Every few years we are treated to glowing news stories about “Darwin’s finches.” The latest, published today in The Washington Post, is titled “200 years after Darwin, this is how the iconic Galápagos finches are still evolving,” and, as usual, it is full of hype.
When Charles Darwin visited the Galápagos Islands in 1835, he collected specimens of the local wildlife. These included some finches that he threw into bags, many of them mislabeled. Although the Galápagos finches had little impact on Darwin’s thinking (he doesn’t even mention them in The Origin of Species), biologists who studied them a century later called them “Darwin’s finches” and invented the myth that Darwin had correlated differences in the finches’ beaks with different food sources (he hadn’t). According to the myth, Darwin was inspired by the finches to formulate his theory of evolution, though according to historian of science Frank Sulloway “nothing could be further from the truth.”
In the 1970s, biologists studied a population of medium ground finches on one of the islands in great detail. When a severe drought left only large, hard-to-crack seeds, 85 percent of the birds perished. The survivors had beaks that were about 5 percent larger than the average beak size in the original population. The biologists estimated that if similar droughts occurred once every ten years, the population could become a new species in only 200 years. In a 1999 booklet defending evolution, the U.S. National Academy of Sciences called the finches “a particularly compelling example” of the origin of species.
But after the drought, birds with smaller beaks flourished again, and the average beak size of the population returned to normal. No net evolution had occurred. No matter; Darwin’s finches became an icon of evolution that is still featured in most biology textbooks.
In the 1980s, a population of large ground finches, with larger beaks than the medium ground finches, migrated to the island. When a drought in 2004-2005 again reduced the food supply, the medium and large ground finch populations both declined. But since even the largest beaks among the medium ground finches were no match for the beaks of the large ground finches, the latter pretty much monopolized the larger seeds and the former had to make do with smaller seeds. This time, the medium ground finches that survived the drought had beaks that were smaller than the average size in the original population. Biologists studying the finches argued that birds with smaller beaks were better able to eat the tiny seeds that were left after the large ground finches ate the big ones, and they concluded that this was again an example of “evolutionary change.”
Then those biologists, together with some colleagues, studied DNA sequences in the medium ground finches. They correlated several regions of DNA with the 2004-2005 decrease in average beak size, and they concluded that one region in particular, called HMGA2, is associated with beak size. (Note, however, that HMGA2 did not cause the decrease.)
Enter The Washington Post. According to today’s article, the biologists “pinpointed the bit of finch DNA behind the swift transition” in average beak size and “now have a pretty thorough blueprint of how these famous finches evolve.”
Wait a minute. Average beak size increased slightly during one drought, only to return to normal after the rains return. Then average beak size decreased slightly during another drought. A region of DNA is correlated with beak size. And somehow that tells us how finches evolved in the first place?
As Winston Churchill might say, “Never in the field of science was so much based by so many on so little.”