The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (9 page)

arguments make little evolutionary sense. In most species surplus energy is converted into fat, not creativity. Surplus brain-mass that yielded no survival or reproductive advantages would quickly be eliminated by selection.

If Darwin had found that male animals choose female mates selectively and that many females are highly ornamented to attract male attention, would Wallace and his contemporaries have been so skeptical about sexual choice? I think not. For male Victorian scientists, it was taken for granted that young single ladies should wear brilliant dresses and jewels to attract the attention of eligible bachelors. Male scientists had direct personal insight into male mate choice. They might easily have sympathized with male animals had Darwin credited them with powers of sexual discernment. They did sympathize with male animals engaged in violent contests with other males for the "possession" of females, which is presumably why they were able to accept Darwin's theory that male weaponry evolved for sexual competition. They simply did not like to think of males as sexual objects accepted or rejected by female choice. (This point is often overlooked by Darwin's feminist critics, who unfairly portray him as embodying Victorian social attitudes.)

The rejection of Darwin's female choice theory was, I think, due to ideological biases in 19th-century natural history, especially

the unthinking sexism of most biologists other than Darwin. The rejection was cloaked in scientific argumentation, but the motivations for rejection were not scientific. Many male scientists at the time wrote as if female humans were barely capable of cognition and choice in any domain of life. Female animals were held in even greater contempt, as mere egg repositories to be fought over by males. Male scientists were willing to believe that combat between males, analogous to careerist economic competition in capitalist society, could account for many bodily and behavioral features of male animals. But they could not accept that the sexual whims of female animals could influence the stately progress of evolution.

Wallace paid a high price for his rejection of female choice. He

recognized that the human mind contains many biological adaptations, such as elaborate language, music, and art, that seem impossible to explain as outcomes of natural selection for survival value. With more field experience among the primitive tribes of Oceania than Darwin ever amassed on his
Beagle
voyage, Wallace appreciated more acutely than Darwin how striking these adaptations were. He held the musical talents of the Pacific Islanders and African tribal peoples in the highest regard, but could find no survival value in their songs and dances. By rejecting sexual selection for ornamentation, he rejected the one process that might have explained such adaptations. Wallace found himself allied with anti-Darwinians who claimed that evolution could never account for human consciousness, intelligence, or creativity. Though he remained an evolutionist about everything else, Wallace became a creationist about the "human spirit." He went to seances. He developed interests in mesmerism and spiritualist charlatans. He died convinced that science could never fathom the origins or nature of the human mind.

Mendelian Exile

The years 1871 to 1930 were one long dry spell for sexual selection theory Wallace's criticisms were especially damaging, and gave female choice a bad name. Within a few years of Darwin's death in 1882, sexual selection had already come to be regarded by most biologists as a historical curiosity. Especially hard hit was Darwin's claim that sexual choice played a major role in human evolution. Edward Westermarck's
History of Human Marriage
of 1894 spent hundreds of pages trying to undermine the idea that premodern humans were free to choose their sexual partners. He thought that traditional arranged marriages destroyed any possibility of sexual selection. Like most anthropologists of his era, he saw women as pawns in male power games, and young lovers as dominated by matchmaking parents. He founded the tradition of seeing marriage primarily as a way of cementing alliances between families, a view that dominated anthropology until the last years of the 20th century

Not all biologists were hostile to sexual selection. August Weismann, a leading Darwinian at the University of Freiburg in Germany, included a positive chapter on sexual selection in his
The Evolution Theory
of 1904. After discounting Wallace's surplus-energy theory, and supporting and adding to Darwin's examples of sexual ornamentation, Weismann concluded that "sexual selection is a much more powerful factor in transformation than we should at first be inclined to believe." He added, "Darwin has shown convincingly that a surprising number of characters in animals, from worms upwards, have their roots in sexual selection, and has pointed out the probability that this process has also played an important part in the evolution of the human race." Nonetheless, Weismann's thoughtful assessment was swept away in the rising tide of genetics.

The rediscovery around 1900 of Mendel's work on genetics distracted biologists from Darwin's ideas. For young biologists at the turn of that century, genes were the way forward. Sexual selection was dead, and even natural selection was an unfashionable hobby of the older generation. Biology entered a reductionistic phase of empiricism. Laboratory experiments on mutations attracted more attention and respect than grand theories of natural history. One of the leaders of the new genetics was Thomas Hunt Morgan, a Nobel prize-winner for his work on fruit fly mutations. In his 1903 book
Evolution and Adaptation,
Morgan dismissed sexual selection, concluding that "the theory meets with fatal objections at every turn." He proposed that sex hormones account for all sex differences in ornamentation, failing to realize that the sex hormones and their sex-specific effects themselves require an evolutionary explanation. Morgan's brave new world of mutated flies bred in bottles won over Darwin's world of ornamented butterflies breeding in the wild.

The Fisher King

It was several decades later that the novelty of breeding mutated fruit flies wore off, and some biologists rediscovered Darwin's ideas. One of these young thinkers was Ronald Fisher, whose

career spanned the first half of the 20th century. Fisher was a polymath whose insights shaped many fields. To biologists, he was an architect of the "modern synthesis" that used mathematical models to integrate Mendelian genetics with Darwin's selection theories. To psychologists, Fisher was the inventor of various statistical tests that are still supposed to be used whenever possible in psychology journals. To farmers, Fisher was the founder of experimental agricultural research, saving millions from starvation through rational crop breeding programs. In each case, Fisher brought his powerful mathematical brain to bear on questions that had previously been formulated only vaguely and

verbally.

Fisher considered Darwin's theory of mate choice to be one vague idea worth trying to formalize. In his first paper on sexual choice in 1915, Fisher enthused that "Of all the branches of biological science to which Charles Darwin's life-work has given us the key, few if any, are as attractive as the subject of sexual selection." Fisher understood that to make sexual selection scientifically respectable, he had to explain the origins of sexual preferences. In particular, Darwin failed to offer any explanation for female choice. Why should females bother to select male mates for their ornaments? Fisher's breakthrough was to view sexual preferences themselves as legitimate biological traits that can vary, that can be inherited, and that can evolve. In his 1915 paper he faced the problem squarely: "The question must be answered 'Why have the females this taste? Of what use is it to the species that they should select this seemingly useless ornament?'" Later, in a 1930 book, Fisher emphasized that "the tastes of organisms, like their organs and faculties, must be regarded as the product of evolutionary change, governed by the relative advantages which such tastes confer." While Darwin had left sexual preferences as mysterious causes of sexual selection, Fisher asked how sexual preferences themselves evolved.

In thinking about the evolution of sexual preferences, Fisher developed the two major themes of modern sexual selection theory. The first idea is the more intuitive, and concerns the

information conveyed by sexual ornaments. In the 1915 paper, Fisher speculated thus:

Consider, then, what happens when a clearly marked pattern of bright feathers affords ... a fairly good index of natural superiority. A tendency to select those suitors in which the feature is best developed is then a profitable instinct for the female bird, and the taste for this "point" becomes firmly established ... Let us suppose that the feature in question is in itself valueless, and only derives its importance from being associated with the general vigor and fitness of which it affords a rough index.

Fisher proposed that many sexual ornaments evolved as indicators of fitness, health, and energy. Suppose that healthier males have brighter plumage. Females may produce more and healthier offspring if they mate with healthier males. If they happen to have a sexual preference for bright plumage, their offspring will automatically inherit better health from their highly fit fathers. Over time, the sexual preference for bright plumage would become more common because it brings reproductive benefits. Then, even if bright male plumage is useless in all other respects, it will become more common among males simply because females prefer it. Fisher understood that preferences for fitness indicators could hasten the effect of natural selection, and could potentially affect both sexes. Unfortunately, Fisher's fitness-indicator idea was forgotten until the 1960s.

Fisher's other idea, the concept of runaway sexual selection, attracted more interest because it sounded much stranger. In fact,

it was so strange that Thomas Hunt Morgan had first aired the

idea in 1903 as a counter argument against sexual selection. Morgan asked what would happen if female birds had a tendency

to prefer plumage slightly brighter man the males of their species currently possess. He realized that the males would evolve brighter plumage under the pressure of female choice, but that the females would still not be satisfied. They would just move the

goal posts, demanding still more extreme ornamentation. Morgan mocked, "Shall we assume that ... the two continue heaping up the ornaments on one side and the appreciation of these ornaments on the other? No doubt an interesting fiction could be built up along these lines, but would anyone believe it, and, if he did, could he prove it?" To Morgan, the possibility of an endless arms rate between female preferences and male ornaments was an evolutionary impossibility that exposed the whole idea of sexual selection as a fallacy. But Fisher was used to integrating equations for exponential growth, and understood the speed and power of positive-feedback processes. He realized that an arms race between female preferences and male ornaments, far from undermining the theory of sexual selection, could offer an exciting possibility for explaining sexual ornamentation.

The idea of runaway sexual selection appeared in Fisher's masterpiece of 1930,
The Genetical Theory of Natural Selection.
Whenever attractive males can mate with many females and leave many offspring, the sexual preferences of females can drive male ornaments to extremes. Fisher suggested that when this happens, female preferences will evolve to greater extremes as well. This is because a female who prefers a super-ornamented male will tend to produce super-ornamented sons, who will be super-attractive to other females, and who will therefore produce more grandchildren. Evolution will favor super-choosy females for this reason. Yet the choosier the females become, the more extreme the male ornamentation will become in response. Both sexes end up on an evolutionary treadmill. The female preferences and male ornaments become caught up in a self-reinforcing cycle, a positive-feedback loop.

Fisher speculated that whenever the most ornamented individuals gain a large reproductive advantage, there is "the

potentiality of a runaway process, which, however small the

beginnings from which it arose, must, unless checked, produce great effects, and in the later stages with great rapidity." This

runaway process, Fisher claimed, could make ornaments evolve with exponentially increasing speed. They would evolve until the

ornaments become so cumbersome that their massive survival costs finally outweigh their enormous sexual benefits: "both the feature preferred and the intensity of preference will be augmented together with ever-increasing velocity, causing a great and rapid evolution of certain conspicuous characteristics, until the process can be arrested by the direct or indirect effects of Natural Selection." I shall explore the runaway process more thoroughly in the next chapter.

Like many mathematical geniuses presenting startling ideas, Fisher thought that runaway sexual selection was so obviously plausible that he did not need to present a detailed proof that it could work. He left that as an exercise for the reader. However, most mathematically talented scientists of the 1930s probably took up the challenge of quantum physics rather than evolutionary biology, and of those who went into biology, nobody took up Fisher's challenge.

Modern Exile

Sexual selection theory has been haunted by unconstructive critics. Whenever a new sexual selection idea raised its head, there was always an eminent biologist ready to knock it down. Wallace attacked female choice in animals, and Westermark attacked female choice in humans. After Fisher proposed his ideas about fitness indicators and the runaway process, the eminent biologist Julian Huxley attacked those too, in two widely influential papers criticizing sexual selection in 1938.