Comparative tests of the role of dewlap size in Anolis lizard speciation.
Understanding the patterns of diversification in sexual traits and the selection underlying such diversification represents a major unresolved question in evolutionary biology. We examined the phylogenetic diversification for courtship and external genitalic characters across ten species of Timema walking-sticks, to infer the tempos and modes of character change in these sexual traits and to draw inferences regarding the selective pressures underlying speciation and diversification in this clade. Rates of inferred change in male courtship behaviours were proportional to speciation events, but male external genitalic structures showed a pattern of continuous change across evolutionary time, with divergence proportional to branch lengths. These findings suggest that diversification of courtship behaviour is mediated by processes that occur in association with speciation, whereas diversification of genitalia occurs more or less continuously, most likely driven by forces of sexual selection.