Evolution's Rainbow. Joan Roughgarden
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At times I’ve loved writing this book; at other times I’ve felt afraid of what I have to say. The view of our bodies, of gender and sexuality, that emerges is strikingly new. But I’ve carried on because I’ve found the message to be positive and liberating. I hope you enjoy this book. I hope it betters your life.
I thank the staff of the Falconer Biology Library at Stanford University for extensive help with research. I am deeply grateful for reviews from Blake Edgar, Patricia Gowaty, Scott Norton, Robert Sapolsky, and Bonnie Spanier, together with editorial improvements from the staff of the University of California Press, especially Elizabeth Berg and Sue Heinemann. I’ve been blessed by love from my closest friend, Trudy, and my sisters at Trinity Episcopal Church in Santa Barbara, especially Terry.
PART ONE
ANIMAL RAINBOWS
1
Sex and Diversity
All species have genetic diversity—their biological rainbow. No exceptions. Biological rainbows are universal and eternal. Yet biological rainbows have posed difficulties for biologists since the beginnings of evolutionary theory. The founder of evolutionary biology, Charles Darwin, details his own struggle to come to terms with natural variation in his diaries from The Voyage of the Beagle.1
In the mid 1800s, living species were thought to be the biological equivalent of chemical species, such as water or salt. Water is the same everywhere. Countries don’t each have water with a unique color and boiling temperature. For biological species, though, often each country does have a unique variant. Darwin saw that finches change in body size from island to island in the Galápagos. We can see that robins in California are squat compared to robins in New England, and lizards of western Puerto Rico are gray compared to the brownish ones near San Juan. Darwin recognized that the defining properties of biological species, unlike physical species, aren’t the same everywhere. This realization, new and perplexing in the mid 1800s, remains at times perplexing today.
In Darwin’s time, the Linnaean classification system, which is based on phyla, genera, species, and so forth, was just becoming established. Naturalists mounted expeditions to foreign places, collecting specimens for museums and then pigeonholing them into Linnaeus’s classification system. At the same time, physicists were developing a periodic table for elements—their classification scheme for physical species—and chemists were classifying recipes for various compounds on the basis of chemical bonds. But the biological counterpart of physical classification didn’t work very well. If Boston’s robin is different San Francisco’s, and if intermediates live at each gas station along Route 80, what do we classify? Who is the “true” robin? What does “robin” mean? Biological names remain problematic in zoology and botany today. Biological rainbows interfere with any attempt to stuff living beings into neat categories. Biology doesn’t have a periodic table for its species. Organisms flow across the bounds of any category we construct. In biology, nature abhors a category.
Still, a robin is obviously different from a blue jay. Without names, how can we say whether it is a robin or a blue jay at the bird feeder? The work-around is to collect enough specimens to span the full range of colors in the species’ rainbow. Then specialists in biological classification, taxonomists, can say something like, “A robin is any bird between six and seven inches in length with a red to orange breast.”2 No single robin models the “true robin” all robins are true robins. Every robin has first-class status as a robin. No robin is privileged over others as the exemplar of the species.
DIVERSITY-GOOD OR BAD?
Rainbows subvert the human goal of classifying nature. Even worse, variability in a species might signify something wrong, a screwup. In chemistry a variation means impurity, a flaw in the diamond. Doesn’t variability within a species also indicate impurity and imperfection? The most basic question faced by evolutionary biology is whether variation within a species is good in its own right or whether it is simply a collection of impurities every species is stuck with. Evolutionary biologists are divided on this issue.
Many evolutionary biologists are positive about the rainbow. They view it as a reservoir of genes that can come to the forefront at different times and places to guarantee a species’ survival under changing conditions. The rainbow represents the species’ genetic assets.3 According to this view, the rainbow is decidedly good. This view is optimistic about the capability of species to respond to ever-changing environmental conditions. This view affirms diversity.
Other evolutionary biologists are negative about the rainbow, believing that all gene pools—even our own—are loaded with deleterious mutations, or bad genes. During the 1950s, studies claimed that every person has three to five lethal recessive genes that would surface if they chose the wrong marriage partner, causing their children to die.4 This view is pessimistic about the future, suggesting that evolution has already reached its pinnacle and all variation is useless or harmful.5 This school of evolutionists believed in a genetic elite, advocating artificial insemination from sperm banks stocked with genes from great men. This view represses diversity.
Darwin himself was ambivalent on the value of rainbows. Darwin argued that natural selection is the mechanism that causes species to evolve. On the one hand, because natural selection depends on variation, Darwin viewed the rainbow as a spectrum of possibilities constituting the species’ future. A species without variability has no evolutionary potential, like a firm with no new products in the pipeline. On the other hand, Darwin viewed females as shopping around for mates with desirable genes while rejecting those with inferior genes. This view demeans the variation among males and implies a hierarchy of quality, suggesting that female choice is about finding the best male rather than the best match. Darwin both affirmed and repressed diversity at different times within his career.
The philosophical conflict over whether to affirm or to repress diversity is still with us today, permeating everything from the way biologists interpret motives for an animal’s choice of a particular mate to how medical doctors handle newborn babies in the hospital.
THE COSTS VERSUS THE BENEFITS OF SEX
How, then, are we to decide whether rainbows are good or bad? Who is correct, the diversity affirmers or the diversity repressers? To answer this most fundamental question of evolutionary biology, let’s compare species with full rainbows to species with very limited rainbows. Species who manage to reproduce without sex have limited rainbows. By sex, I mean two parents mixing genes to produce offspring. Lots of species propagate without sex. In such species, everyone is female and offspring are produced without fertilization. In addition, in many species offspring may be produced either with or without fertilization, depending on the season.
If you go to Hawaii, look at the cute geckoes on the walls. You’re seeing an asexual species—all these geckoes are female.6 Females in all-female species produce eggs that have all the needed genetic material to begin with. In sexual species, like humans, an egg has only half the genetic material needed to produce a baby; a sperm has the other half, so combining these yields the required material. In addition, eggs from an all-female species don’t need fertilization by a sperm to trigger the cell divisions that generate an embryo. Females in all-female species clone themselves when they reproduce.
The Hawaiian all-female geckoes are locally abundant and widespread throughout the South Pacific, from the lovely Society Islands of French Polynesia to the Marianas Islands near New Guinea. More all-female species live in Mexico, New Mexico, and Texas—all varieties of whiptail lizards These small, sleek tan and brown-striped animals dart quickly along the ground looking for food. The all-female species of whiptail lizards live along streambeds, while sexually reproducing relatives typically live up-slope from the streams in adjacent woods or other vegetation. Every major river drainage basin in southwestern North America is a site where an all-female whiptail lizard species has evolved. More than eight all-female species are found in this area. Still more all-female species of lizards are found in the Caucasus Mountains of Armenia and along the Amazon River in Brazil. All-female fish occur too. Indeed, all-female animal species are found among most major groups of vertebrates.7
Also, some species have two kinds