Faster-Z evolution is predominantly due to genetic drift

Genes linked to sex chromosomes may show different levels of functional change than autosomal genes due to different evolutionary pressures. We used whole-genome data from zebra finch-chicken orthologs to test for Faster-Z evolution, finding that Z-linked genes evolve up to 50% more rapidly than autosomal genes. We combined these divergence data with information about sex-specific expression patterns in order to determine whether the Faster-Z Effect that we observe was predominantly the result of positive selection of recessive beneficial mutations in the heterogametic sex or primarily due to genetic drift attributable to the lower effective population size of the Z chromosome compared with an autosome. The Faster-Z Effect was no more prevalent for genes expressed predominantly in females; therefore, our data indicate that the largest source of Faster-Z Evolution is the increased levels of genetic drift on the Z chromosome. This is likely a product of sexual selection acting on males, which reduces the effective population size of the Z relative to that of the autosomes. Additionally, this latter result suggests that the relative evolutionary pressures underlying Faster-Z Evolution are different from those in analogous Faster-X Evolution.