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

Keiichi Morita, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan and Masato Yamamichi, School of Biological Sciences, The University of Queensland, Brisbane, QLD, Australia

Background/Question/Methods
Ecological and reproductive character displacement, where trait divergence weakens resource competition and reproductive interference, respectively, has been studied intensively in ecology and evolutionary biology. While previous studies on character displacement tended to focus on trait divergence and convergence, recent studies suggest that character displacement can be seen as a special case of evolutionary rescue, where rapid evolution prevents population extinction by weakening the 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 theoretically examine the effects of the magnitude of genetic variation on the two types of evolutionary rescue (i.e., ecological and reproductive character displacement). We conducted numerical simulations of a discrete-time eco-evolutionary model where two species interact with resource competition and reproductive interference (Schreiber et al. 2019). We assumed that increasing the interspecific difference of quantitative traits weakens the negative interspecific interactions, and the quantitative traits evolve along the fitness gradient.
Results/Conclusions
Our simulations showed that large additive genetic variance of the quantitative traits is more important in reproductive character displacement than in ecological character displacement for preventing population extinction. The difference arises because reproductive character displacement produces a locally stable coexistence equilibrium whereas ecological character displacement produces a globally stable coexistence equilibrium. Therefore, in transient dynamics to reproductive character displacement, divergence of reproductive traits needs to be rapid enough for population dynamics to move to the basin of attraction toward the coexistence equilibrium. When trait divergence is slow due to small genetic variance, minor species may go extinct by reproductive interference (i.e., positive frequency-dependence in community dynamics). On the other hand, there is no threshold in the process of ecological character displacement, and minor species does not go extinct as long as we ignore demographic stochasticity. Furthermore, extinction becomes less likely when ecological and reproductive character displacement occur simultaneously with positive genetic covariance between ecological and reproductive traits. Our results suggest that reproductive character displacement may be rarer than ecological character displacement in the wild, but 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.