Positive selection of a gene family during the emergence of humans and African apes

Gene duplication followed by adaptive evolution is one of the primary force s for the emergence of new gene function(1). Here we describe the recent pr oliferation, transposition and selection of a 20-kilobase (kb) duplicated s egment throughout 15 Mb of the short arm of human chromosome 16. The disper sal of this segment was accompanied by considerable variation in chromosoma l-map location and copy number among hominoid species. In humans, we identi fied a gene family (morpheus) within the duplicated segment. Comparison of putative protein-encoding exons revealed the most extreme case of positive selection among hominoids. The major episode of enhanced amino-acid replace ment occurred after the separation of human and great-ape lineages from the orangutan. Positive selection continued to alter amino-acid composition af ter the divergence of human and chimpanzee lineages. The rapidity and bias for amino-acid-altering nucleotide changes suggest adaptive evolution of th e morpheus gene family during the emergence of humans and African apes. Mor eover, some genes emerge and evolve very rapidly, generating copies that be ar little similarity to their ancestral precursors. Consequently, a small f raction of human genes may not possess discernible orthologues within the g enomes of model organisms.