Although empirical evidence has shown that many male traits have evolved via sexual selection by female mate choice, our understanding of the adaptive value of female mating preferences is still very incomplete. It has recently been suggested that female mate choice may result from females evolving resistance rather than attraction to males, but this has been disputed. Here, we develop a quantitative genetic model showing that sexual con£ict over mating indeed results in the joint evolution of costly female mate choice and exaggerated male traits under a wide range of circumstances. In contrast to traditional explanations of costly female mate choice, which rely on indirect genetic bene¢ts, our model shows that mate choice can be generated as a side-e¡ect of females evolving to reduce the direct costs of mating.

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Speciation rates among extant lineages of organisms vary extensively, but our understanding of the causes of this variation and, therefore, the processes of speciation is still remarkably incomplete. Both theoretical and empirical studies have indicated that sexual selection is important in speciation, but earlier discussions have focused almost exclusively on the potential role of female mate choice. Recent findings of postmating reproductive conflicts of interest between the sexes suggest a quite different route to speciation. Such conflicts may lead to perpetual antagonistic coevolution between males and females and may thus generate rapid evolutionary divergence of traits involved in reproduction. Here, we assess this hypothesis by contrasting pairs of related groups of insect species differing in the opportunity for postmating sexual conflict. Groups where females mate with many males exhibited speciation rates four times as high as in related groups where females mate only once. Our results not only highlight the general importance of postmating sexual selection in speciation, but also support the recent suggestion that sexual conflict is a key engine of speciation. Sexual selection has been shown to be a key component in the process of speciation (1-6), but reproductive competition may contribute to the evolution of reproductive isolation in several ways (3). Recent research has suggested that differences in the evolutionary interests of males and females may provide an important route to speciation (7-16). Because conflicts of interest between interacting loci residing within the same genome, but favoring different sexes can result in very rapid antagonistic coevolution (9-10, 16), such conflicts may be capable of generating reproductive isolation (14). Sexual conflict over the postmating interests of males and females is virtually ubiquitous, and stems from competition between males over the fertilization of eggs (17). Males stand to gain from adaptations that increase sperm competition success (7) and female shortterm egg production (18), as well as from those that act to decrease female remating rate (9), even if these benefits are achieved at the expense of female fitness (19). Many of these postmating conflicts are mediated by various components transferred to the female in the male seminal fluid and by female receptivity to these substances (9, 20). Whenever female interests are compromised by males, the female reproductive system will evolve to depress these costs, in turn creating perpetual or episodic postmating sexual selection (by sperm competition and͞or cryptic female choice) for novel male adaptations by biasing postmating fertilization success among males toward males most able to manipulate female reproduction in their own interests. Such sexually antagonistic adaptations will generate rapid coevolution between male and female reproductive physiology (9) and morphology (8), eventually resulting in reproductive isolation between allopatric populations (11, 14). Here we assess the general i...

One of Ernst Mayr's legacies is the consensus that the allopatry model is the predominant mode of speciation in most sexually reproducing lineages. In this model, reproductive isolation develops as a pleiotropic byproduct of the genetic divergence that develops among physically isolated populations. Presently, there is no consensus concerning which, if any, evolutionary process is primarily responsible for driving the specific genetic divergence that leads to reproductive isolation. Here, we focus on the hypothesis that inter-locus antagonistic coevolution drives rapid genetic divergence among allopatric populations and thereby acts as an important ''engine'' of speciation. We assert that only data from studies of experimental evolution, rather than descriptive patterns of molecular evolution, can provide definitive evidence for this hypothesis. We describe and use an experimental approach, called hemiclonal analysis, that can be used in the Drosophila melanogaster laboratory model system to simultaneously screen nearly the entire genome for both standing genetic variation within a population and the net-selection gradient acting on the variation. Hemiclonal analysis has four stages: (i) creation of a laboratory ''island population''; (ii) cytogenetic cloning of nearly genomewide haplotypes to construct hemiclones; (iii) measurement of additive genetic variation among hemiclones; and (iv) measurement of the selection gradient acting on phenotypic variation among hemiclones. We apply hemiclonal analysis to test the hypothesis that there is ongoing antagonistic coevolution between the sexes in the D. melanogaster laboratory model system and then discuss the relevance of this analysis to natural systems.sexual conflict ͉ inter-locus contest evolution ͉ sexually antagonistic coevolution ͉ reproductive isolation ͉ genetic divergence