Charles Darwin wrote in 1860 that “there seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.” Although many features of living things appear to be designed, Darwin’s theory was that they are actually the result of undirected processes such as natural selection and random variation.
Scientific theories, however, must fit the evidence. Two examples of the evidence for Darwin’s theory of evolution — so widely used that I have called them “icons of evolution” — are Darwin’s finches and the four-winged fruit fly. Yet both of these, it seems to me, show that Darwin’s theory cannot account for all features of living things.
Darwin’s finches consist of several species on the Galápagos Islands that differ mainly in the size and shape of their beaks. Beak differences are correlated with what the birds eat, suggesting that the various species might have descended from a common ancestor by adapting to different foods through natural selection. In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos to observe this process in the wild.
In 1977 the Grants watched as a severe drought wiped out 85 percent of a particular species on one island. The survivors had, on average, slightly larger beaks that enabled them to crack the tough seeds that had endured the drought. This was natural selection in action. The Grants estimated that twenty such episodes could increase average beak size enough to produce a new species.
When the rains returned, however, average beak size returned to normal. Ever since, beak size has oscillated around a mean as the food supply has fluctuated with the climate. There has been no net change, and no new species have emerged. In fact, the opposite may be happening, as several species of Galápagos finches now appear to be merging through hybridization.
Darwin’s finches and many other organisms provide evidence that natural selection can modify existing features — but only within established species. Breeders of domestic plants and animals have been doing the same thing with artificial selection for centuries. But where is the evidence that selection produces new features in new species?
New features require new variations. In the modern version of Darwin’s theory, these come from DNA mutations. Most DNA mutations are harmful and are thus eliminated by natural selection. A few, however, are advantageous — such as mutations that increase antibiotic resistance in bacteria and pesticide resistance in plants and animals. Antibiotic and pesticide resistance are often cited as evidence that DNA mutations provide the raw materials for evolution, but they affect only chemical processes. Major evolutionary changes would require mutations that produce advantageous anatomical changes as well.
Normal fruit flies have two wings and two “balancers” — tiny structures behind the wings that help stabilize the insect in flight. In the 1970s, geneticists discovered that a combination of three mutations in a single gene produces flies in which the balancers develop into normal-looking wings. The resulting four-winged fruit fly is sometimes used to illustrate how mutations can produce the sorts of anatomical changes that Darwin’s theory needs.
But the extra wings are not new structures, only duplications of existing ones. Furthermore, the extra wings lack muscles and are therefore worse than useless. The four-winged fruit fly is severely handicapped — like a small plane with extra wings dangling from its tail. As is the case with all other anatomical mutations studied so far, those in the four-winged fruit fly cannot provide raw materials for evolution.
In the absence of evidence that natural selection and random variations can account for the apparently designed features of living things, the entire question of design must be reopened. Alongside Darwin’s argument against design, students should also be taught that design remains a possibility.
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