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A shortcut to a species

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Science  18 Nov 2016:
Vol. 354, Issue 6314, pp. 818
DOI: 10.1126/science.354.6314.818

Hybrids that start mating with just other hybrids can become isolated and different enough from their parents that they begin to speciate.

“Big Bird,” a hybrid Darwin's finch, could be on its way to becoming a new species.

PHOTO: PETER GRANT

A hefty finch with an outsized head is the poster child for a recently recognized source of new species: hybridization. For decades, biologists have explored how cross-species matings can accelerate evolution by introducing genetic novelty into the parent lineages (see main story, p. 817). But they now realize that the hybrid offspring themselves can thrive and set off on their own evolutionary path. “Big Bird,” one of Darwin's finches on the Galápagos Islands, may be the best documented animal example.

Peter and Rosemary Grant, evolutionary biologists at Princeton University, and their graduate student Trevor Price noticed the unusual male in 1981, after he arrived on the Galápagos island of Daphne Major. Weighing 28 grams instead of the typical 18 grams for male finches, sporting a big head, and singing an unusual song, Big Bird was probably born on neighboring Santa Cruz Island from a mating between a cactus finch and medium ground finch. At first, the immigrant and its young consorted with Daphne Major's medium ground finches. But after a severe drought from 2003 to 2005 killed 90% of the island's avian inhabitants, the two Big Bird descendants that survived and their 26 offspring crowded into one corner of the island and have kept to themselves ever since.

This group breeds just among their own kind, and even though male Big Bird finches are very territorial, they react aggressively only when other Big Bird males infringe, suggesting they don't see those other male finches as reproductive rivals. They also have a distinctive food source: Thanks to their intermediate-sized beaks, they are able to crack certain seed cases that other birds can't. “It's been going on like that for a few generations,” Peter Grant says.

Until recently, biologists insisted that a species has to be reproductively isolated—unable to produce viable offspring when it mates with another species. The Big Birds are far from meeting that definition, but since many scientists no longer insist on it, “It's provocative to call [these birds] a new species,” Peter Grant says. Still, the Grants are unwilling make that final call on the Big Birds.

DNA analysis is turning up more examples of hybrid species. Between 4% and 10% of plant species appear to have arisen this way. And researchers are finding new examples among birds, insects, fish, and marine mammals. The clymene dolphin (Stenella clymene), found in the Atlantic Ocean, emerged from dalliances between the striped and spinner dolphins, and at least two of five cichlid species—a group known for being very diverse—in Africa's Lake Victoria originated from hybrids.

Among birds, the Italian sparrow is recognized as a distinct species—it's a hybrid of the Spanish and house sparrows. The Hawaiian duck is descended from a hybridization event 10,000 years ago between the mallard (Anas platyrhynchos) and Laysan ducks (A. laysanensis). And the root-knot nematode, a deadly plant pest, adds another layer of complexity: One of its parent species was itself a hybrid species.

As evolutionary biologist Scott Taylor of the University of Colorado in Boulder puts it, hybridization is “making it seem that speciation is more complex than we thought.”

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