Relative-rate tests and biological causes of molecular evolution in hummingbirds

Abstract

Hummingbirds (Trochilidae) present extreme variation in several factors thought to affect rates of molecular evolution, including generation time, species diversity, body mass, and metabolic rate. A published DNA hybridization phylogeny was used to examine experimental and biological causes of apparent rate variation among 26 species representing the principal lineages in the family. Molecular evolutionary rates (fitted path lengths based on ΔT(m)H-C) among the various lineages differed significantly as determined by Felsenstein's F ratio test. Parametric and nonparametric correlations between relative rates and various predictor variables were qualitatively similar for outgroup species within and across different lineages except for outgroups that required comparisons among a small set of ingroups. Thus, the relative-rate tests appeared to be less sensitive to outgroup specification than to ingroup sampling. Correlations and analyses of covariance with predictor variables and outgroup species nested within the principal lineage indicated consistently significant associations of relative rates with various measures of body mass (negative) and with some mass- specific measures of basal metabolic rate (positive), but not with generation time or species diversity. These patterns held even if correlations among predictor variables were taken into account. Overall, these results for hummingbirds are consistent with hypotheses that relate metabolic processes associated with oxygen consumption to rates of molecular evolution. The results are incompatible with demographic (generation time, speciation) or body temperature effects on rates of DNA evolution. As DNA hybridization distances index the entire single-copy genome, the results also provide evidence for metabolic effects on evolutionary rates of the nuclear germ line.