Domesticated birds as a model for the genetics of speciation by sexual selection.

In theory, even populations occupying identical environments can diverge in sexually selected traits, as a consequence of different mutational input. I evaluate the potential of this process by comparing the genetics of breeds of domesticated birds to what is known about the genetics of differences among species. Within domesticated species there is a strong correlation of time since domestication with the number of breeds. Descendants of the rock dove, Columba livia (the oldest domesticate) show differences in courtship, vocalizations, body shape, feather ornaments (crests and tails) and colors and color patterns. When nine other domesticated species are included there is a striking hierarchy, with more recent domesticates having a nested subset of these traits: the youngest domesticated species have breeds distinguished only by color. This suggests that selection of new, visible, mutations is driving the process of breed diversification, with mutations that appeal to the breeder happening the most frequently in color. In crosses among related species, color, feather ornaments and many vocalizations and displays show both intermediate dominance and pure dominance. Although the number of loci affecting each of these traits is typically unknown, limited evidence of the genetics of species' differences suggests that some differences are due to the substitution of single genes of major effect. While neither the genetics of breeds nor the genetics of species provide a perfect model for the genetics of speciation, similarities between the two are sufficiently striking to infer that major, visible, mutations can provide the impetus underlying new directions of sexual selection.