Kimball, R.T., and E.L. Braun. 2008. A Multigene Phylogeny of Galliformes Supports a Single Origin of Erectile Ability in Non-feathered Facial Traits. Journal of Avian Biology 39: 438-445.

Many species in the avian order Galliformes have bare (or “fleshy”) regions on their head, ranging from simple featherless regions to specialized structures such as combs or wattles. Sexual selection for these traits has been demonstrated in several species within the largest galliform family, the Phasianidae, though it has also been suggested that such traits are important in heat loss. These fleshy traits exhibit substantial variation in shape, color, location and use in displays, raising the question of whether these traits are homologous. To examine the evolution of fleshy traits, we estimated the phylogeny of galliforms using sequences from four nuclear loci and two mitochondrial regions. The resulting phylogeny suggests multiple gains and/or losses of fleshy traits. However, it also indicated that the ability to erect rapidly the fleshy traits is restricted to a single, well-supported lineage that includes species such as the wild turkey Meleagris gallopavo and ring-necked pheasant Phasianus colchicus. The most parsimonious interpretation of this result is a single evolution of the physiological mechanisms that underlie trait erection despite the variation in color, location, and structure of fleshy traits that suggest other aspects of the traits may not be homologous.