Planet Earth

A Career Among The Finches

The LoomBy Carl ZimmerOct 1, 2008 8:36 PM


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I just got back from a pretty remarkable lecture by the husband-and-wife team of Peter and Rosemary Grant. The Grants started studying Darwin's finches on the Galapagos Islands in 1973, and they made some of the most detailed studies of evolution in the wild ever carried out. Their adventures were chronicled 14 years ago by Jonathan Weiner in the Beak of the Finch, which won the Pulitzer Prize. But the Grants did not stop. They continued to observe the birds evolve, and make fascinating new discoveries. In 2002, I wrote an article on what they'd learned after some three decades of research. Six years have passed since then, and at today's lecture I learned that they retired from Princeton just last month. But apparently they're still going back to the Galapagos. So their lecture today was not a summing up, but yet another status report. In precisely one hour, they ripped through 35 years of research on natural selection and the origin of species. These were, of course, the obsessions of Charles Darwin, and it was the finches of the Galapagos Islands that helped him develop his theory of evolution. The Grants returned 114 years after the publication of the Origin of Species, and also after the subsequent discovery of how to measure natural selection with statistics. Darwin's finches, the Grants realized, would be a good group of birds in which to study how natural selection takes place, and how new species arise. They arrived a few million years ago on the islands from the mainland, and have since evolved into 14 species. On many of the islands, they still enjoy a relatively pristine life, and so far as anyone knows, none have become extinct since human contact. Since they arrived, they've evolved a wide range of beaks, from giant plier-like forms to slender needle-nose ones. The Grants have some clues to how these different beaks evolved--starting from within. Working with Harvard experiments on genes that build embryos, they found that a couple genes have a major effect on the shape of beaks. A gene called Bmp4 is active in different patterns in different beak shapes. In large, deep beaks, it's active sooner, in a wider patch of cells on a bird embryo's face, and makes more proteins. When the Harvard researchers mimicked the differences in Bmp4 activity in chicken embryos, they produced a similar range of giant and shrunken beaks. The Harvard team found that a different protein, called calmodulin, influences how pointing the beak gets. The more calmodulin in a species, the pointier the beak. These kinds of differences between Darwin's finches had to have evolved after their ancestors arrived on the Galapagos Islands. The Grants predicted that certain variations would be favored by natural selection. As a population of birds spread from one species to another, certain variations would be favored, because birds that carried them would be able to get more food. After hundreds of thousands of years, the birds from one island might arrive on another one, where they would compete with the resident finches. The Grants expected that the two populations might then evolve to become different from each other, because the ones that avoided competition would do best.

At the lecture, the Grants offered a striking case of a bird adapted to different foods on different islands: the sharp-beaked ground finch. On one island, they eat hard seeds and insects. On another island they probe flowers or eat smaller seeds. And on yet another island they hammer at the eggs of other birds or peck the birds themselves until they bleed, and then drink up the blood like feathered vampires. On each island, the sharp-beaked ground finches have differently shaped beaks adapted for their particular diet. To show that beaks really do evolve by natural selection, the Grants focused their attention on a desolate island called Daphne Major, where medium ground finches split their time eating big hard spiked seeds, or small soft seeds. They measured the size of beaks on all the finches from one generation to the next. Birds with big beaks tended to have chicks with big beaks, and small beaks begat small beaks. So the beaks fulfilled the first major requirement of natural selection: they were a traits with variations that could be inherited. A major drought hit the island in 1977, and 85% of the birds died. Having big beak raised the odds of a bird surviving, because it meant the animal could crack the hard spiked seeds. The Grants discovered that within a few years the population of finches the recovered. But now their beaks were, on average, 4% deeper. Finches with big beaks had a better chance of surviving the drought and could thus produce a bigger fraction of the next generation. In other words, natural selection caused the average size of medium ground finch beaks to increase. Five years later, the Grants were able to see natural selection at work again. At the end of 1982, heavy rains came to the islands. Daphne major turned green, and the plants with small seeds were particularly abundant. Now small-beaked birds had the advantage. They could eat small seeds more efficiently than the big-beaked ones, allowing them to grow faster and have more energy for producing offspring. Now natural selection favored them. The average size of beaks decreased by 2.5%. Long-term studies like those of the Grants are all too rare, and it's a shame. If the Grants had stopped going to Daphne Major after 30 years, they would have missed an astonishing event that occurred just the year after. In 2004, another drought came. And this time the beaks of the finch did not get big, as they had in 1977. Instead, they got a lot smaller. What was different was that a new bird had arrived--the large ground finch. These bruisers showed up in 1982 on Daphne Major but their numbers grew very slowly, in part because the wet 1980s favored birds that could eat small seeds rather than big hard ones. But by the time the drought of 2004 struck, there were enough large ground finches to compete with the medium ground finches for the big hard seeds. A lot of birds from both species died, but, once more, the deaths were not random. Medium ground finches could not compete with the large ground finches, so the survivors had smaller beaks. This process is known as character displacement, and it may well have helped the finches diverge into distinct forms. But Darwin's finches are still very early in the journey to distinct species. Studies on many birds have consistently shown that it takes about 32 million years before two diverging lineages of birds can no longer interbreed. On Daphne major, the medium ground finches interbreed with another species, called the cactus finch, which has a long beak it uses to pick out food from cactuses without getting poked by the cactus spines. The two species don't interbreed a lot, though. To explain why, the Grants explained how they learn to sing. Male chicks aren't born with their song programmed in their heads; they must learn their songs from adult males--typically, their own father. The Grants described one male finch that got a cactus spine stuck in his throat in the 1970s and proceeded to sing a croaky song. Today, his descendants still sing with a croak, even with perfectly healthy throats. On rare occasion, a male medium ground finch may end up learning the song of the cactus finch (or vice versa). This mismatch may happen if, for example, a cactus finch takes over a medium ground finch nest and dumps out all the eggs to lay its own--but accidently leaves a medium ground finch egg behind. A father finch may die, leaving his sons to learn songs of males in the neighborhood. When these birds start to sing the wrong song, they attract females from the other species and mate. The hybrids do just fine when food is abundant, but they are not producing a new species of their own. No hybrid song has emerged, so they end up mating with a member of one of the parental species. The Grants argue that this hybridization is pretty common on the Galapagos Islands. They compared the genetic similarity between the two species on several other islands, and found that a species on any given island was more closely related to the other species on its own island than to the other species on other islands. Ironically, though, the Grants don't believe that hybridizing is going to make two budding species collapse back in on each other. Instead, they suspect that the hybridization infuses each species with new variations on their genes. A general rule of natural selection is that it is stronger when it has more genetic variation to act on. So interbreeding may be allowing the birds to adapt faster to their own niches. Only much later, when they can no longer interbreed, will they become more at risk of extinction because they've lost this supply of fresh genes. It's an intriguing hypothesis, one that could be tested with further study. Given the excitement and vigor with which the Grants gave their lecture, I expect they'll be spending their retirement back on the islands, trying to test it. (For more on the Grants' recent work, see their new book, How and Why Species Multiply.)

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