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But what good is half a wing? - The Evolution of Flight

One of the problems faced by evolutionary scientists is the origin of flight. For small animals, like insects, it is not too difficult. Sufficiently small organisms fly automatically. They have a hard time staying on the ground. As mass increases, the problem reverses. An animal the size of an elephant could never get enough muscle power and airlift to fly. 

Bird-size organisms, which obviously sometimes fly, provide us with the obvious problem: how did wings first evolve? Naturally, no creature was one day born with functional wings. And since evolution can't plan ahead, and will only select for traits that are benefitial here and now, anti-evolutionists have argued that organs like wings (and, even more popularly, the eye) never could have evolved by natural selection. How good is half a wing?

The most obvious possible answer to the origin of flight we can find from observing animals like the squirrel. It has a big tail that helps it float in the air as it jumps from tree to tree. Some species actually have wings that are sufficient to help it glide, but not good enough to fly. It is not hard to imagine a gradual evolution where the gliding ability increased slowly until one day one of them could fly.

An alternative hypothesis is that predators evolved wings to increase their speed while running, gradually being able to make longer and longer jumps. Since flying has evolved independently countless times on Earth, both of these scenarios may explain some occurences, but there are also instances where both scenarios raise some serious objections.

Kenneth Dial has just launched a third theory, perhaps most closely related to the second above. Accidentally, while doing experiments on partridge chicks, he observed that even young birds with partially formed wings were aided by them when in increasing running speed. Small improvements will, as it mostly does in evolution, mean the difference between life and death. And even adults sometimes prefer running while flapping its wings to flying when trying to avoid a pursuer, as the picture shows.

The answer to the original question is that half a wing is half as good as a whole wing, and sometimes better than no wings.

There is an interesting chapter devoted to the evolution of wings in Richard Dawkins' Climbing Mount Improbable.

Posted on my blog on Jan 19, 2003.


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Last update: 07.04.2004; 20:56:46.