Reinforcement: the Road Not Taken (original) (raw)
Related papers
CAN REINFORCEMENT COMPLETE SPECIATION?
Evolution, 2012
Hybridization is common in nature, even between "good" species. This observation poses the question of why reinforcement is not always successful in leading to the evolution of complete reproductive isolation. To study this question, we developed a new "quasi-linkage disequilibrium" (QLD) approximation to obtain the first analytic results for the evolution of modifiers that increase mate discrimination against hybrids and heterospecifics. When such modifiers have small effects, they evolve more readily under a one-allele than a two-allele mechanism (sensu Felsenstein 1981). The strength of selection on the modifier decreases as hybrids decrease in frequency, and so further reinforcement may not occur once hybridization is sufficiently rare. The outcome is qualitatively different when modifiers have large effects, however, for example, when a single mutation can cause complete reproductive isolation. In this case, modifiers in a two-allele mechanism can be selected as or more strongly than those in a one-allele mechanism. Furthermore, they can spread under quite general conditions. Thus, whether complete closure of genetic introgression by reinforcement occurs may depend on the size of effects that mutations have on the sensory systems used in mate choice.
Evolution, 2001
The evolution of premating isolation after secondary contact is primarily considered in the guise of reinforcement, which relies on low hybrid fitness as the driving force for mating preference divergence. Here I consider two additional forces that may play a substantial role in the adaptive evolution of premating isolation, direct selection on preferences and indirect selection against postmating, prezygotic incompatibilities. First, I argue that a combination of ecological character displacement and sensory bias can cause direct selection on preferences that results in the pattern of reproductive character displacement. Both analytical and numerical methods are then used to demonstrate that, as expected from work in single populations, such direct selection will easily overwhelm indirect selection due to low hybrid fitness as the primary determinant of preference evolution. Second, postmating, prezygotic incompatibilities are presented as a driving force in the evolution of premating isolation. Two classes of these mechanisms, those increasing female mortality after mating but before producing offspring and those reducing female fertility, are shown to be identical in their effects on preference divergence. Analytical and numerical techniques are then used to demonstrate that postmating, prezygotic factors may place strong selection on preference divergence. These selective forces are shown to be comparable if not greater than those produced by the low fitness of hybrids.
2018
The impact of different reproductive barriers on species or population isolation may vary in different stages of speciation depending on evolutionary forces acting within species and through species' interactions. Genetic incompatibilities between interacting species are expected to reinforce prezygotic barriers in sympatric populations and create character displacement between conspecific populations living within and outside the area of sympatry. The outcome of reinforcement has been suggested to be affected by the strength of postzygotic barriers, the history of species coexistence, and the impact of species abundancies on females' discrimination against heterospecific males. We tested these predictions in Drosophila montana and Drosophila flavomontana populations from different geographic regimes. All barriers between D. montana females and D. flavomontana males were extremely strong, while in the reciprocal cross postzygotic isolation was less effective and the target of reinforcement varied according to population type. In long-established sympatric populations, where D. flavomontana is abundant, reinforcement targeted sexual isolation, and in populations, where this species is a new invader and rare, reinforcement targeted postmating prezygotic barriers. Reinforcement of these barriers also created respective barriers between different D. flavomontana populations. These findings show that interspecies interactions have far-reaching effects on strengthening species barriers and promoting speciation. .
The counterintuitive role of sexual selection in species maintenance and speciation
Proceedings of the National Academy of Sciences, 2014
The pronounced and elaborate displays that often differ between closely related animal species have led to the common assumption that sexual selection is important in speciation, especially in geographically separated populations. We use population genetic models to examine the ability of Fisherian sexual selection to contribute to lasting species differentiation by isolating its effect after the onset of gene flow between allopatric populations. We show that when sexually selected traits are under ecologically divergent selection, the situation most favorable to speciation, mating preferences tend to introgress faster than trait alleles, causing sexual selection to counter the effects of local adaptation. As a consequence, the net amount of trait divergence often drops with stronger Fisherian sexual selection. Furthermore, alleles for progressively weaker preferences spread in this context until sexual selection is removed. The effects of pure Fisherian sexual selection on species maintenance are thus much more inhibitory than previously assumed.
Natural selection and divergence in mate preference during speciation
Genetica, 2007
Sexual isolation can evolve due to natural selection against hybrids (reinforcement). However, many different forms of hybrid dysfunction, and selective processes that do not involve hybrids, can contribute to the evolution of sexual isolation. Here we review how different selective processes affect the evolution of sexual isolation, describe approaches for distinguishing among them, and assess how they contribute to variation in sexual isolation among populations of Timema cristinae stick-insects. Pairs of allopatric populations of T. cristinae living on different host-plant species exhibit greater sexual isolation than those on the same host, indicating that some sexual isolation has evolved due to host adaptation. Sexual isolation is strongest in regions where populations on different hosts are in geographic contact, a pattern of reproductive character displacement that is indicative of reinforcement. Ecological costs to hybridization do occur but traits under ecological selection (predation) do not co-vary strongly with the probability of between-population mating such that selection on ecological traits is not predicted to produce a strong correlated evolutionary response in mate preference. Moreover, F1 hybrid egg inviability is lacking and the factors contributing to reproductive character displacement require further study. Finally, we show that sexual isolation involves, at least in part, olfactory communication. Our results illustrate how understanding of the evolution of sexual isolation can be enhanced by isolating the roles of diverse ecological and evolutionary processes.
Reinforcement and the Genetics of Nonrandom Mating
Evolution, 2000
The occurrence of reinforcement is compared when premating isolation is caused by the spread of a gene causing females to prefer to mate with males carrying a population-specific trait (a ''preference'' model) and by a gene that causes females to prefer to mate with males that share their own trait phenotype (an ''assortative mating'' model). Both two-island models, which have symmetric gene flow, and continent-island models, which have one-way gene flow, are explored. Reinforcement is found to occur much more easily in a two-island assortative mating model than in any of the other three models. This is due primarily to the fact that in this model the assortative mating allele will automatically become genetically associated in each population with the trait allele that is favored by natural selection on that island. In contrast, natural selection on the trait both favors and opposes the evolution of premating isolation in the two-island preference model, depending on the particular population. These results imply that species recognition in the context of mating may evolve particularly easily when it targets cues that are favored by natural selection in each population. In the continent-island models, reinforcement is found to occur more often under the preference model than the assortative mating model, thus reversing the trend from the two-island models. Patterns of population subdivision may therefore play a role in determining what types of premating isolation may evolve.
Reproductive isolation driven by the combined effects of ecological adaptation and reinforcement
Proceedings of the Royal Society B: Biological Sciences, 2003
Recent years have seen a resurgence of interest in the process of speciation but few studies have elucidated the mechanisms either driving or constraining the evolution of reproductive isolation. In theory, the direct effects of reinforcing selection for increased mating discrimination where interbreeding produces hybrid offspring with low fitness and the indirect effects of adaptation to different environments can both promote speciation. Conversely, high levels of homogenizing gene flow can counteract the forces of selection. We demonstrate the opposing effects of reinforcing selection and gene flow in Timema cristinae walking-stick insects. The magnitude of female mating discrimination against males from other populations is greatest when migration rates between populations adapted to alternate host plants are high enough to allow the evolution of reinforcement, but low enough to prevent gene flow from eroding adaptive divergence in mate choice. Moreover, reproductive isolation is strongest under the combined effects of reinforcement and adaptation to alternate host plants. Our findings demonstrate the joint effects of reinforcement, ecological adaptation and gene flow on progress towards speciation in the wild.
Speciation by Natural and Sexual Selection: Models and Experiments
The American Naturalist, 2002
A large number of mathematical models have been developed that show how natural and sexual selection can cause prezygotic isolation to evolve. This article attempts to unify this literature by identifying five major elements that determine the outcome of speciation caused by selection: a form of disruptive selection, a form of isolating mechanism (assortment or a mating preference), a way to transmit the force of disruptive selection to the isolating mechanism (direct selection or indirect selection), a genetic basis for increased isolation (a one-or two-allele mechanism), and an initial condition (high or low initial divergence). We show that the geographical context of speciation (allopatry vs. sympatry) can be viewed as a form of assortative mating. These five elements appear to operate largely independently of each other and can be used to make generalizations about when speciation is most likely to happen. This provides a framework for interpreting results from laboratory experiments, which are found to agree generally with theoretical predictions about conditions that are favorable to the evolution of prezygotic isolation.
Reinforcement and the Genetics of Hybrid Incompatibilities
Genetics, 2006
Recent empirical studies suggest that genes involved in speciation are often sex-linked. We derive a general analytic model of reinforcement to study the effects of sex linkage on reinforcement under three forms of selection against hybrids: one-locus, two-locus, and ecological incompatibilities. We show that the pattern of sex linkage can have a large effect on the amount of reinforcement due to hybrid incompatibility. Sex linkage of genes involved in postzygotic isolation generally increases the strength of reinforcement, but only if genes involved in prezygotic isolation are also sex linked. We use exact simulations to test the accuracy of the approximation and find that qualitative predictions made assuming weak selection can hold when selection is strong. Our simulations also show that incompatibilities that evolve in allopatry by drift or weak selection are likely to be lost by swamping during secondary contact, even when selection against hybrids is strong.