The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (11 page)

The controversy over Zahavi's idea marked the true revival of sexual selection theory. Within ten years of his 1975 paper, more research was published on sexual selection than in the previous hundred years. Fisher's fitness-indicator idea was finally in play, its share value boosted by Zahavi's takeover bid. Soon Fisher's runaway process attracted more intellectual capital as well. In 1980 Peter O'Donald published
Genetic Models of Sexual Selection,
summarizing twenty years of thinking about the mathematics of sexual selection. This inspired a spate of new mathematical modeling. In the early 1980s Russell Lande and Mark Kirkpatrick showed that Fisher's runaway process could indeed work. The genes underlying female choice really could get swept up in a positive-feedback loop with the genes underlying male sexual ornaments. Species could even split apart into new species entirely as a result of diverging sexual preferences. Critics attacked these runaway models, leading to the kind of rapid revision and rethinking that marks the most productive epochs of science.

Evolutionary controversies attract experimental biologists. For most of the 20th century, the experimental techniques existed for testing Darwin's basic idea that females choose their mates for their ornamentation. Experimental psychology had developed sophisticated methods and statistical tests for investigating how

people make choices. These could have easily been applied to animals. But the work was not done, because biologists thought that sexual selection had been dismissed by the leading theorists. Once the theorists revived the ideas of fitness indicators and runaway processes, the experimenters took a fresh look at mate choice. In species after species, females were seen to show preferences for one male over another, for beautiful ornaments over bedraggled ones, for a higher level of fitness over a lower. Female choice was observed by Linda Partridge in fruit flies, by Malte Andersson in widowbirds, and by Michael Ryan in Tungara frogs. David Buss even showed evidence of mate choice in humans. Wherever males had sexual ornaments, females seemed to show sexual choice, just as Darwin predicted.

Sexual Selection Triumphant

Within a few years, sexual selection became the hottest area of evolutionary biology and animal behavior research. Before this revival, sexual selection was caught in a double bind. Nobody did experiments on mate choice because theorists doubted its existence. And nobody did theoretical work on sexual selection because there was no experimental evidence for mate choice. Once this vicious circle was broken by John Maynard Smith, George Williams, Amotz Zahavi, Robert Trivers, and other pioneers, Darwin's favorite idea was free to succeed.

Sexual selection's revival has been swift, dramatic, and unique. It may be the only major scientific theory to have become accepted after a century of condemnation, neglect, and misinter-

pretation. Throughout the 1990s, sexual selection research became one of the most successful and exciting areas of biology,

dominating the leading evolution journals and animal behavior

conferences. Helena Cronin's
The Ant and the Peacock
put sexual selection in its historical context, reminding biologists where it

came from and where it might go, Malte Andersson's 1994 textbook
Sexual Selection
reviewed the state of the art for a new generation of scientists. Sexual selection became the most fruitful idea in the emerging science of evolutionary psychology. After a

hundred years of neglect,
The Descent of Man
was once more being read—and not just for what it has to say on human evolution.

What Sexual Selection's Exile Costs the Human Sciences

Sexual selection's century of exile from biology had substantial costs for other sciences. Anthropologists paid little attention to human mate choice in the tribal peoples they studied for most of this century. By the time mate choice was accepted as an important evolutionary factor, most of those tribal peoples had been exterminated or assimilated. Psychologists had little evolutionary insight into human sexuality and their discipline was dominated for decades by Freudianism. Almost all of 20th-century psychology developed without considering the possibility that sexual selection through mate choice might have played a role in the evolution of human behavior, the human mind, human culture, or human society Following Marx, the social sciences saw a culture's mode of production as more important than its mode of reproduction. Economists had no explanation for the importance of "positional goods" that advertise one's wealth and rank in comparison to sexual rivals. In the other human sciences as well—archeology, political science, sociology, linguistics, cognitive science, neuroscience, education, and social policy—there was a blind spot where the theory of sexual selection should have been.

When these sciences did try to trace the evolutionary roots of human behavior, they have usually come up with theories based on "survival of the fittest" and "the goods of the species." Mate choice was simply not on the intellectual map as an evolutionary force. Darwin's broader vision, in which most of nature's ornamentation arises through sexual courtship, was never used to explain the ornamental aspects of human behavior and culture.

For example, without sexual selection theory, 20th-century science had great difficulty in explaining the aspects of human nature most concerned with display status, and image. Economists could not explain our thirst for luxury goods and conspicuous consumption. Sociologists could not explain why

men seek wealth and power more avidly than women. Educational psychologists could not explain why students became so rebellious and fashion-conscious after puberty Cognitive scientists could not fathom why human creativity evolved. In each case, apparent lack of "survival value" made human behavior appear irrational and maladaptive.

More generally, the sciences concerned with human nature have often lamented their incompleteness, fragmentation, and isolation. People are certainly complicated entities to study, but other sciences such as organic chemistry climate modeling, and computer science have coped with high degrees of complexity. The limited success of the human sciences may not have resulted from the complexity of human behavior, but from overlooking Darwin's crucial insight about the importance of sexual competition, courtship, and mate choice in human affairs.

Today, evolutionary biology is proclaiming that the old map of evolution was wrong. It put too much weight on the survival of the fittest and, until the 1980s, virtually ignored sexual selection through mate choice. Yet in the human sciences we are still using the old map, and we still do not know where we came from, or where we are going. The next few chapters offer a new map of evolution to help us find our way.

3

The Runaway Brain

The worlds of academia, high fashion, religion, and modern art produce sublime wonders, and sometimes monstrous absurdities. They can afford such creative freedom because their systems of self-regulation and self-perpetuation are insulated from the mundane pragmatics of the outside world. Their autonomy endows them with liberty and creative power. They are free to evolve under their own momentum, along lines of their own choosing, without having to justify themselves at every step to outside critics.

Sexual selection can work similarly. One of sexual selection's central processes allows species to evolve in arbitrary directions under their own momentum. We shall see how this process, Fisher's runaway process, can provide a pretty good first model for how the human mind evolved.

Evolution's Autarch

Under natural selection, species adapt to their environments. When the environment refers to a species' physical habitat, this seems simple enough. If a species lives in the Arctic, it had better evolve some warm fur. Under sexual selection, species adapt too, but they adapt to themselves. Females adapt to males, and males adapt to females. Sexual preferences adapt to the sexual ornaments available, and sexual ornaments adapt to sexual preferences.

This can make things quite confusing. In sexual selection, genes do not code just for the adaptations used in courtship, such as sexual ornaments. They also code for the adaptations used in mate choice, the sexual preferences themselves. What the physical

environment is to natural selection, sexual preferences are to sexual selection. They are not only the tastes to which sexual ornaments must appeal, but the environment to which they must adapt.

With sexual selection, genes act as both the fashion models and the fashion critics, both the apostates and the inquisitors. This creates the potential for the same kind of feedback loops that drive progress in high fashion and modern theology. These feedback loops are the source of sexual selection's speed, creativity, and unpredictability. Yet they also raise the classic problem of runaway corruption in autarchies: who watches the watchmen? How can mere genes be trusted as both selectors and selectees in evolution under sexual selection? The world of mate choice plays by its own rules, and though survival is a prerequisite for mating (as it is for scholarship, fashion, and faith), the principles of sexual selection cannot be reduced to the principles of survival. The biologist seems to have no point of entry into this protean wonderland where genes build brains and bodies, which pick the genes that build the next generation's brains and bodies, which in turn pick the genes that pick the genes. . .

Imagine the headaches if natural selection worked that way. Organisms would select which environments exist, as well as environments selecting which organisms exist. Strange, unpredictable feedback loops would arise. Would the feedback loop between polar bears and Arctic tundra result in a tundra of Neptunian frigidity where bears have fur ten feet thick, or a tundra of Brazilian sultriness where bears run nude? Would migratory birds select for more convenient winds, lower gravity, and more intelligible constellations? Or just an ever-full moon that pleasingly resembles an egg? Evolutionary prediction seems impossible under these conditions. Yet this is just what happens with sexual selection: species capriciously transform themselves into their own sexual amusements.

Introducing sexual selection in this way is more than just an attempt to encourage you to share my belief that it is one of the weirdest and more wonderful of nature's phenomena. That I

could achieve simply by presenting the standard catalog of sexual selection's "greatest hits": the peacock's tail, the nightingale's song, the bowerbird's nest, the butterfly's wing, the Irish elk's antlers, the baboon's rump, and the first three Led Zeppelin albums. By presenting sexual selection as a strange world of genes selecting other genes, I have tried to provoke a different question: How could one ever make a science out of sexual selection? Darwin showed that sexual selection exists and documented its effects, but it took another century before biologists had the scientific tools for explaining why sexual selection produces certain kinds of traits and not others. To understand how sexual selection shaped human mental evolution, we need to become familiar with this new toolbox of ideas and models. Let's first have a better look at Fisher's runaway process. It is the best example of how sexual selection exercises a power distinct from natural selection.

How Runaway Works

When Fisher's runaway process first appeared in print in 1930, other scientists greeted it with suspicion. Runaway did not fit the prevailing emphasis on the good of the species, the efficiency of survival adaptations, and the modernist machine aesthetic. Yet despite its frosty initial reception, runaway has finally been invited back to the center of the evolutionary stage. Theoretical biologists in the 1980s showed that Fisher was right: runaway can work. Indeed, it works so well that it is hard to avoid when sexual selection is in play. Because runaway may have had an important role in the evolution of the human mind, it is important to understand it as fully as possible. What follows is the simplest example of runaway I can offer, although the theory is subtle, and demands some concentrated attention.

Imagine a population of birds with short tails, in which the males contribute nothing to raising the offspring. Although this makes life hard for females after mating, it allows females to choose any male they want, even a male who has been chosen by many other females already The most attractive male could mate with many females. He has no reason to turn down a sexual

invitation from any female, because copulation costs so little time and energy.

Within this population, different males inevitably have different tail lengths Just as they have different wingspans, and different leg lengths. All biological traits show variation. Usually, much of that variation is heritable (that is, due to genetic differences between individuals), so longer-tailed males will tend to produce longer-tailed offspring. In other words, tail length varies and tail length is heritable, satisfying two out of Darwin's three requirements for evolution.

Now, suppose that some of the females become sexually attracted to tails that are longer than average. (It doesn't matter why they evolve this preference—perhaps there was a mutation affecting their sexual preferences, or their vision happened to respond more positively to large than to small objects.) Once this female preference for long tails arises, we have the third requirement for evolution: selection. In this case, it is sexual selection through mate choice. The choosy females who prefer long tails will tend to mate with long-tailed males, who are happy to copulate with all their admirers. The non-choosy females mate randomly, usually ending up with an average-tailed male.