November 21, 2009


Birth of New Species Witnessed by Scientists (Brandon Keim, November 16, 2009, Wired)

This miniature evolutionary saga is described in a paper published Monday in the Proceedings of the National Academy of Sciences. It’s authored by Peter and Rosemary Grant, a husband-and-wife team who have spent much of the last 36 years studying a group of bird species known collectively as Darwin’s finches.

The finches — or, technically, tanagers — have adapted to the conditions of each island in the Galapagos, and they provided Darwin with a clear snapshot of evolutionary divergence when he sailed there on the HMS Beagle. The Grants have pushed that work further, with decades of painstaking observations providing a real-time record of evolution in action. In the PNAS paper, they describe something Darwin could only have dreamed of watching: the birth of a new species.

The species’ forefather was a medium ground finch, or Geospiza fortis, who flew from a neighboring island to the Grants’ island of Daphne Major, and into their nets, in 1981. He “was unusually large, especially in beak width, sang an unusual song” and had a few gene variants that could be traced to another finch species, they wrote. This exotic stranger soon found a mate, who also happened to have a few hybrid genes. The happy couple had five sons.

In the tradition of finches, for whom songs are passed from father to son and used to serenade potential mates, the sons learned their immigrant father’s tunes. But their father’s vocalizations were strange: he’d tried to mimick the natives, but accidentally introduced new notes and inflections, like a person who learns a song in a language he doesn’t understand.

These tunes set the sons apart, as did their unusual size. Though they found mates, it may only have taken a couple generations for the new lineage to ignore — or be ignored by — local finches, and breed only with each other. The Grants couldn’t tell for certain when this started, but they were certain after four generations, when a drought struck the island, killing all but a single brother and sister. They mated with each other, and their children did the same.

No exact rule exists for deciding when a group of animals constitutes a separate species. That question “is rarely if ever asked,” as speciation isn’t something that scientists have been fortunate enough to watch at the precise moment of divergence, except in bacteria and other simple creatures. But after at least three generations of reproductive isolation, the Grants felt comfortable in designating the new lineage as an incipient species.

The future of the species is far from certain. It’s possible that they’ll be out-competed by other finches on the island. Their initial gene pool may contain flaws that will be magnified with time. A chance disaster could wipe them out. The birds might even return to the fold of their parent species, and merge with them through interbreeding.

Of course, the ease with which they interbreed with other finches means they aren't a separate species by any common understanding. Not for nothing do even fellow Darwinists joke that only God and the Grants can tell his finches apart from each other.

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Posted by Orrin Judd at November 21, 2009 1:33 PM
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