Ornaments evolve faster than weapons

Abstract

We investigate the evolutionary dynamics of sexually selected traits using both a mathematical model and comparative analyses, focusing on the relative rates of evolution between male ornaments and weapons. In our framework, ornaments evolve jointly with female preferences, whereas weapons evolve under direct male–male competition. Our theoretical model predicts that coevolution with mate preference is the key driver of the accelerated evolution of ornaments. Stochastic drift in female preferences causes fluctuating selection on male ornaments, resulting in rapid evolutionary change in ornament size. In contrast, weapons evolve more slowly due to the more stable selection generated by male–male competition. In the empirical part of the study, we used Brownian motion and Ornstein–Uhlenbeck models to measure the rate of trait evolution across various taxa and found that ornaments generally evolve at faster rates than weapons, consistent with the predictions of our theoretical modelling. Together, these results demonstrate how preference-driven dynamics can accelerate ornament evolution, revealing a key asymmetry between the inter- and intrasexual mechanisms of sexual selection. Our findings contribute to a deeper understanding of how different forms of sexual selection shape trait evolution on macroevolutionary timescales.