Genome reveals three more species of Darwin's finches

There's more to Darwin's finches than meets the eye. Famously, the 14 species found on the Galapagos islands are distinguished from one another largely by differences in beak shape. But the first full genome analysis of the birds shows the approach isn't foolproof, because some characteristic beak shapes appear to have evolved on two or three separate occasions.

The finches are named in Darwin's honour because he was the first to collect them during his time on the Beagle and later referred to them when he was formulating his theory of natural selection.

The Galapagos islands owe their reputation as a hotbed of adaptation to their geographical isolation and to the considerable changes in their climate and environments over the last few million years. The finches that inhabit the islands and which featured in Darwin's writings are generally distinguished from one another by the size and beak shape. Some species, for instance, have deep and blunt beaks for cracking nuts while others have long and pointed beaks for feeding on nectar.

Leif Andersson at Uppsala University, Sweden, and his colleagues, have now sequenced the full genomes of 120 birds, representing all 14 established species of Galapagos finch, a 15th species that lives on nearby Cocos Island, and two closely related species that live in the Caribbean. Theirs is the most comprehensive genetic analysis of the famous birds to date, says Andersson. Earlier studies have examined just the bird's mitochondrial DNA or small regions of the genome.

When comparing the full genetic sequences to build an evolutionary tree, the researchers found that what ornithologists going back to Darwin's time thought were just two species, based on their beak shape, may in fact be five separate species, based on their genomes.

One species of finch Geospiza difficilis pops up in three completely different branches of the family tree, so it should be counted as three separate species. Another species Geospiza conirostris sits on two separate evolutionary branches, so should be treated as two species.

That means there are 17 not 14 species of finch living on the Galapagos, says Andersson. "We hope that taxonomists will accept our suggestions."

The genetic evidence revealed something else. A single gene, ALX1, has played a vital role in shaping the evolution of beak shape in the finches in particular, whether they are long and pointed or deep and blunt, says Andersson. The find is remarkable because more often than not, important traits are encoded by hundreds of genes, each playing such a small role that linking single genes to a particular trait is very difficult.

"For instance, if you consider stature in humans, genetic studies have shown that there are hundreds of genes that each explain a very small part of the population variance," says Andersson.

So why was it easy to link ALX1 to beak shape? It may be down to the fact that the finch beak has been the focus of exceptionally strong natural selection during the evolutionary radiation of the finches, says Andersson. This means that any genes that are particularly important in its development will have evolved substantially, making them easier to spot in genetic studies.

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Genome reveals three more species of Darwin's finches