MYTH 14

THAT AFTER DARWIN (1871), SEXUAL SELECTION WAS LARGELY IGNORED UNTIL ROBERT TRIVERS (1972) RESURRECTED THE THEORY

Erika Lorraine Milam

Like almost everything else important in evolutionary theory, Charles Darwin discovered sexual selection.… After Darwin, the concept of sexual selection was largely ignored until 1972, when Robert L. Trivers resurrected the topic.

—John Alcock, Animal Behavior: An Evolutionary Approach (1989)

It took time for Robert Trivers’s (b. 1943) professional reputation to grow. Before enrolling as a biology graduate student at Harvard University in the late 1960s, Trivers worked for a small Cambridge-based educational organization helping to produce a yearlong program for elementary school children called Man: A Course of Study (or MACOS). He first learned about evolution within this context, crafting short books such as Animal Adaptation, Natural Selection, and Innate and Learned Behavior.¹ He had majored in history as an undergraduate, and his experience with MACOS gave him new hope for elaborating the reasons why humans (and animals) behave as they do. Thus inspired, Trivers published several papers in the early 1970s that became crucial to the burgeoning field of sexual selection over the course of the next decade. As increasing numbers of biologists focused their research on the mating behavior of animals, they cited Trivers’s papers—on reciprocal altruism, parental investment, the sex ratios of offspring, and parent-offspring conflict. As sexual selection found its footing as a field, so, too, did Trivers as one of its intellectual leaders. The suggestion, however, that Trivers single-handedly “resurrected the topic” following a century of neglect could make sense only after redefining sexual selection as a concept studied solely in animals under natural conditions.2

Given his commitment to natural selection, Charles Darwin (1809–1882) needed to explain how members of the same species could differ dramatically in their appearance and behavior. If natural selection shaped traits that fostered survival, then surely all members of the population would come to look rather similar. Sexual selection affected traits that improved reproduction, however, and created room for the evolution of stable differences between the sexes.³ Darwin included two elements in his original formulation of sexual selection.⁴ The first, male-male competition, specified that just as members of a species vie for access to resources such as food or territory, males compete for access to fertile females. The second, female choice, laid the basis of this competition at the feet of females, who prefer to mate with some males rather than others.

For Darwin, the give-and-take of these two forces—male competition for mates and female choice of prospective suitors—explained why in some species males and females look so different from each other: competition between males led to war armaments, such as horns, whereas female choice accounted for male beauty. For humans, moreover, Darwin used sexual selection to elucidate the origin of the apparent physical differences that contemporary racial scientists used to define ethnic groups.⁵ Both mechanisms implied a mental awareness of self and others that natural historians were reluctant to attribute to animals, a problem accentuated by Darwin’s use of anthropomorphic language to elaborate female choice. That human men and women make such judgments all the time remained uncontroversial and indeed became a crucial element of the flourishing eugenics movement in Great Britain as well as abroad.6

In later decades, biologists explored the effects of mate choice in lizards, fish, and especially the ubiquitous, quickly breeding fruit flies that came to embody research in experimental population genetics after the Second World War.⁷ Rather than conceiving of sexual selection as a mechanism creating morphological differences between the sexes, however, these investigators typically suggested that mate choice would act to maintain reproductive isolation between species in evolutionary time. Many population geneticists and theorists came to question a clear distinction between sexual and natural selection. Additionally, they studiously avoided the anthropomorphic language that had contributed to the skepticism with which Darwinian female choice was initially greeted.

For biologists such as Lee Ehrman (b. 1935) and Claudine Petit (1920–2007), female fruit flies were notoriously particular in accepting suitors, mating only with males of the same species (by way of contrast, such discrimination did not characterize the mating behavior of male flies). By the 1960s, even mathematically inclined zoologists, such as Peter O’Donald (b. 1935) and John Maynard Smith (1920–2004), modeled the conditions under which sexual selection might have measurable effects on the mating success of individuals and whether it might serve as a mechanism for speciation without geographic isolation.⁸

Even if these investigations were far from common within the zoological community, Trivers knew of their existence and drew on them in composing his first forays into evolutionary theory. To see Trivers as Darwin’s long-awaited heir, in other words, required redefining research on female choice in previous decades as something other than investigations into sexual selection—discrediting it either because of its association with eugenic theory or because it merely established the mathematical plausibility of sexual selection as an evolutionary mechanism but provided no empirical proof. None of this, however, explains the skyrocketing importance of Trivers himself to the identity of sexual selection as a field.

In the first of his seminal papers on evolutionary biology, Trivers described his theory of reciprocal altruism as an intuitive concept—“you scratch my back and I’ll scratch yours”—that could form the basis of social cooperation in human (and animal) societies.9 Cooperation among relatives made sense to Trivers because any effort individuals expended on their relatives, even at great personal cost, would help spread the genes they shared in the next generation. More difficult was the question of why nonrelatives behaved altruistically toward one another, when individuals that cheated the system could gain all of the benefit at no cost. By linking present cooperation to future benefit (a kind of anthropological gift culture applied to evolutionary theory), Trivers noted that cheaters, if caught, would be excluded from such mutually beneficial networks of exchange.¹⁰ He thus explained the evolution of cooperative social behavior as a balance between innate tendencies to behave kindly and to cheat, and held in check with a healthy dose of suspicion, all without recourse to a universal altruistic impulse.¹¹

Turning his attention to mate choice, Trivers reasoned that males and females invested different amounts of resources in their offspring—whereas a male peacock might father numerous chicks with several females, expending little energy taking care of his young, a female peacock devoted far more time, attention, and resources to ensuring the survival of the few hatchlings she could produce in a season.¹² This difference in parental investment meant that a male’s reproductive capacity would far outstrip any female’s potential reproduction. Thus, we should expect females to be far pickier in their mate choices than males, and Darwin had been right to emphasize female choice and male competition as the dominant mechanisms of sexual selection. Trivers further put his finger on the pulse of the times: men and women, in his scenario, wanted different things out of marriage and life. These underlying evolutionary urges might then provide one basis for the conflict between the sexes in contemporary society.13

Browsing through the pages of iterative editions of John Alcock’s (b. 1942) textbook on Animal Behavior reveals both the centrality of Trivers’s research to the expanding field of sexual selection and Trivers’s mythic role within that community.¹⁴ Alcock published the first edition of his textbook in 1975, only six years after he had earned his PhD at Harvard, where—given their mutual interests in animal behavior—he likely knew Trivers. Certainly Alcock greatly admired the sociobiological theories that Edward O. Wilson (b. 1929) generated at Harvard in the years after he left and the theoretical heft his former colleagues derived from applying game theory and mathematical models to the evolutionary process.¹⁵ Animal Behavior ultimately went through nine editions and became one of the most popular college textbooks on the subject.¹⁶ Not until the second edition (1979), however, did Alcock mention Trivers’s contributions as “exceptionally important.”¹⁷ In the next rewrite, he expanded this view, suggesting that Trivers’s work on parental investment had been “extremely influential in the rebirth of interest in sexual selection.”¹⁸ By the late 1980s, Alcock invoked Darwin as the founder of all “important” aspects of evolutionary theory, including sexual selection, and a couple of paragraphs later ascribed to Trivers the theory’s “resurrection.” Even to Alcock, then, Trivers’s key role as an evolutionary theorist had not been obvious from the outset.

Over the course of the 1980s, a new generation of biologists appropriated sexual selection as a theory explaining the evolution of reproductive behavior in animals and humans. Their field-based research on animal courtship provided an important foil for the even more remarkable growth of molecular biology and genetics. These zoologists saw the previous decades as lacking both careful observations of sexual behavior in wild species and a theoretical frame by which to interpret them. By demarcating their discipline as newly important within the biological sciences, they simultaneously redefined its history as characterized by a substantial eclipse in sexual selection from the time of Darwin to the 1970s. In this context, Trivers became a hero who had vanquished the ignorance of earlier generations.19

In subsequent editions of Alcock’s textbook, however, Trivers’s prominence receded. Alcock began by distancing Darwin from Trivers, inserting between them several pages that described the nuances of sexual selection for current research.²⁰ Then he removed Trivers entirely as a central figure uniting the field, opting instead to review his contributions in four separate places, each associated with one of his main papers.²¹ Why? As the study of sexual selection gained institutional grounding, garnered professional authority, and expanded dramatically, Trivers’s early studies no longer held the community together. Additionally, Trivers had not produced more groundbreaking work along the lines of his first publications, and he was plagued by personality conflicts with colleagues.²² Given the widely acknowledged eclipse of his theory, Darwin took precedence as the sole iconic figure in the history of sexual selection.²³

A large number of issues played a role in building the myth that sexual selection had been eclipsed for a century, primarily because of female choice. Ideologically, after the Second World War, biologists sought to distance themselves from theories associated with eugenics. As professional scientists turned their attention to animal behavior, anthropomorphism became associated with amateurs and armchair scientists. Shifting dynamics within professional biology made a difference, too. Researchers investigating sexual selection in the 1970s worked primarily in natural environments and self-consciously dissociated their work from earlier laboratory research.

Even so, scientific heroes in textbooks last only as long as they are useful pedagogically or disciplinarily. For a brief time Trivers’s story fulfilled both functions. Parental investment provided an easy conceptual bridge to more recent work in the field, and Trivers’s theories anchored a methodologically diverse community. In succeeding decades, research on sexual selection has branched out in many new directions.24 Trivers, in turn, has once again picked up questions over which he first puzzled while working for MACOS: In a world where clear perceptions matter, how can we understand the value of deceit and self-deception in our lives?²⁵ Despite the increasing divergence of these legacies, biologists continue to demarcate sexual selection from other ways of exploring the natural world, treating it as a static set of tools developed by Darwin to explain differences between the sexes. As a result, the myth that sexual selection was virtually forgotten for almost a century persists, albeit without Trivers as its sole redeemer.