Genomes reveal Darwin finches' messy family tree

Darwin's drawing of finches from Galapagos islands drawn in 1845 by J.Gould
image Wikimedia Commons

Special reporter Jonathan Webb writes in BBC News Science&Environment about genetic study of Darwin's finches by scientists from Upisala University in Sweden, Princeton and elsewhere. Below are some quotes from the article with subtitles added by me - read the entire BBC report here.

Darwin
This "most singular group of finches" appeared in Charles Darwin's famous journal from the voyage of the Beagle. Darwin collected specimens which were later classified by the ornithologist John Gould, back in London.

"Darwin was amazed by the beak diversity in species that were otherwise very similar," said the new study's senior author Prof Leif Andersson, from Uppsala University in Sweden.

According to Darwin's now-famous theory of natural selection, the birds rapidly adapted to the different food sources that were available in their new home, where they faced little competition from other birds. Chief among these adaptations were different beak shapes: stronger, blunter beaks for cracking tough seeds or insects, for example.

Sample
They sequenced the genomes of multiple individuals from all 15 species of Darwin's finches, one of which inhabits the rather more distant Cocos Island, along with two related bird species that live on the South American mainland and the West Indies.

One of the things that the team looked for, after a detailed comparison of 120 individual genomes, was genetic changes that could explain the diversity of beak shapes.

The study also revealed a surprisingly large amount of "gene flow" between the branches of the family.

Genetic key
"What we discovered is basically two variants of ALX1," Prof Andersson explained.

"One is associated with pointed beaks - that is the ancestral form. Then there is a variant associated with the blunt beaks. That's the derived form.

"The blunt beak version is a genetic innovation that occurred on the islands."

Hybrids, not separate species
This indicates that the species have continued to interbreed or hybridise, after diversifying when they first arrived on the islands.

Normally when two species start to develop independently, they reach a point where there are so many genetic differences that animals from the different lineages no longer mate, or their hybrid offspring are sterile - as is the case when a horse and a donkey produce a mule.

"It's been observed that the species of Darwin's finches sometimes hybridise - Peter and Rosemary Grant have seen that during their fieldwork," Prof Andersson told the BBC.

"But it's difficult to say what the long-term evolutionary significance of that is. What does it contribute?"

In fact, the team's findings suggest that hybridisation is very important to these birds' evolution. In particular, they saw that the rise and fall of pointed beaks in one species - once again, the medium ground finch - was driven by hybridisation with a different, pointed-beak species.

Geographically not as isolated as thought
Prof Peter Keightley from the University of Edinburgh, though largely convinced by the results, was less surprised that the finches had interbred so extensively.

"These islands are pretty close together. So it's not surprising that they are flying from one island to the other," he said.

Some of the traditional species might not, in fact, be genuinely distinct, he added.
Jonathan Webb

Darwin's finches continue to be in the focus of evolutionary biology as a case study. Modern advances in genetics provide accurate new methods to go and try to answer the questions and evaluate the suggestions originally made by Charles Darwin. As so often is case with living things, matters are not that black and white as we might think.

The reported research is published in Nature.