Phylogeny, body mass, and genetic consequences of lek-mating behavior in hummingbirds

Abstract

The overt mate competition manifested in lek-type mating systems is thought to impart strong historical constraints on the origin and loss of lek behavior as well as to accelerate genetic evolution in lekking taxa. A DNA hybridization-based phylogeny for 26 hummingbirds (Trochilidae) was used to test these expectations through ancestral character state reconstruction and relative-rate tests (based on the index ΔT(m)H-C) using an outgroup swift (Apodidae). Lekking developed at least eight times across the hummingbird lineages represented in the phylogeny, suggesting that lek behavior in these birds is not subject to strong historical constraints. Moreover, rate patterns differed from expectations for faster rates among lekkers in that: (1) the 2 lekking hermits (subfamily Phaethornithinae) were evolving significantly more slowly than all 24 (lekking and nonlekking) nonhermits (subfamily Trochilinae); (2) analysis of covariance on the more numerous nonhermits alone or on the entire sample of species from both subfamilies indicated a significant negative effect of body mass (as covariate), but no significant effect of breeding system (as main effect), on rates of molecular evolution. Thus, molecular rate variation within and among breeding systems reflects both clade-specific effects on mean rates and a superimposed covariance between rates and body mass such that rates may be faster or slower depending on whether relatively smaller or larger size is favored. It is predicted that the frequent association of larger size with intense mate competition in many organisms may more often decrease the rate of molecular evolution under sexual selection, at least at the entire single-copy genome level measured by DNA hybridization.