Darwin’s finches are of course famous for being studied by Charles Darwin during his voyage on the Beagle; their remarkable biodiversity, for instance in their beak shapes, being one of the major pieces of evidence leading him to formulate the theory of evolution and natural selection. (The beaks of these finches, incidentally, formed the topic of a Pulitzer Prize-winning book, based in large part on the Grants’ research.) They consist of 14 different species, which are distributed separately and together in different combinations of islands in the Galápagos archipelago, which has the fortune to be a relatively pristine environment (it is part of Ecuador’s national park system). Their importance to evolutionary biology is due in part to the fact that their evolutionary radiation is very recent by evolutionary standards; indeed, data from their mitochondrial DNA and microsatellites, combined with an allozyme molecular clock, have shown that these species all evolved from a common ancestor about 2 or 3 million years ago. (For comparison, geological data shows that each of the islands of the archipelago emerged above sea level between 5 to 9 million years ago.)
Grant and her fellow researchers have tracked the finches on the island of Daphne Major in the Galápagos for the past 35 years – banding them, measuring them, and taking blood samples for DNA analysis. There are two major species of finches on this island: Geospiza scandens (common cactus-finch), which is a larger finch with a more pointed beak that feeds primarily on cactus seeds, and Geospiza fortis (medium ground-finch), which is a smaller finch with a stronger, blunter beak and a more varied diet, feeding on pollen and nectar as well as the smaller of the cactus seeds. Both finches have a fairly rapid life cycle, being fully grown after about two months, breeding every year, and living up to 16 years. They are genetically very similar, and in fact interbreed very occasionally (about 1% of the time), but are distinct enough to be considered different species. Here is a picture of the ancestry of these and other finches from the Encyclopedia Britannica: