IN 1981 PETER and Rosemary Grant, a hus-band-and-wife team of evolutionary biologists, spotted something odd on Daphne Major. Every year for the previous decade they had travelled from Princeton University to this island in the Galapagos, to study its three endemic tanager species, part of a group known colloquially as "Darwin's finches". On this occasion their eyes were drawn to an unusual male that sported dark feathers and sang a unique song. Genetic analysis later identified him as a large cactus finch, probably blown in from Espanola, another part of the archipelago that is over 100km away. Intrigued, the Grants followed the castaway as he explored his new home. They watched him mate with a local female medium ground finch. That produced five fit, healthy offspring. Those offspring were also surprisingly sexually selective. A single male excepted, they and their descendants mated only among themselves-and they have continued to do so ever since.
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