Disruptive selection and the genetic basis of bill size polymorphism in the African finch Pyrenestes
MECHANISMS producing and maintaining discrete polymorphisms have long fascinated evolutionary biologists1,2. Despite the ubiquity of non-sex-limited polymorphisms in vertebrates, the evolutionary factors maintaining them are well understood in only a few instances3. The African finch Pyrenestes is unique among birds in exhibiting a non-sex-determined polymorphism in bill size4,5. Morphs breed randomly with respect to bill size and differ in diet and feeding performance on soft and hard seeds4,6. I present here: (1) new data showing that the polymorphism appears to result from a single genetic factor; (2) support from long-term field studies for earlier suggestions that disruptive selection is acting on bill size; and (3) data revealing the presence of a possible third, much larger morph. Results suggest that the polymorphism may have arisen through single mutations, where morphs occupy distinct adaptive peaks through differences in feeding performance on seeds differing in hardness.