Evolution's Rainbow. Joan Roughgarden
view as both theoretically impossible and empirically vacuous. The scenario envisioned by the diversity-repressing theory can’t exist. In an asexual species, when a bad gene arises, the line where the mutation originated is lost to natural selection, whereas the lines without the mutation prosper. The entire stock never deteriorates, because natural selection doesn’t look the other way while a bad gene spreads. Instead, natural selection eliminates a bad gene when it first appears, preserving the overall functionality of the species. No evidence whatsoever shows asexual species becoming extinct because of a progressive accumulation of disabilities and loss of functionality. A bad gene never gets going in an asexual species, and sex’s supposed pruning of the gene pool is unnecessary and mythical.
On the other hand, the environment does change from year to year, and individuals who don’t do well one year may shine when conditions change, and vice versa. Butterflies whose enzymes work at cold temperatures thrive in dark, damp years, while butterflies whose enzymes function best at hot temperatures do better in sunny drought years. All butterflies are perfectly good butterflies, even if the abilities of some don’t match the opportunities currently supplied by the environment.
I don’t see any grounds for dignifying the diversity-repressing view for the benefit of sex as a viable alternative to the diversity-affirming view. To be agreeable, one might say both theories are valid. But this compromise isn’t true. Conceding, even slightly, that one function of sex is to prune diversity puts forth a view that hasn’t earned its place scientifically. Accepting a diversity-repressing view of sex simply to be polite admits through the back door a philosophical stance that may later be used to justify discrimination.
Therefore, I accept as a working premise that a species’ biological rainbow is good—good because diversity allows a species to survive and prosper in continually changing conditions. I further accept that the purpose of sex is to maintain the rainbow’s diversity, resynthesizing that diversity each generation in order to continually rebalance the genetic portfolio of the species. I reject the alternative theory that sex exists to prune the gene pool of bad diversity.
Darwinists have to take a consistent stand on the value of diversity. They can’t maintain on the one hand that most variation is good because it’s needed for natural selection and on the other hand also maintain that females must continually shop for males with the best genes as though most genes could be ranked from good to bad. Instead, I argue that almost all diversity is good and that female choice is more for the best match than for the best male.
How then should we assess the rainbows in our own species? We should be grateful that we do reproduce sexually, although we probably take this gift for granted. I feel too that we should conserve and embrace our rainbows. Affirming diversity is hard, very hard. We must come to accept ourselves and love our neighbors, regardless of color in the rainbow.
Overall, sex is essentially cooperative—a natural covenant to share genetic wealth. Sexual reproduction is not a battle.
2
Sex versus Gender
To most people, “sex” automatically implies “male” or “female.” Not to a biologist. As we saw in the last chapter, sex means mixing genes when reproducing. Sexual reproduction is producing offspring by mixing genes from two parents, whereas asexual reproduction is producing offspring by one parent only, as in cloning. The definition of sexual reproduction makes no mention of “male” and “female.” So what do “male” and “female” have to do with sex? The answer, one might suppose, is that when sexual reproduction does occur, one parent is male and the other female. But how do we know which one is the male? What makes a male, male, and a female, female? Indeed, are there only two sexes? Could there be a third sex? How do we define male and female anyway?
“Gender” also automatically implies “male” and “female” to most people. Therefore, if we define male and female biologically, do we wind up defining gender as well? Similarly, for adjectives like “masculine” and “feminine,” can we define these biologically? Moreover, among humans, is a “man” automatically male and a “woman” necessarily female? One might think, yes, of course, but on reflection these key words admit lots of wiggle room. This chapter develops some definitions for all these words, definitions that will come in handy later on.
When speaking about humans, I find it’s helpful to distinguish between social categories and biological categories. “Men” and “women” are social categories. We have the freedom to decide who counts as a man and who counts as a woman. The criteria change from time to time. In some circles, a “real man” can’t eat quiche. In other circles, people seize on physical traits to define manhood: height, voice, Y chromosome, or penis. Yet these traits don’t always go together: some men are short, others are tenors, some don’t have a Y chromosome, and others don’t have a penis. Still, we may choose to consider all such people as men anyway for purposes like deciding which jobs they can apply for, which clubs they can join, which sports they may play, and whom they may marry.
For biological categories we don’t have the same freedom. “Male” and “female” are biological categories, and the criteria for classifying an organism as male or female have to work with worms to whales, with red seaweed to redwood trees. When it comes to humans, the biological criteria for male and female don’t coincide 100 percent with present-day social criteria for man and woman. Indeed, using biological categories as though they were social categories is a mistake called “essentialism.” Essentialism amounts to passing the buck. Instead of taking responsibility for who counts socially as a man or woman, people turn to science, trying to use the biological criteria for male to define a man and the biological criteria for female to define a woman. However, the definition of social categories rests with society, not science, and social categories can’t be made to coincide with biological categories except by fiat.
MALE AND FEMALE DEFINED
To a biologist, “male” means making small gametes, and “female” means making large gametes. Period! By definition, the smaller of the two gametes is called a sperm, and the larger an egg.* Beyond gamete size, biologists don’t recognize any other universal difference between male and female. Of course, indirect markers of gamete size may exist in some species. In mammals, males usually have a Y chromosome. But whether an individual is male or not comes down to making sperm, and the males in some mammalian species don’t have a Y chromosome. Moreover, in birds, reptiles, and amphibians, the Y chromosome doesn’t occur. However, the gamete-size definition is general and works throughout the plant and animal kingdoms.
Talk of gamete size may seem anticlimactic. Among humans, for example, centuries of poetry and art speak of strength and valor among men, matched by beauty and motherhood among women. Saying that the only essential difference between male and female is gamete size seems so trivial. The key point here is that “male” and “female” are biological categories, whereas “man” and “woman” are social categories. Poetry and art are about men and women, not males and females. Men and women differ in many social dimensions in addition to the biological dimension of gamete size.
Yet, biologically, the gamete-size definition of “male” and “female” is far from anticlimactic. In fact, this definition is downright exciting. One could imagine species whose members all make gametes of the same size, or several gamete sizes—small, medium, and large—or a continuum of gamete sizes ranging from small to large. Are there any such species? Almost none. Some species of algae, fungi, and protozoans have gametes all the same size. Mating typically occurs only between individuals in genetic categories called “mating types.” Often there are more than two mating types.1 In these cases, sex takes place between the mating types, but the distinctions of male and female don’t apply because there is only one gamete size.2 By contrast, when gametes do come in more than one size, then there are generally only two sizes, one very small and the other very large. Multicellular organisms with three or more distinct gamete sizes are exceedingly rare, and none is known to have a continuum of gamete sizes.
More than two gamete sizes occur in some colonial single-celled organisms, the protozoans. In the green ciliate Clamydomonas euchlora,