Introgressive hybridization and natural selection in Darwin's finches

Introgressive hybridization, i.e. hybridization with backcrossing, can lead to the fusion of two species, but it can also lead to evolution of a new trajectory through an enhancement of genetic variation in a new or changed ecological environment. On Daphne Major Island in the Galapagos archipelago, similar to 1-2% of Geospiza fortis finches breed with the resident G.scandens and with the rare immigrant species G.fuliginosa in each breeding season. Previous research has demonstrated morphological convergence of G.fortis and G.scandens over a 30-year period as a result of bidirectional introgression. Here we examine the role of hybridization with G.fuliginosa in the evolutionary trajectory of G.fortis. Geospiza fuliginosa (similar to 12g) is smaller and has a more pointed beak than G.fortis (similar to 17g). Genetic variation of the G.fortis population was increased by receiving genes more frequently from G.fuliginosa than from G.scandens (similar to 21g). A severe drought in 2003-2005 resulted in heavy and selective mortality of G.fortis with large beaks, and they became almost indistinguishable morphologically from G.fuliginosa. This was followed by continuing hybridization, a further decrease in beak size and a potential morphological fusion of the two species under entirely natural conditions.