Excerpt from The Mating Mind by Geoffrey F. Miller, plus links to reviews, author biography & more

Giving so much credit to sexual choice can make sexual selection sound almost too powerful. If sexual selection can act on any trait that we can notice in other individuals, it can potentially explain any aspect of human nature that scientists can notice too. Sexual selection's reach seems to extend as far as psychology's subject matter. So be it. Scientists don't have to play fair against nature. Physics is full of indecently powerful theories, such as Newton's laws of motion and Einstein's theory of general relativity. Darwin gave biology two equally potent theories: natural selection and sexual selection. In principle, his two theories explain the origins of all organic complexity, functionality, diversity, and beauty in the universe. Psychologists generally believe that so far they have no theories of comparable power. But sexual selection can also be viewed as a psychological theory, because sexual choice and courtship are psychological activities. Psychologists are free to use sexual selection theory just where it is most needed: to explain mental abilities that look too excessive and expensive to have evolved for survival.

This sexual choice view also sounds rather circular as an explanation of human mental evolution. It puts the mind in an unusual position, as both selector and selectee in its own evolution. If the human mind catalyzed its own evolution through mate choice, it sounds as though our brains pulled themselves up by their own bootstraps. However, most positive-feedback processes look rather circular, and a positive-feedback process such as sexual selection may be just what we need to explain unique, highly elaborated adaptations like the human mind. Many theorists have accepted that some sort of positive-feedback process is probably required to explain why the human brain evolved to be so large so quickly. Sexual selection, especially a process called runaway sexual selection, is the best-established example of a positive-feedback process in evolution.

Positive-feedback systems are very sensitive to initial conditions. Often, they are so sensitive that their outcome is unpredictable. For example, take two apparently identical populations, let them undergo sexual selection for many generations, and they will probably end up looking very different. Take two initially indistinguishable populations of toucans, let them choose their sexual partners over a thousand generations, and they will evolve beaks with very different colors, patterns, and shapes. Take two populations of primates, and they will evolve different hairstyles. Take two populations of hominids (bipedal apes), and one may evolve into us, and the other into Neanderthals. Sexual selection's positive-feedback dynamics make it hard to predict what will happen next in evolution, but they do make it easy to explain why one population happened to evolve a bizarre ornament that another similar population did not.

Excerpted from The Mating Mind by Geoffrey Miller Copyright© 2000 by Geoffrey Miller. Excerpted by permission of Doubleday, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.