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Rethinking Sex

Robert L. Dorit

Evolution's Rainbow: Diversity, Gender, and Sexuality in Nature and People. Joan Roughgarden. viii + 474 pp. University of California Press, 2004. $27.50.

More than 2,400 years ago, Socrates was charged with using philosophy to "study things in the sky and beneath the earth"—with seeking a material understanding of the world, in effect, rather than accepting well–established theistic narratives. He defended himself in an Apologia—a passionate and intellectually rigorous speech that made clear the grounds for his opinions and actions. As recorded by Plato, it stands as one of the defining documents of Western culture.

Evolution's Rainbow is Joan Roughgarden's Apologia, an extraordinary book that entwines a radical attack on the Darwinian concept of sexual selection with a personal narrative written from her perspective as a transgendered woman (until six years ago, she was Jonathan Roughgarden). The book is thought–provoking, even at times profound, although some of its arguments are infuriatingly extraneous or superficial. Some critics will dismiss it as a book with an agenda—polemic tainted by the author's unwillingness to detach her scientific analyses from her personal experience. To take that narrow view, however, does grave injustice to Roughgarden's ambitious undertaking.

The book's scope is broad and inclusive. Part I, titled "Animal Rainbows," surveys the natural history of two thorny subjects: sex and gender. Consider, for instance, the fact that sexual reproduction is itself a fundamental evolutionary paradox. Parthenogenetic reproduction—cloning—produces offspring that are virtually genetically identical to their mother. In contrast, making offspring with a partner may seem like a good idea for a variety of reasons, but those offspring will share only half of a given parent's genes. Sex thus comes at a cost. Although a number of competing theories for the evolution of sexual reproduction have been put forth, we still have no unanimously accepted explanation.

The matter only gets more complicated when we consider the myriad ways in which evolution has seen fit to embody the sexes, in apparent disregard of our binary expectations. Despite the determination of many of us to partition the living world into neat, discrete categories of male and female, the living world does not comply. Sex is itself a fuzzy term: Is sex determined by the chromosomal makeup of cells, so that any mammal with a Y chromosome is male? Should sexual distinctions be made with regard to the set of developmental events that result in recognizably male and female characteristics (genitalia and secondary sexual traits)? Or should definitions be based on behavior—that is, should some unambiguous set of displays be considered the mark of maleness or femaleness?

Roughgarden's expertise as an evolutionary ecologist is apparent in her fascinating accounts of species in which no simple binary division of sex applies. Nearly half of the females in certain species of South American hummingbirds, for instance, are characterized by male coloration, and on occasion a small percentage of the males sport female colors. Roughgarden also has the reader consider the aptly named hamlets (small bass that live around coral reefs), which are simultaneously male and female, and can switch between producing sperm and producing eggs in the span of a single mating episode. To be or what to be, indeed.

What are we to make of the many examples of hermaphroditism and sex–role reversal, of intersexed deer and pouched male kangaroos? One choice, which Roughgarden soundly rejects, is to consider these phenomena oddities—or worse, pathologies. As she indicates, the central philosophical legacy of the Darwinian revolution supplies an alternative perspective. Diversity of sexual forms, in a post–Darwinian world, is not simply noise, nor is it the flawed expression of some underlying Platonic dichotomy. Instead, diversity is the message itself—the very stuff of evolution, the rainbow alluded to in the title of the book.

Roughgarden urges a broad and liberating definition of sex and gender. Sex, she argues, boils down to the size of the haploid gametes produced: Males produce small gametes, females produce large ones. Gender, she suggests, is "the appearance, behavior and life history of a sexed body." In this formulation, the many reproductive strategies, appearances, behaviors and social systems cease to be departures from the norm and become instead examples of the rich tapestry of adaptation in nature.

Roughgarden, however, has more in mind than making a plea for a more expansive perspective on sex and gender in the natural world. Ambitiously, she pronounces dead the theory of sexual selection, an important beam in the Darwinian edifice. In its stead, she sketches a theory of social selection that makes possible a more expansive and diversity–valuing vision of sex and gender. All of this before the reader is midway through the book.

Roughgarden's objections to Darwinian sexual selection deserve careful analysis. Sexual selection, which Darwin first proposed in On the Origin of Species in 1859 and elaborated on to a much greater extent in The Descent of Man in 1871, accounts for features of organisms that appear to have little or no functional significance and that sometimes even appear to hinder survival: the preposterous feathers of the peacock, the predator–attracting call of the bullfrog and the overdecorated nests built by male bowerbirds, to name a few.

Darwin ably argued that the evolution of such traits was driven not by their effect on survival but instead by their contribution to reproductive success. When females choose males that exhibit ever–more–elaborate versions of a particular trait, they set in motion a process that can result in extraordinarily baroque structures in one sex (usually, but not always, males) and is virtually absent in the other sex. The theory of sexual selection, like so much in Darwin, is framed in the language of economics: Sperm are cheap, whereas eggs are expensive. Females invest more in reproduction, which forces them to be more selective, whereas males diversify their reproductive investment.

Roughgarden, claiming that "sexual selection theory has now been discredited," proposes a different, more globalized economic model of social selection, in which competition among males for access to females is replaced by cooperation, both within and between sexes. In this model, females are no longer seen as the sole resource limiting reproduction. Instead, we are urged to consider all the resources—food, nesting sites, parental care and partners—that are necessary to ensure successful reproduction. Any strategies that increase an individual's access to and control of such resources—including, under certain ecological circumstances, same–sex mating, sex–role reversals and multigendered societies—will be favored.

Roughgarden argues that binary sexes thus become a special case in which males are simply a "special adaptation for the home delivery of sperm"; the ancestral condition is one in which male and female functions coexist in a single body. Similarly, Roughgarden's model suggests that most of the elaborate sex–specific traits we marvel at are not the result of male competition for female attention but rather are social–inclusionary traits—membership signals guaranteed access to valuable resources (think "secret handshake"). Elaborate feather displays and decorated bowers are not simply billets–doux from one sex to the other but are instead subtle social signals to all members of the group.

It is much too early to tell whether Roughgarden's theory of social selection will prove to be a productive revision of evolutionary theory or will simply contribute to a broadening of existing models of sexual and kin selection. I don't anticipate that the prevailing theory of sexual selection will simply ride off quietly into the sunset: Many of its predictions have been borne out over the past 150 years. Yet the rise and fall of theories in science is not simply an objective matter of weighing the evidence. The success of social selection theory may well depend on its ability to subsume the theory of sexual selection.

But regardless of the eventual success of the formulation, Evolution's Rainbow will change the way many biologists view the world, making it easier for them to see additional instances of diversity in genders, sexual phenotypes and sex roles. More important, Roughgarden provides a theoretical framework into which such observations can be placed, turning anomalies into useful data. That's real progress.

The second section, "Human Rainbows," reviews the biology of sex determination and the diversity of gender expression in humans, emphasizing how our 30,000 or so genes interact with one another and with the cellular, physiological and ecological environment in which development takes place. Human diversity thus derives both from the genetic uniqueness of individuals and from the number of developmental interactions involved in transforming a fertilized zygote into a reproductive adult. The sheer quantity and complexity of those interactions virtually guarantee that no two trajectories—and thus no two outcomes—will be alike.

This interactionist perspective contributes much to our understanding of sex determination and sex differences. We live in an age wedded both to the primacy of genes as causes of biological phenomena and to the notion that sex is binarily and irreversibly determined at the moment of conception. We imagine sex to be determined by tightly linked causes and effects that lead, for example, from the presence of a Y chromosome to the development of a male body and then to a set of behaviors, all in the service of reproduction. This book reminds the reader that these connections are flexible and modifiable. Seen in this light, sex differences look like almost all data in biology: Males and females vary, and male and female distributions for the vast majority of traits overlap.

In a chapter titled "Gender Identity," Roughgarden brings observations about natural history and developmental biology to bear on an exploration of human gender identity and sexual orientation. I confess I always feel some trepidation when evolutionary biologists take on human behavior. Hapless academics often find themselves caught between the Scylla of anthropomorphizing animal behavior and the Charybdis of reducing human culture and behavior to a set of fitness–enhancing traits. Roughgarden is no exception. When I read about "lesbian lizards," I flinch, because the expression suggests that an evolutionary explanation that accounts for female–female courtship in lizards will have something useful to say about human lesbianism. It won't unless lizard same–sex courtship and human same–sex courtship have the same evolutionary origins (that is, they are homologous behaviors) or unless same–sex courtship evolved in lizards and in humans in response to similar ecological pressures (making them convergent behaviors). I worry that human and lizard behaviors may be neither homologous nor convergent but may simply share some superficial similarities. Human behavior, driven by our greatest evolutionary innovation, the 20–billion–neuron neocortex, has made the search for homology far more complicated.

But my most serious disagreement with Roughgarden concerns her attempts to find an adaptive explanation for the varieties of human sexuality and gender orientation she so ably describes. To make of all human behavioral diversity an adaptation risks returning the analysis of sex and gender to the very reductive and speculative quagmire from whence she has just extracted it.

I applaud her use of a Darwinian framework to de–pathologize difference. Exploding the notion that all sexual and gender diversity results from conditions or syndromes that require a cure is an undertaking both scientifically sound and long overdue. Individuals who cannot easily be classified as either male or female continue to be squeezed by reductive biological explanations (a gene that causes homosexuality, a cluster of neurons that is larger or smaller in transgendered individuals, and so forth), ill–defined or overly rigid medical and psychiatric classifications (penis length at birth of less than 1.5 centimeters makes an intersexed baby "female") and political forces that rabidly cling to a cultural construct of what supposedly is or is not "natural." This book will help readers question their assumptions and examine their prejudices in the bright light of reason.

Perhaps everything we do has an effect on our survival and reproduction, but that does not make every behavior an adaptation, evolved to ensure future representation. Roughgarden's apparent insistence on viewing all human behavior as adaptation forces her into an unconvincing quantitative argument. She suggests that gay, lesbian and transsexual individuals together amount to about 5 percent of the U.S. population—a frequency that is, she says, too high for those gender expressions to be considered "genetic defects," because traits that reduce fitness by even a mild 5 percent become increasingly rare, eventually stabilizing at a frequency of around 1 in 50,000 (0.002 percent).

Leaving aside the many demographic and chromosomal circumstances under which selection cannot purge genetic defects, this argument abandons the rigor and subtlety in the classification of sex and gender that the book pleads for, implying that gender orientation can be thought of as an obvious trait with a simple underlying basis. The frequency of gay, lesbian and transgender people does not require an adaptive explanation, and seeking one pulls Roughgarden into needless and untestable speculation.

The final section of the book, "Cultural Rainbows," presents comparative anthropological perspectives on sex and gender. This material is a tantalizing, if superficial, reminder of how greatly human constructions of gender and sexuality vary across cultures. By introducing the vestidas (transgender sex workers) of Mexico City and the hijras of India (a castelike group of more than one million transgender people), Roughgarden reminds us that we have a choice: We can accept the struggles and discourse of all people at face value, or we can try to fit them into preexisting categories. More important, these examples underscore the extent to which those categories are contingent products of our own culture, and not something natural. Autres pays, autres moeurs—every time and place defines and redefines sex, gender and normalcy. Reread the Bible or the Koran or the legend of Joan of Arc carefully, Roughgarden tells us, and you'll find a more nuanced understanding of sexuality and gender than you perhaps expected.

I hope this book will be widely read. It combines the combustible power of a keen intellect with powerful conviction and ethical courage. Scientists are trained to create the illusion of objectivity by hiding their hopes and expectations. Roughgarden tries a different approach, openly injecting her reasoned convictions and policy agenda into her analyses. I don't agree with all of her conclusions, but she has written an important and honest book about a subject that matters.

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Related Internet Resources

An Interview with Joan Roughgarden

Chapter 7 of Evolution's Rainbow

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