As a graduate student in Boston, I was enlisted to help a senior scientist who had written a paper about whether it was more efficient for warm-blooded animals to run on two legs or four. He planned to submit the paper to Nature, one of the most prestigious scientific journals, and asked me to help him take a photograph striking enough to land on the journal cover and call attention to his work. Eager to get out of the laboratory, I spent an entire afternoon chasing a horse and an ostrich around a corral, hoping to get them to run side by side, demonstrating both types of running in a single frame. Needless to say, the animals refused to cooperate, and, all species being exhausted, we finally gave up. Although we never got the picture, the experience did teach me a biology lesson: ostriches can't fly, but they can still use their wings. When they're running, they use their wings for balance, extending them to the sides to keep from toppling over. And when an ostrich becomes agitated—as it tends to do when you chase it around a corral—it runs straight at you, extending its wings in a threat display. That's a sign to get out of the way, for a miffed ostrich can easily disembowel you with one swift kick. They also use their wings in mating displays, and spread them out to shade their chicks from the harsh African sun.

The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co-opted for new uses. Like all flightless birds, ostriches are descended from flying ancestors. We know this from both fossil evidence and from the pattern of ancestry that flightless birds carry in their DNA. But the wings, though still present, can no longer help the birds take flight to forage or escape predators and bothersome graduate students. Yet the wings are not useless—they've evolved new functions. They help the bird maintain balance, mate, and threaten its enemies.

The African ostrich, isn't the only flightless bird. Besides the ratites—the large flightless birds that include the South American rhea, the Australian emu, and the New Zealand kiwi—dozens of other bird species have independently lost the ability to fly. These include flightless rails, grebes, ducks, and, of course, penguins. Perhaps the most bizarre is the New Zealand kakapo, a tubby flightless parrot that lives mainly on the ground but can also climb trees and "parachute" gently to the forest floor. Kakapos are critically endangered: fewer than 100 still exist in the wild. Because they can't fly, they are easy prey for introduced predators like cats and rats.

All flightless birds have wings. In some, like the kiwi, the wings are so small—only a few inches long and buried beneath their feathers— that they don't seem to have any function. They're just remnants. In others, as we saw with the ostrich, the wings have new uses. In penguins, the ancestral wings have evolved into flippers, allowing the bird to swim underwater with amazing speed. Yet they all have exactly the same bones that we see in the wings of species that can fly. That's because the wings of flightless birds weren't the product of deliberate design (why would a creator use exactly the same bones in flying and flightless wings, including the wings of swimming penguins?), but of evolution from flying ancestors.

Opponents of evolution always raise the same argument when vestigial traits are cited as evidence for evolution. "The features are not useless," they say. "They are either useful for something, or we haven't yet discovered what they're for." They claim, in other words, that a trait can't be vestigial if it still has a function, or a function yet to be found.

But this rejoinder misses the point. Evolutionary theory doesn't say that vestigial characters have no function. A trait can be vestigial and functional at the same time. It is vestigial not because it's functionless, but because it no longer performs the function for which it evolved. The wings of an ostrich are useful, but that doesn't mean that they tell us nothing about evolution. Wouldn't it be odd if a creator helped an ostrich balance itself by giving it appendages that just happen to look exactly like reduced wings, and which are constructed in exactly the same way as wings used for flying?

Indeed, we expect that ancestral features will evolve new uses: that's just what happens when evolution builds new traits from old ones. Darwin himself noted that "an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose."

But even when we've established that a trait is vestigial, the questions don't end. In which ancestors was it functional? What was it used for? Why did it lose function? Why is it still there instead of having disappeared completely? And which new functions, if any, has it evolved?

Let's take wings again. Obviously, there are many advantages to having wings, advantages shared by the flying ancestors of flightless birds. So why did some species lose their ability to fly? We're not absolutely sure, but we do have some powerful clues. Most of the birds that evolved flightlessness did so on islands—the extinct dodo on Mauritius, the Hawaiian rail, the kakapo and kiwi in New Zealand, and the many flightless birds named after the islands they inhabit (the Samoan wood rail, the Gough Island moorhen, the Auckland Island teal, and so on). As we'll see in the next chapter, one of the notable features of remote islands is their lack of mammals and reptiles—species that prey on birds. But what about ratites that live on continents, like ostriches? All of these evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.

The long and short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you're flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also, wings are large appendages that are easily injured. If they're unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly.

So why haven't they disappeared completely? In some cases they nearly have: the wings of the kiwi are functionless nubs. But when the wings have assumed new uses, as in the ostrich, they will be maintained by natural selection, though in a form that doesn't allow flight. In other species, wings may be in the process of disappearing, and we're simply seeing them in the middle of this process.

Excerpt from Why Evolution is True. © Jerry A. Coyne, 2009. All rights reserved.