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)
Remark by Moos: This seems like the situation of which followers of the Intelligent Design theory say that it is too coincidental and therefore considered unlikely to happen in real life. The example above proves that it could do without a designer!