REINFORCEMENT OF STICKLEBACK MATE PREFERENCES: SYMPATRY BREEDS CONTEMPT

Genetic divergence and reproductive isolation in the Ochthebius (Calobius) complex (Coleoptera: Hydraenidae)

The reproductive isolation in hydrenid beetles of the Ochthebius complex was studied by analysing gene exchange in natural populations of O. quadricollis, Ochthebius sp. A and O. brevicollis steinbuehleri collected along the Mediterranean coasts. The ranges of these three species are largely allopatric, but sympatric areas were detected between contiguous taxa, ie, O. quadricollis and Ochthebius sp. A; Ochthebius sp. A and O. b. steinbuehleri. Three levels of reproductive isolation and genetic divergence were observed. One level involves extensive intraspecific genetic divergence within the biological species O. quadricollis, Ochthebius sp. A and O. brevicollis, associated with both physical barriers (eg, sea and sand stretches) and the low dispersal capacity of larvae and adults. The finding of transitional samples between the most differentiated population groups should indicate, however, that there is still some gene flow between the populations of the three taxa. Another level is found between Ochthebius sp. A and O. b. steinbuehleri, whose gene pools appear to be fairly distinct in spite of the fact that reproductive isolation is still incomplete: in their few syntopic sites, some F1 hybrids appeared indeed to have lower fitness, since backcrosses or recombinant genotypes were never observed. The final level in the evolution of reproductive isolation (full reproductive isolation) has been achieved by the species O. quadricollis and Ochthebius sp. A. No F1 or F(n) hybrids, nor backcrosses were found in their sympatric areas. The relative importance of ecological factors and evolutionary forces in the prevention of gene exchange between taxa are discussed.

Degree of male ornamentation affects female preference for conspecific versus heterospecific males

Several studies have shown female preference for conspecific males with the attached artificial ornaments of more elaborate heterospecifics. However, preference for heterospecifics under natural conditions is relatively rare. We tested what factors affect behavioural mechanisms of species isolation using three species of estrildid finch (genus Uraeginthus) that occur in both sympatry and allopatry. These finches differ in degree of sexual dimorphism; male ornamentation; behavioural and morphological similarity; and phylogenetic distance. Paired mate-choice trials were used in which females were presented with a conspecific and heterospecific male to test which of the above between-species differences best predicted the degree of premating isolation. The three species differed in the degree of species-specific mate preference shown. Females from the brighter two species discriminated against dull males, independently of sympatry-allopatry, similarity and phylogenetic distance. Females from the dull species reacted to conspecific males and brighter heterospecific males equally strongly, independently of similarity and phylogenetic distance. In contrast to previous studies, an equal preference for heterospecific and conspecific males was found under natural conditions. It is suggested that differences between closely related species in male ornamentation affect the likelihood that premating isolation will occur due to the fact that sexual selection tends to drive preferences for exaggerated ornamentation.

Geographical variation in reproductive character displacement in mate choice by male sailfin mollies

Female Amazon mollies, Poecilia formosa, are a unisexual species that reproduce by gynogenesis. They must coexist and mate with males of other species (usually the mollies Poecilia latipinna or Poecilia mexicana) to induce embryogenesis, but inheritance is strictly maternal. We examined the mating preference of the male sailfin molly, P. latipinna, for female sailfin mollies versus Amazon mollies, P. formosa. We compared the mating preferences of sympatric and allopatric populations collected throughout the Gulf Coast of North America. Male P. latipinna from six populations sympatric with Amazon mollies showed a significantly greater strength of preference for conspecific sailfin females than males from five populations that were allopatric with Amazon mollies. These results provide strong evidence for reproductive character displacement of male mate choice in sympatry. Furthermore, the large geographical range of populations that we tested revealed variation among populations within sympatry and allopatry, indicating that it is important to evaluate a large number of populations when examining reproductive character displacement.

Sexual selection and speciation in field crickets

Recent theoretical work has shown that sexual selection may cause speciation under a much wider range of conditions than previously supposed. There are, however, no empirical studies capable of simultaneously evaluating several key predictions that contrast this with other speciation models. We present data on male pulse rates and female phonotactic responses to pulse rates for the field cricket Gryllus texensis; pulse rate is the key feature distinguishing G. texensis from its cryptic sister species G. rubens. We show (i) genetic variation in male song and in female preference for song, (ii) a genetic correlation between the male trait and the female preference, and (iii) no character displacement in male song, female song recognition, female species-level song discrimination, or female song preference. Combined with previous work demonstrating a lack of hybrid inviability, these results suggest that divergent sexual selection may have caused speciation between these taxa.

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

Natural selection and the reinforcement of mate recognition

Natural selection on mate recognition may often contribute to speciation, resulting in reproductive character displacement. Field populations of Drosophila serrata display reproductive character displacement in cuticular hydrocarbons when sympatric with Drosophila birchii. We exposed field sympatric and allopatric populations of D. serrata to experimental sympatry with D. birchii for nine generations. Cuticular hydrocarbons of field allopatric D. serrata populations evolved to resemble the field sympatric populations, whereas field sympatric D. serrata populations remained unchanged. Our experiment indicates that natural selection on mate recognition resulted in the field pattern of reproductive character displacement.

Evolutionary Branching and Sympatric Speciation Caused by Different Types of Ecological Interactions

Evolutionary branching occurs when frequency-dependent selection splits a phenotypically monomorphic population into two distinct phenotypic clusters. A prerequisite for evolutionary branching is that directional selection drives the population toward a fitness minimum in phenotype space. This article demonstrates that selection regimes leading to evolutionary branching readily arise from a wide variety of different ecological interactions within and between species. We use classical ecological models for symmetric and asymmetric competition, for mutualism, and for predator-prey interactions to describe evolving populations with continuously varying characters. For these models, we investigate the ecological and evolutionary conditions that allow for evolutionary branching and establish that branching is a generic and robust phenomenon. Evolutionary branching becomes a model for sympatric speciation when population genetics and mating mechanisms are incorporated into ecological models. In sexual populations with random mating, the continual production of intermediate phenotypes from two incipient branches prevents evolutionary branching. In contrast, when mating is assortative for the ecological characters under study, evolutionary branching is possible in sexual populations and can lead to speciation. Therefore, we also study the evolution of assortative mating as a quantitative character. We show that evolution under branching conditions selects for assortativeness and thus allows sexual populations to escape from fitness minima. We conclude that evolutionary branching offers a general basis for understanding adaptive speciation and radiation under a wide range of different ecological conditions.

Reinforcement and divergence under assortative mating

Traits that cause assortative mating such as the flowering time in plants and body size in animals can produce reproductive isolation between hybridizing populations. Can selection against unfit hybrids cause two populations to diverge in their mean values for these kinds of traits? Here I present a haploid analytical model of one population that receives gene flow from another. The partial pre-zygotic isolation between the two populations is caused by assortative mating for a trait that is influenced by any number of genes with additive effects. The post-zygotic isolation is caused by selection against genetic incompatibilities that can involve any form of selection on individual genes and gene combinations (epistasis). The analysis assumes that the introgression rate and selection coefficients are small. The results show that the assortment trait mean will not diverge from the immigrants unless there is direct selection on the trait favouring it to do so or there are genes of very large effect. The amount of divergence at equilibrium is determined by a balance between direct selection on the assortment trait and introgression from the other population. Additional selection against hybrid genetic incompatibilities reduces the effective migration rate and allows greater divergence. The role of assortment in speciation is discussed in the light of these results.

Genetic compatibility, mate choice and patterns of parentage: invited review

There is growing interest in the possibility that genetic compatibility may drive mate choice, including gamete choice, particularly from the perspective of understanding why females frequently mate with more than one male. Mate choice for compatibility differs from other forms of choice for genetic benefits (such as 'good genes') because individuals are expected to differ in their mate preferences, changing the evolutionary dynamics of sexual selection. Recent experiments designed to investigate genetic benefits of polyandry suggest that mate choice on the basis of genetic compatibility may be widespread. However, in most systems the mechanisms responsible for variation in compatibility are unknown. We review potential sources of variation in genetic compatibility and whether there is any evidence for mate choice driven by these factors. Selfish genetic elements appear to have the potential to drive mate compatibility mate choice, though as yet there is only one convincing example. There is abundant evidence for assortative mating between populations in hybrid zones, but very few examples where this is clearly a result of selection against mating with genetically less compatible individuals. There are also numerous cases of inbreeding avoidance, but little evidence that mate choice or differential fertilization success driven by genetic compatibility occurs between unrelated individuals. The exceptions to this are a handful of situations where both the alleles causing incompatibility and the alleles involved in mate choice are located in a chromosome region where recombination is suppressed. As yet there are only a few potential sources of genetic compatibility which have clearly been shown to drive mate choice. This may reflect limitations in the potential for the evolution of mate choice for genetic compatibility within populations, although the most promising sources of such incompatibilities have received relatively little research.

Natural selection and parallel speciation in sympatric sticklebacks

Natural selection plays a fundamental role in most theories of speciation, but empirical evidence from the wild has been lacking. Here the post-Pleistocene radiation of threespine sticklebacks was used to infer natural selection in the origin of species. Populations of sticklebacks that evolved under different ecological conditions show strong reproductive isolation, whereas populations that evolved independently under similar ecological conditions lack isolation. Speciation has proceeded in this adaptive radiation in a repeatable fashion, ultimately as a consequence of adaptation to alternative environments.

Reinforcement and other consequences of sympatry

The pattern of greater species mating discrimination between sympatric taxa than between allopatric taxa has been attributed to the strengthening of mate discrimination to avoid maladaptive hybridization. This process, termed reinforcement, has been highly contentious, particularly with regard to its role in speciation. Here, I review some recent studies of reinforcement, discuss alternative explanations for the pattern of greater species discrimination in sympatric taxa, and point to some new directions that may help to clarify the evolutionary forces involved. In particular, we need more ecological work on putative cases of reinforcement, more theoretical models that give diagnostic predictions of reinforcement relative to other modes of divergence, and empirical studies to evaluate these diagnostic predictions.

Sexual imprinting, learning and speciation

Learned mate preferences may play an important role in speciation. Sexual imprinting is a process whereby mate preferences are affected by learning at a very young age, usually using a parent as the model. We suggest that while the origins of learning appear to lie in the advantages of individual recognition, sexual imprinting results from selection for recognition of conspecifics. This is because efficient early learning about one's own species is favoured in the presence of heterospecifics. If different species are hybridizing, both sexual imprinting and learning to avoid heterospecifics during adulthood promote assortative mating and hence speciation. As a result of sexual imprinting, speciation may also be completed in allopatry when divergence between populations is sufficient to prevent interbreeding when the populations reunite, even in the absence of genetic evolution of mate preference. The role of behaviour and learning in completing the speciation process is relatively overlooked. In particular the evolution of sexual imprinting as a result of selection against hybridization warrants more study.