Monday, January 24, 2011

Monkeys and Weasels, or, Why Evolution Can Occur Rapidly

Many people, quite reasonably, wonder why they cannot see evolution happening. The usual scientific answer is that evolution happens so slowly that you cannot see it. While this is true, it is not because evolution has to be slow. Evolution can be very rapid.

Creationists never tire of saying that evolution cannot happen, because it would take forever for something to evolve. They like to use the monkey and typewriter (or, nowadays, word processor) example. Suppose you had a room—or a mall, or a continent—full of monkeys typing away at keyboards, randomly striking the keys. How long would it take for one of the monkeys to, by chance, type an entire Shakespeare play? I will not attempt the calculation, but we all know it would be a very, very long time. If the monkey and typewriter scenario were really the way that evolution works, then clearly evolution would be impossible. If we had to wait for evolution to, purely by chance, cough up a jellyfish, or a sequoia tree, or a human, we would be waiting essentially forever.

But evolution is not just a matter of chance. The mutations are produced by chance, but natural selection accumulates the good mutations non-randomly. This considerably changes the monkey and typewriter scenario. If a monkey types a letter at random, but it is the correct letter, natural selection will save it, and do the same with the second letter, the third, and so on. The monkeys, typing at random, may in this way quickly produce a work of Shakespeare, provided that each correct keystroke is preserved. Selection is a ratchet: it saves the good key strokes, and discards the bad ones. In one famous example of this process, evolutionary biologist Richard Dawkins used a computer program that started with a random series of letters. Each generation, the program introduced random mutations into the series of letters, and saved whichever mutations caused the string of letters to more closely resemble the Shakespearean phrase, “Methinks it is like a weasel.” This quote comes from Hamlet who was looking at the shapes of clouds. The clouds had random shapes but Hamlet was trying to select the ones that looked like animals. It took only about forty generations for the computer program, now popularly called the “weasel program,” to transform a string of random letters into the Shakespearean sentence. Forty generations is not very long. Therefore, in theory, evolution should proceed rapidly. The monkeys could go home before lunch.

This entry will appear in my book Life of Earth: Portrait of a Beautiful, Middle-Aged, Stressed-Out World, just released by Prometheus Books.

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