating or fertilization. One potential consequence is maintenance of polymorphism in these proteins, which might result in reproductive isolation between sympatric subpopulations. I investigate the conditions for polymorphism maintenance in a series of mathematical models of sexual conflict over mating or fertilization frequency. The models represent a male or sperm ligand and a female or egg receptor, and they differ in whether expression of either protein is haploid or diploid. For diploid expression, the conditions imply that patterns of dominance, which involve neither overdominance nor underdominance, can determine whether polymorphism is maintained. For example, suppose ligand expression is diploid, and consider ligand alleles L1 and L2 in interactions with a given receptor genotype; if L1/L1 males are fitter than L2/L2 males in these interactions, then polymorphism is more likely to be maintained when L1/L2 males more closely resemble L1/L1 males in these interactions. Such fitter-allele dominance might be typical of a ligand or its receptor due to their biochemistry, in which case polymorphism might be typical of the pair.

THE EVOLUTION OF PREMATING ISOLATION: LOCAL ADAPTATION AND NATURAL AND SEXUAL SELECTION AGAINST HYBRIDS

Although reinforcement is ostensibly driven by selection against hybrids, there are often other components in empirical cases and theoretical models of reinforcement that may contribute to premating isolation. One of these components is local adaptation of a trait used in mate choice. I use several different comparisons to assess the roles that local adaptation and selection against hybrids may play in reinforcement models. Both numerical simulations of exact recursion equations and analytical weak selection approximations are employed. I find that selection against hybrids may play a small role in driving preference evolution in a reinforcement model where the mating cue is separate from loci causing hybrid incompatibilities. When females have preferences directly for purebreds of their own population, however, selection against hybrids can play a large role in premating isolation evolution. I present some situations in which this type of selection is likely to exist. This work also illustrates shortfalls of using a weak selection approach to address questions about reinforcement.