This is what Peter and Rose­mary Grant observed on the small Galá­pa­gos Island of Daphne Major. The process by which two species form from one (spe­ci­a­tion) they observed involved the devel­op­ment of repro­duc­tive iso­la­tion of two diver­gent lin­eages. They fol­lowed the fate of an immi­grant medium ground finch (Geospiza for­tis), which arrived on Daphne Major in 1981, for seven gen­er­a­tions and over 28 years.

It was unusu­ally large, espe­cially in beak width and sang an unusual song com­pared with the res­i­dent pop­u­la­tion of ground finches. This made it, and its descen­dants, unat­trac­tive to the local birds. This repro­duc­tive iso­la­tion cre­ated an ideal sit­u­a­tion for either extinc­tion or spe­ci­a­tion. In the fourth gen­er­a­tion, after a severe drought, the lin­eage was reduced to a sin­gle brother and sis­ter, who bred with each other. Their descen­dants dif­fered that much from the orig­i­nal finch pop­u­la­tion on Daphne major, that inter­breed­ing was not seen any­more. A new species was born.

(Source:Pro­ceed­ings of the National Acad­emy of Sci­ences, 16 Novem­ber 2009, pub­lished online before print)