How does the magnitude of genetic variation affect eco-evolutionary dynamics in character displacement?

While previous studies on character displacement tended to focus on trait divergence and convergence as a result of long-term evolution, recent studies suggest that character displacement can be a special case of evolutionary rescue, where rapid evolution prevents population extinction by weakening negative interspecific interactions. When the magnitude of genetic variation is small, however, the speed of trait divergence can be slow and populations may go extinct before the completion of character displacement. Here we analyzed a simple model to examine how the magnitude of genetic variation affects evolutionary rescue via ecological and reproductive character displacement that weakens resource competition and reproductive interference, respectively. We found that the large additive genetic variance is more important for preventing extinction in reproductive character displacement than in ecological character displacement. This is because reproductive interference produces a locally stable coexistence equilibrium with positive frequency-dependence (i.e., minority disadvantage) whereas ecological character displacement results in a globally stable coexistence equilibrium. Furthermore, population extinction becomes less likely when ecological and reproductive character displacement occur simultaneously due to positive covariance between ecological and reproductive traits. Our results suggest that while reproductive character displacement may be rarer than ecological character displacement, it is more likely to occur when there exists positive trait covariance, such as the case of a magic trait in reinforcement of speciation processes.

Ecological character displacement destabilizes food webs

Ecological character displacement is an adaptive process that generally increases phenotypic diversity. Despite the fact that this diversification is due to an eco-evolutionary feedback between consumers competing for shared resources, its consequences for food-web dynamics have received little attention. Here, I study a model of two consumers competing for two shared resources to examine how character displacement in consumer attack rates affects resource abundances and the resilience of food webs to perturbations. I found that character displacement always strengthened consumer-resource interactions whenever consumers competed for resources that occurred in different habitats. This increase in interaction strength resulted in lower resource abundances and less resilient food webs. This occurred under different evolutionary tradeoffs and in both simple and more realistic foraging scenarios. Taken together, my results show that the adaptive process of character displacement may come with the ecological cost of decreasing food-web resilience.

Understanding the Evolution of Ecological Sex Differences: Integrating Character Displacement and the Darwin-Bateman Paradigm

Sex differences in selection arise for two possible reasons: 1) differences originating from anisogamy – the Darwin-Bateman paradigm – and 2) competition-driven ecological character displacement (ECD), agnostic of anisogamy. Despite mounting evidence of ECD and increasing focus on the ecological causes and consequences of sexual dimorphism, progress in understanding the evolution of ecological sex differences has likely been hindered because ecological dimorphisms are not exclusive to ECD. I argue that embracing non-exclusivity of causal models of sexual dimorphism itself provides insights into evolution of sex differences. This integrated view of the evolution of sexual dimorphism leads to four predictions for how sex-specific selection and phenotypic divergence between the sexes change over the course of the evolution of sexual dimorphism. First, dimorphism resulting directly from anisogamy likely precedes evolution of ecological dimorphism driven by ECD. Second, ecological sexual dimorphism driven by ECD may (initially) evolve in directions in trait space favored by other sources of sex-specific selection. Third, we may expect correlated evolution of ecological dimorphism and other forms of sexual dimorphism. Finally, ecological optima may be sex specific even when competition plays a role in reaching them. Rather than simply a less-parsimonious alternative explanation for ecological sex differences, ECD should be seen as one likely contributor to sex-specific selection that could act at predictable times during the evolution of ecological sexual dimorphisms.

Habitat partitioning during character displacement between the sexes

Ecological differences between the sexes are often interpreted as evidence of within-species ecological character displacement (ECD), a hypothesis with almost no direct tests. Here, we experimentally test two predictions that are direct corollaries of ECD between the sexes, in a salamander. First, we find support for the prediction that each sex has a growth rate advantage in the aquatic microhabitat where it is most commonly found. Second, we test the prediction that selection for ECD in the breeding environment may affect partial migration out of this environment. We found that phenotype-dependent migration resulted in a shift in the phenotypic distribution across treatments, with the highest sexual dimorphism occurring among residents at high founding density, suggesting that migration and ECD can both be driven by competition. Our work illustrates how complex patterns of habitat partitioning evolve during ECD between the sexes and suggest ECD and partial migration can interact to effect both ecological dynamics and evolution of sexual dimorphism.