Ecology not genetics explains correlated trait divergence during speciation

Abstract

The formation of new species often involves the correlated divergence of multiple traits and genetic regions. However, the mechanisms by which such trait covariation builds up remain poorly understood. In this context, we consider two non-exclusive hypotheses. First, genetic covariance between traits can cause divergent selection on one trait to promote population divergence in correlated traits (a genetic covariation hypothesis). Second, correlated environmental pressures can generate selection on multiple traits, facilitating the evolution of trait complexes (an environmental covariation hypothesis). Here, we test these hypotheses using cryptic coloration (controlled by an incipient supergene) and chemical traits (i.e., cuticular hydrocarbons, CHCs) involved in desiccation resistance and mate choice in Timema cristinae stick insects. We first demonstrate that population divergence in color-pattern is correlated with divergence in some (but not all) CHC traits. We show that when correlated population divergence does occur, it is unlikely to be explained by genetic covariation because within-population genetic covariance between color-pattern and CHCs traits is weak. In contrast, we find that correlated variation in climate and host plant likely generates selection jointly on color-pattern and some CHC traits. This supports the environmental covariation hypothesis, likely via the effects of two correlated environmental axes selecting on different traits. Finally, we provide evidence that misalignment between natural and sexual selection also contributes to patterns of correlated trait divergence. Our results shed light into transitions between phases of speciation by showing that environmental factors can promote population divergence in trait complexes, even without strong genetic covariance.