The evolution of female-biased sexual size dimorphism: A population-level comparative study in horned lizards (Phrynosoma)

Female-biased sexual size dimorphism is uncommon among vertebrates and trad itionally has been attributed to asymmetric selective pressures favoring la rge fecund females (the fecundity-advantage hypothesis) and/or small mobile males (the small-male advantage hypothesis). I use a phylogenetically base d comparative method to address these hypotheses for the evolution and main tenance of sexual size dimorphism among populations of three closely relate d lizard species (Phrynosoma douglasi, P. ditmarsi, and P. hernandezi). Wit h independent contrasts I estimate evolutionary correlations among female b ody size, male body size, and sexual size dimorphism (SSD) to determine whe ther males have become small, females have become large, or both sexes have diverged concurrently in body size during the evolutionary Xhistory of thi s group. Population differences in degree of SSD are inversely correlated w ith average male body size, but are not correlated with average female body size. Thus, variation in SSD among populations has occurred predominantly through changes in male size, suggesting that selective pressures on small males may affect degree of SSD in this group. I explore three possible evol utionary mechanisms by which the mean male body size in a population could evolve: changes in size at maturity, changes in the variance of male body s izes, and changes in skewness of male body size distributions. Comparative analyses indicate that population differentiation in male body size is achi eved by changes in male size at maturity, without changes in the variance o r skewness of male and female size distributions. This study demonstrates t he potential of comparative methods at lower taxonomic levels (among popula tions and closely related species) for studying microevolutionary processes that underlie population differentiation.