from encarta 2001:
Finch Hybrids
This article from Discover Magazine discusses how finches interbreed and why the resulting hybrids are unusually "fit" to survive.
Any Finch in a Pinch
By Lori Oliwenstein
Plants hybridize with other species of plant fairly regularly, but as a rule animals are less free with their genes. Birds are an exception. Of the 9,672 known bird species, about 10 percent are known to interbreed with other species. Ornithologists have generally figured, though, that hybridization doesn't make much difference-that hybrid birds tend to be "unfit," which is a biologist's way of saying they don't have kids. The reason for this assumption, it turns out, is that until now no one had really documented the effects of hybridization in the wild.
Every year for the past 16 years, ecologists Peter and Rosemary Grant of Princeton have visited Daphne Major, an 83-acre island in the Galápagos archipelago, to study populations of Darwin's famous finches. Because the island is so small, the Grants have been able to inspect every nest and tag just about every finch with a leg band. That has allowed them to observe which finches survive and which don't, which finches produce offspring and which fail.
There are three species of finch on Daphne: two that are native to the island (the medium ground finch and the cactus finch) and one that visits occasionally (the small ground finch). The Grants found that the medium ground finch interbred with both of the other species, which did not interbreed with each other. The interesting result, though, was the fate of the hybrid offspring. Both types of hybrid had a higher survival rate than their purebred rivals. What's more, the hybrids went on to produce as many or more offspring.
All in all, the hybrid finches were fittest. That raises the question of whether the finches on Daphne really are distinct species-after all, a species is supposed to be a population of individuals who can't mate successfully outside the group. It's possible, say the Grants, that the three populations on Daphne are in the process of fusing into one.
But it's also possible that they just haven't had time to define themselves as species. Darwin's finches are young species, descended from birds that migrated from South America only recently. (They're a classic instance of evolution in action.) "The species we are studying have not been isolated very long-just tens of thousands of years," says Peter Grant. "In time, they may develop the genetic incompatibilities that prevent interbreeding."
Source: Discover Magazine, August 1992.
"Finch Hybrids." Microsoft® Encarta® Encyclopedia 2001. © 1993-2000 Microsoft Corporation. All rights reserved.
