Elevated rates of nonsynonymous substitution in island birds.

Slightly deleterious mutations are expected to fix at relatively higher rates in small populations than in large populations. Support for this prediction of the nearly-neutral theory of molecular evolution comes from many cases in which lineages inferred to differ in long-term average population size have different rates of nonsynonymous substitution. However, in most of these cases, the lineages differ in many other ways as well, leaving open the possibility that some factor other than population size might have caused the difference in substitution rates. We compared synonymous and nonsynonymous substitutions in the mitochondrial cyt b and ND2 genes of nine closely related island and mainland lineages of ducks and doves. We assumed that island taxa had smaller average population sizes than those of their mainland sister taxa for most of the time since they were established. In all nine cases, more nonsynonymous substitutions occurred on the island branch, but synonymous substitutions showed no significant bias. As in previous comparisons of this kind, the lineages with smaller populations might differ in other respects that tend to increase rates of nonsynonymous substitution, but here such differences are expected to be slight owing to the relatively recent origins of the island taxa. An examination of changes to apparently "preferred" and "unpreferred" synonymous codons revealed no consistent difference between island and mainland lineages.