EVOLUTION OF A COLOR PATTERN - HISTORY, DEVELOPMENT, AND SELECTION

Patterns of melanin pigmentation in birds are extremely varied. Nevert heless it is easy to think of many patterns that are never observed, a nd others that frequently recur in diverse and distantly related speci es. Using as our model the avian genus Phylloscopus we ask how the res tricted range of observed patterns might be attributable to a restrict ed range of variants produced by developmental perturbations. The patt erns we consider consist of unmelanized patches on the wings, crown an d rump on otherwise pigmented upperparts. We use reaction-diffusion mo dels to show that gross features of the pattern can be simply predicte d from considerations of embryo shape. We suggest that birds are expec ted to have more patterned heads, because the head region is relativel y larger than other regions in the developing embryo. A comparative an alysis across many species of birds and a phylogenetic analysis within the genus Phylloscopus show that the component elements of the patter n have repeatedly been lost and gained during evolution. A shift in a threshold reading could explain the appearance and disappearance of th e unmelanized patches, perhaps through changes in the sensitivity of m elanocytes to epidermal signals. Such threshold shifts would make the transition between patterned and unpatterned forms particularly easy o nce the patterns have been exposed to selection in some distant ancest or. This partitioning of the roles of selection and development implie s that many features of the patterns reflect developmental mechanisms in both immediate and more distant ancestors.