SLOW RATE OF MOLECULAR EVOLUTION IN HIGH-ELEVATION HUMMINGBIRDS

Estimates of relative rates of molecular evolution from a DNA-hybridiz ation phylogeny for 26 hummingbird species provide evidence for a nega tive association between elevation and rate of single-copy genome evol ution, This effect of elevation on rate remains significant even after taking into account a significant negative association between body m ass and molecular rate. Population-level processes do not appear to ac count for these patterns because (i) all hummingbirds breed within the ir first year and (ii) the more extensive subdivision and speciation o f bird populations living at high elevations predicts a positive assoc iation between elevation and rate, The negative association between bo dy mass and molecular rate in other organisms has been attributed to h igher mutation rates in forms with higher oxidative metabolism. As amb ient oxygen tensions and temperature decrease with elevation, the slow rate of molecular evolution in high-elevation hummingbirds also may h ave a metabolic basis, A slower rate of single-copy DNA change at high er elevations suggests that the dynamics of molecular evolution cannot be separated from the environmental context.