Abstract
The evolution of birds from theropod dinosaurs took place approximately 150 mya, and was associated with a number of specific adaptations that are still evident among extant birds, including feathers, song and extravagant secondary sexual characteristics. Knowledge about the molecular evolutionary background to such adaptations is lacking. We have taken a comparative genomic approach to study this by analysing the evolution of >5,000 orthologous protein-coding gene sequences in zebra finch and chicken. We find that genes related to apoptosis, recombination and centromere-function are overrepresented among the top 10% of genes ranked by their dN/dS value in birds, indicative of adaptive evolution.
After alignment to mammalian orthologues, we identify a catalogue of 186 genes that show a significantly higher rate of protein evolution in birds than in mammals. These fast bird genes, representing candidates for avian-specific adaptations, include genes related to traits that morphologically (skeletal development) or physiologically (lipid binding) distinguish birds from mammals. Moreover, colouration genes evolve faster in birds than in mammals, something which may have been driven by sexual selection for extravagant plumage characteristics. Furthermore, sex-linked genes are overrepresented among those genes that evolve faster in birds than in mammals, providing large-scale genomic support for a disproportionate role of sex chromosomes in adaptive divergence.
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