On the origin of Darwin's finches

Darwin's finches comprise a group of 15 species endemic to the Galapagos (1 4 species) and Cocos (1 species) Islands in the Pacific Ocean. The group is monophyletic and originated from an ancestral species that reached the Gal apagos Archipelago from Central or South America. Descendants of this ances tor on the Archipelago then colonized Cocos Island. In the present study, w e used sequences of two mitochondrial (mt) DNA segments (922 bp of the cyto chrome b gene and 1,082 bp of the control region), as well as two nuclear m arkers (830 bp of numt2, consisting of 140 bp of mtDNA control region and 6 90 bp of flanking nuclear DNA; and 740 bp of numt3, consisting of 420 bp of mt cytochrome b sequence flanked by 320 bp of nuclear DNA) to identify the species group most closely related to the Darwin's finches. To this end, w e analyzed the sequences of 28 species representing the main groups (tribes ) of the family Fringillidae, as well as 2 outgroup species and 13 species of Darwin's finches. In addition, we used mtDNA cytochrome b sequences of s ome 180 additional Fringillidae species from the database for phylogeny rec onstruction by maximum-parsimony, maximum-likelihood, minimum-evolution, an d neighbor-joining methods. The study identifies the grassquit genus Tiaris , and specifically the species Tiaris obscura, as the nearest living relati ve of Darwin's inches among the species surveyed. Darwin's finches diverged from the Tiaris group shortly after the various extant species of Tiaris d iverged from one another. The initial adaptive radiation of the Tiaris grou p apparently occurred on the Caribbean islands and then spread to Central a nd South America, from where the ancestors of Darwin's finches departed for the Galapagos Islands approximately 2.3 MYA, at the time of the dramatic c limatic changes associated with the closure of the Panamanian isthmus and t he onset of Pleistocene glaciation.