The stages of vertebrate evolutionary radiation

Evolutionists and ecologists are motivated to understand the forces that generate and maintain biological diversity. In turn, attention has focused on the relative roles of natural and sexual selection in vertebrate groups that are considered to be exemplars of evolutionary radiation. Here, we argue that recent phylogenetic and population genetic evidence shows that seemingly different vertebrate radiations follow similar evolutionary trajectories. Groups diverge along axes of habitat, trophic morphology and communication, often in that order. Notably, clades proceed to different stages of the radiation process, with the endpoint being correlated with species richness. Divergence along axes one and two (habitat and trophic morphology, respectively) is likely to follow ecological selection models; diversification along axis three (communication) probably proceeds according to sexual selection models. We speculate that a paucity of genetic variation for means of signaling that are independent of fitness traits might be the most important constraint limiting the diversification of vertebrate groups. The stages of evolutionary radiation discussed here do not apply to all organisms, but might represent a common trend.