This finch's odd beak and song make it unpopular with the locals. Credit: Grant, P./PNAS

A husband and wife team has spotted what could be the beginning of a new species of finch on one of the Galapagos Islands, where Charles Darwin developed his ideas about evolution.

Peter and Rosemary Grant, evolutionary biologists at Princeton University in New Jersey, have spent nearly four decades watching finches on Daphne Major, in the Galapagos archipelago where Darwin, too, studied finches. The birds later figured prominently in his discussions of variation and natural selection.

Over the decades, the Grants have measured and tagged the vast majority of the finches that inhabit Daphne Major, and as a result have been able to observe evolution in real time (see 'Evolution caught in the act').

It was in 1981, that the Grants spotted an unusually heavy medium ground-finch (Geospiza fortis). At 29.7 grams, the male was more than 5 grams heavier than any they had seen on Daphne Major before. Genetic analysis showed that it probably came from the neighbouring island of Santa Cruz.

The Grants numbered the bird 5110 and followed it and all its known descendants over seven generations. Many of its descendants stuck out from the other G. fortis on Daphne Major: they had unusually shaped beaks and their songs differed from those of the other finches.

All in the family

In the fourth generation, a severe drought hit the island and 5110's descendants were reduced to one male and one female — a brother and sister. From then on the immigrant lineage isolated itself, breeding with no other G. fortis on the island, they report in an article in this week's Proceedings of the National Academy of Sciences1.

"No study of this sort has been done before, and it shows one way in which speciation can get started," say the Grants from Japan, where they are receiving the Kyoto Prize for basic science for their life work (see 'Kyoto prize for evolution while you watch').

The distance between Daphne Major and Santa Cruz had made contact between finches on the two islands a rare event. 5110's arrival is an example of a phenomenon called 'secondary contact', when separated populations of the same species meet after a period of time, during which they may have evolved.

"The secondary contact phase is typically discussed as a thought experiment," says Jeff Podos, a biologist at the University of Massachusetts in Amherst who has worked extensively with Darwin's finches. "The real strength of the paper is that it documents, in natural field circumstances, the arrival and fate in secondary contact of a previously separated lineage."

He adds that "the authors' ability to document the reproductive fate of the immigrant bird is nothing short of remarkable."

Second coming

The fact that 5110's descendants haven't mixed could be because they differ from the natives. The Grants note that the descendants have a differently shaped beak from those native to Daphne Major. As finch beaks are vital in identifying potential mates, this could serve to keep them reproductively isolated.

5110's offspring also have the avian equivalent of a strange accent. These finches learn their songs from their father, and the Grants suggest that 5110 sang the songs from his birth home of Santa Cruz then modified his come-hither ballad by roughly copying the Daphne Major birds'. This imperfect copying, they suggest, has over time acted as a barrier to interbreeding.

Lukas Keller of the Zoological Museum at the University of Zurich, Switzerland, agrees that 5110's case is special. However, he thinks it may be possible to generalize to other species who learn behaviours such as singing in a similar way. "For me it's a very exciting paper," he says.

Whereas Darwin thought that a new species would take a considerable amount of time to appear, Keller says that this paper "shows how rapidly reproductive isolation can develop".

The Grant's aren't yet ready to call 5110's lineage a new species, a term fraught with difficulty for evolutionary biologists. "There is no non-arbitrary answer to the question of how many generations should elapse before we declare the reproductively isolated lineage to be a new species," they say. "For the present it is functioning as a [separate] species because its members are breeding only with each other."

The Grants think there is only a small chance that 5110's descendants will remain isolated long enough to speciate. If they do, the new species will have to be named: "When discussing these birds we call them 'big birds'," the Grants say. "That could be translated into Latin."

Credit: Finch