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
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
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
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
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.