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In theory, females of many species choose mates based on traits that are indicators of male genetic quality. A fundamental question in evolutionary biology is why genetic variation for such indicator traits persists despite strong persistent selection imposed by female preference, which is known as the lek paradox. One potential solution to the lek paradox suggests that the traits that are targets of mate choice should evolve condition-dependent expression and that condition should have a large genetic variance. Condition is expected to exhibit high genetic variance because it is affected by a large number of physiological processes and hence, condition-dependent traits should 'capture' variation contributed by a large number of loci. We suggest that a potentially important cause of variation in condition is competition for limited resources. Here, we discuss a pair of models to analyze the evolutionary genetics of traits affected by success in social competition for resources. We show that competition can contribute to genetic variation of 'competition-dependent' traits that have fundamentally different evolutionary properties than other sources of variation. Competition dependence can make traits honest indicators of genetic quality by revealing the relative competitive ability of males, can provide a component of heritable variation that does not contribute to trait evolution, and can help maintain heritable variation under directional selection. Here we provide a general introduction to the concept of competition dependence and briefly introduce two models to demonstrate the potential evolutionary consequences of competition-dependent trait expression.

The paradoxical persistence of heritable variation for fitness-related traits is an evolutionary conundrum that remains a preeminent problem in evolutionary biology. Here we describe a simple mechanism in which social competition results in the evolutionary maintenance of heritable variation for fitness related traits. We demonstrate this mechanism using a genetic model with two primary assumptions: the expression of a trait depends upon success in social competition for limited resources; and competitive success of a genotype depends on the genotypes that it competes against. We find that such social competition generates heritable (additive) genetic variation for "competition-dependent" traits. This heritable variation is not eroded by continuous directional selection because, rather than leading to fixation of favored alleles, selection leads instead to allele frequency cycling due to the concerted coevolution of the social environment with the effects of alleles. Our results provide a mechanism for the maintenance of heritable variation in natural populations and suggest an area for research into the importance of competition in the genetic architecture of fitness related traits.

In many species, females abandon mate choice to ensure that eggs are fertilized before they are lost. But why do females not just maintain oocytes longer if there is a benefit to mate choice? We conducted a quantitative genetic study in the cockroach Nauphoeta cinerea to test whether genetic constraints prevent the evolution of oocyte maintenance or selection against oocyte loss is weak when females mate late relative to sexual maturity. We found standing genetic variation within the population and no evidence for genetic constraints. Levels of genetic variation are of the magnitude found for life-history traits in general, suggesting that this trait has been exposed to selection. We unexpectedly found two categories of females: those that delay reproduction and those that reproduce at a normal time when mating late, which could indicate alternative strategies. However, frequency-dependent selection does not maintain this variation as females that delay always reproduce less well. Given these findings, we suggest that there may be advantages to egg degradation. The evolution of maintenance of fertilizable oocytes over time would then be constrained by the need to maintain the mechanism by which females control the distribution of resources between current and future reproductive events.

Ejaculates function as an integrated unit to ensure male fertility and paternity, can have a complex structure, and can experience multiple episodes of selection. Current studies on the evolution of ejaculates typically focus on phenotypic variation in sperm number, size, or related traits such as testes size as adaptations to postcopulatory male-male competition. However, the evolution of the integrated nature of ejaculate structure and function depends on genetic variation in and covariation between the component parts. Here we report a quantitative genetic study of the components of the ejaculate of the cockroach Nauphoeta cinerea, including those we know to experience postcopulatory sexual selection, in the context of functional integration of ejaculate characters. We use the patterns of genetic variation and covariation to infer how the integration of the functions of the ejaculate constrain and shape its evolution. Ejaculate components were highly variable, showed significant additive genetic variance, and moderate to high evolvability. The level of genetic variation in these characters, despite strong directional or truncating selection, may reflect the integration of multiple episodes of selection that occur in N. cinerea. There were few significant phenotypic correlations, but all the genetic correlations among ejaculate characters were significantly different from zero. The patterns of genetic variation and covariation suggest that there are important trade-offs among individual traits of the ejaculate and that evolution of ejaculate characteristics will not proceed unconstrained. Fully describing the genetic relationships among traits that perform as an integrated unit helps us understand how functional relationships constrain or facilitate the evolution of the complex structure that is the ejaculate.