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

One difference is that sexual selection through mate choice can be much more intelligent than natural selection. I mean this quite literally. Natural selection takes place as a result of challenges set by an animal's physical habitat and biological niche. The habitat includes the factors that matter to farmers: sunlight, wind, heat, rain, and land quality. The niche includes predators and prey, parasites and germs, and competitors from one's own species. Natural selection is just something that happens as a side-effect of these factors influencing an organism's survival chances. The habitat is inanimate and doesn't care about those it affects. Biological competitors just care about making their own livings. None of these selectors cares whether it imposes evolutionary selection pressures that are consistent, directional, efficient, or creative. The natural selection resulting from such selectors just happens, willy-nilly.

Sexual selection is quite different, because animals often have very strong interests in acting as efficient agents of sexual selection. The genetic quality of an animal's sexual partner determines, on average, half the genetic quality of their offspring. (Most animals inherit half their genes from mother and half from father.) As we shall see, one of the main reasons why mate choice evolves is to help animals choose sexual partners who carry good genes. Sexual selection is the professional at sifting between genes. By comparison, natural selection is a rank amateur. The evolutionary pressures that result from mate choice can therefore be much more consistent, accurate, efficient, and creative than natural selection pressures.

As a result of these incentives for sexual choice, many animals are sexually discriminating. They accept some suitors and reject others. They apply their faculties of perception, cognition, memory' and judgment to pick the best sexual partners they can. In particular, they go for any features of potential mates that signal their fitness and fertility.

In fact, sexual selection in our species is as bright as we are. Every time we choose one suitor over another, we act as an agent of sexual selection. Almost anything that we can notice about a person is something our ancestors might have noticed too, and might have favored in their sexual choices. For example, some of us fall in love with people for their quick wits and generous spirits, and we wonder how these traits could have evolved. Sexual choice theory suggests that the answer is right in front of us. These traits are sexually attractive, and perhaps simpler forms of them have been attractive for hundreds of thousands of years. Over many generations, those with quicker wits and more generous spirits may have attracted more sexual partners, or higher-quality partners. The result was that wits became quicker and spirits more generous.

Of course, sexual selection through mate choice cannot favor what its agents cannot perceive. If animals cannot see the shapes of one another's heart ventricles, then heart ventricles cannot be directly shaped by sexual selection--vivisection is not a practical method for choosing a sexual partner. A major theme of this book is that before language evolved, our ancestors could not easily perceive one another's thoughts, but once language had arrived, thought itself became subject to sexual selection. Through language, and other new forms of expression such as art and music, our ancestors could act more like psychologists--in addition to acting like beauty contest judges--when choosing mates. During human evolution, sexual selection seems to have shifted its primary target from body to mind.

This book argues that we were neither created by an omniscient deity, nor did we evolve by blind, dumb natural selection. Rather, our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could. We have inherited both their sexual tastes for warm, witty, creative, intelligent, generous companions, and some of these traits that they preferred. We are the outcome of their million-year-long genetic engineering experiment in which their sexual choices did the genetic screening.

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.