When Darwin was visiting the Galapagos Islands, he came across a strange assortment of birds which subsequent investigation revealed to be an assortment of different kinds of finches. These finches were so different from the finches found on the mainland that Darwin didn't even recognize them as finches and, while on the Galapagos Islands, didn't pay that much attention to them. It was only when he returned to England and shared his collection with fellow scientists that he realized what he'd missed. But it didn't take him long to realize the significance of the little beasties.
The finches on different islands have different morphological characteristics — things like the size and shape of the beak — that might reasonably be thought to have something to do with feeding. Darwin hypothesized that different selective pressures on the different islands had led the birds to diverge from each other morphologically. Specifically, he surmised that natural selection was at work (refer to Chapter 2).
Darwin hasn't been the only scientist interested in finches. Rosemary and Peter Grant of Princeton University are, too. And what's more, they set out to watch natural selection happen, and they've been doing it successfully for about thirty years.
In one particularly nice example, one of the larger finches which had not previously been present on the island where the Grants work took up residence there and began to compete for food with the smaller finch that was already there.
To grasp the significance of what happened next, you need to know a little bit about the structure of finches' beaks. A finch's beak structure determines what the bird can most efficiently eat: A bird can't crack large seeds with the tiny beak, and it can't pick up tiny seeds with a large beak. Both the larger and smaller finches had a preference for some large, yummy seeds found on the island. And both have bills of sufficient size to crack and eat these seeds.
Prior to the arrival of the larger finch on the island, the small finches had all the big nuts to themselves. When the larger finches arrived, they took over the prime feeding areas as a result of bigger beak size and hogged all the good food. This led to tough times for the smaller birds, especially during periods of low food availability; they weren't getting enough of the big seeds, and they couldn't easily forge on the smaller seeds. Tough times, and a lot of the smaller birds didn't make it, but some of the smaller birds had an easier time of it than others.
Which of the smaller birds were most successful? The ones with slightly smaller beaks. There was variation in beak size, and in the absence of the large seed food resource, the small finches with smaller beaks were able to get more food because they could more efficiently forage on smaller size seeds. As a result of this selective pressure, the big size of the smaller finch decreased. And that's natural selection in action!
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