Difference in Plumage Color Used in Species Recognition between Incipient Species Is Linked to a Single Amino Acid Substitution in the Melanocortin‐1 Receptor. 2009. A. C. Uy et al. American Naturalist 174: 244–254.
Birds use plumage colour to recognize and select potential mates. A mutation of a single DNA base can lead to a striking colour change, as demonstrated by two closely related flycatcher populations in the Solomon Islands. This tiny genetic difference can potentially lead to the evolution of new species.
Two Monarcha castaneiventris sub-species have the same body shape, but different colored bellies and distinct songs. Birds from these sub-species could mate, but these differences stop them recognizing each other as potential sexual partners. This is evidence of incipient speciation: the beginning of the evolution of new species. Other flycatchers in the Solomon Islands also vary their plumage colour, but the genetic basis is not always as clear as this single mutation.
A single gene can cause colour change in birds, affecting the selection of potential sexual partners. This leads to reproductive isolation and eventually speciation. link