Why Sex?

A. Why Sex?

A male peacock struts in front of the camera. "Peahens only mate with well-endowed males," the narrator says. "No fancy display? No sex. No passing of genes to the next generation. That's something every living thing is programmed to do."

The scene shifts to fighting baboons, and the narrator continues: "It's worth fighting for, maybe even dying for. In fact, from an evolutionary perspective, sex is more important than life itself." We watch salmon spawning and dying, then a human family appears. "While we won't trade our lives for sex, most of us will risk death to protect our children, the carriers of our genes. Evolution is a story written over countless generations. To inherit and pass on genes is to be part of that story."

We meet Rutgers University evolutionary geneticist Robert C. Vrijenhoek, who tells us: "That's our immortality. That's what connects us to humans on into the future. That's what's connected us to all of our ancestors in the past. That's what connects us to the ancestors that were fish, the ancestors that were protozoans, and the ancestors that were bacteria. It's the single thread that connects all of life on this planet."

"Sex and genes," the narrator says, "driving behavior, driving evolution."

In southwest Texas, a research crew is rounding up parthenogenetic lizards--a species that consists entirely of females, and does not require males to reproduce. The crew leader says: "Some people think they actually have to have some kind of lesbian behavior where a female mounts a female to get the eggs to develop." We watch as one lizard performs erotic movements on top of another. "That hasn't been really proven yet, but it's an interesting hypothesis." The existence of this all-female species, however, raises a fundamental question: "Why is there sex? I mean, are males really necessary?"

Accompanied by some old movie footage and some wildlife photography, the narrator explains the supposed disadvantages of relying on males to help pass on genes, but notes that almost every life on the planet is nevertheless a result of sexual reproduction. The scene shifts to a husband and wife and their adopted baby, and the narrator concludes: "So males must play a critical role, and sex must offer an advantage. Whatever it is, it's buried deep within us."

We are told that the "biological imperative, as we all know, is to pass on genes." Since sexual reproduction is so common, "there has to be some fundamental biological, evolutionary reason for sexual reproduction. This has been one of the major questions in biology for a very long time."

The emphasis on sex in these opening scenes is striking. Sex is more important than life itself. Sex is our immortality. Even the story of parthenogenetic lizards--for whom sex is irrelevant--is illustrated with a lesbian sex scene. This episode is sure to be an attention-getter. But how much of it will it be science?

B. The Biological Reason for Sex

We accompany Vrijenhoek as he conducts research on minnows that inhabit small streams in Sonora, Mexico. The narrator explains that he "hopes to find clues to the enduring mystery--Why sex?"--by studying a species of fish that includes both sexual and asexual reproducers. According to Vrijenhoek, the sexually reproducing fish are more resistant to infestation with parasites. His hypothesis is that this is due to the greater genetic variability that comes from exchanging genes with other members of the population. Sexual reproduction, he believes, leads to new combinations of genes that confer protection against parasites.

"Here was a solution to the mystery of sex," the narrator says. "It's the best defense against rapidly evolving enemies. Or so it seemed, until a bad drought dried up the pools, killing the minnows and throwing everything into question." After the minnows returned, the parasites were decimating the sexual fish, and the asexual ones were doing quite well. But an experiment convinced Vrijenhoek that this was only because the drought had led to inbreeding among the few survivors, and had thus reduced the genetic variability of the sexual population. He concludes that his hypothesis remains intact, and that the principal benefit of sexual reproduction is that it increases genetic variability.

"That's what sex does," Vrijenhoek says. "Sex generates variability among offspring. And when you take that away from a sexual reproducer by inbreeding them, cloning them, you've lost the very benefit of sex. It's that generation of an immense amount of diversity, that diversity of your offspring, that provides challenges to everything around it: challenges to the parasites, challenges to the viruses, challenges to your competitors. That's the beauty of the sexual process--[it] is the variation and wonderful diversity it creates."

As we watch human families and children, the narrator repeats the point: "Sex generates variation, which improves a species' chances of survival in a world dominated by relentless competition." The scene shifts to a men's basketball game. "For all their down side, males are worth the trouble. Think of them as a female's insurance policy against losing her children to rapidly evolving threats like measles and the flu."

A viewer who knows nothing else about this topic would probably conclude at this point that scientists now know the answer to the question, "Why sex?" But the viewer would be mistaken--misled by an extraordinarily shallow and lopsided account of one of the most controversial topics in evolutionary biology.

The very existence of sexual reproduction presents a problem for Darwin's theory. The easiest way for an organism to reproduce is simply to divide asexually--to make a copy of itself. Bacteria are very successful at this. An organism that reproduces sexually, however, must divert precious energy into making sperm or egg cells; in the process, gene combinations that were quite useful beforehand are sometimes destroyed through "recombination." Then the organism must find a member of the opposite sex and mate with it successfully. From an evolutionary perspective, sex incurs considerable costs that must be offset by advantages to the organism. But what are those advantages?

Various theories have been proposed--including the genetic variability theory to which Vrijenhoek subscribes. But situations such as the one he studied are relatively rare; so why is sexual reproduction so widespread?

A comprehensive review of this topic in 1988 concluded: "While there might well be agreement about the importance of the problem of the evolution of sex, there is no consensus about where its solution lies." Ten years later, in September 1998, the journal Science devoted a special issue to the evolution of sex, and said essentially the same thing: Biologists "haven't solved the mystery of sex yet," partly because of "extremely lousy experimental data." And this was after Vrijenhoek had done his research on Mexican minnows. So evolutionary biologists continue to "scrounge for data to support one or the other of the warring theories of sex." According to Science: "How sex began and why it thrived remain a mystery."See . For a review of the research on Mexican minnows, see Robert C. Vrijenhoek, "Animal Clones and Diversity," BioScience (August, 1998), at:

For people who think biology is boring, the lively debate over the biological reasons for sex could make the field much more interesting. But people won't learn anything about this controversy from Evolution.

C. The Origin of Sex and Gender Differences

The narrator continues: "If the reason for sex is a bit less mysterious these days, its origins remain much more speculative." A whimsical cartoon animation fills the screen. "Some believe it all got started billions of years ago, with two single-celled creatures sharing a chance encounter in the primordial night. They meet, and genes are exchanged. That's what sex is all about. The moment is brief, but it leaves them a little bit stronger, a little more likely to survive and reproduce. Males and females came later, when random change produced a creature that was small and fast, which turned out to be an evolutionary advantage. Organisms with reproductive cells like that are called males. Their goal is to find organisms with a different specialty--providing the nutrients life requires. They're called females. These early pioneers evolved in time into sperm and eggs."

The difference between sperm and eggs is then extrapolated into an account of the differences between male and female sexual attitudes. "Males produce sperm by the millions, with so many potential offspring it doesn't pay to be fussy about eggs. A better strategy is to try to fertilize every egg you can. Eggs are more complex than sperm and take a larger investment of energy. Females make only a limited number of them. Fewer eggs mean fewer chances to pass on genes, and that means females--unlike males--do better if they're choosy. At a deep biological level, males and females want different things, regardless of how things appear on the surface."

The camera pans over a man and woman--apparently naked beneath a sheet that is pulled back to expose as much flesh as network policy permits--who are engaging in sexual foreplay. "Small sperm versus large eggs," the narrator continues. "Quantity versus quality. These are the evolutionary roots of the war between the sexes." This war "can explain a lot about how species evolve, and why they look and act the way they do. Charles Darwin was the first to recognize the evolutionary significance of sex." These statements are graphically illustrated with scenes of various animals engaging in sexual intercourse.

According to the narrator, Darwin's theory of natural selection could explain why "any trait that improves an individual's chances of survival should spread through the entire population. But it offered no help in explaining the wild extravagances found throughout nature, like the peacock's tail."

"It is hard to see a peacock's tail as something other than an impediment to his survival," the narrator says. "Theologians of [Darwin's] day argued that God created ornate flowers and feathers to inspire man's wondering devotion. Darwin was convinced there had to be an evolutionary explanation--just as there had to be an evolutionary explanation for why so many of nature's ornaments are seen only on males."

To solve the problem, Darwin formulated his theory of sexual selection, which psychologist Geoffrey F. Miller calls "Darwin's most ingenious idea." According to Miller, "these ornaments are not for our good. They're to advertise each individual's fitness, its goodness as a mate, to the opposite sex."

We watch scenes of male animals fighting, and a movie scene of John Travolta putting on seductive clothing, as the narrator tells us that females engage in choice, while males engage in competition. Male competition may take the form of fighting, we are told, or it may take "the path of the peacock--seduction through sexual display. This is where female choice comes in."

Commentators take turns emphasizing that the idea of female choice was controversial in Victorian England. Only a century later was this aspect of Darwin's theory tested in peacocks, when experiments showed that males with bigger, flashier tails tend to attract more mates and to have longer-surviving offspring. "It's all a logical consequence," the narrator says, "of the differing reproductive strategies of males, who have lots of sperm, and females, who have fewer eggs."

As the narrator acknowledges, however, all of this is speculation. Science is supposed to rest on evidence, but a whimsical cartoon animation about the evolutionary origin of sex is not evidence. In fact, most of what we have just seen is what evolutionary biologist Stephen Jay Gould would call a "just-so story." About a hundred years ago, Rudyard Kipling wrote a children's book by that name which recounted entertaining but scientifically meaningless stories about how leopards got their spots, and other things. In just-so stories, according to Gould, "virtuosity in invention replaces testability as the criterion for acceptance." Evolution is telling us just-so stories, yet we are expected to regard them as scientific and to draw two far-reaching conclusions: First, that sex originated in "random change;" and second, that men and women behave as they do because the former have lots of small, fast sperm and the latter have a few large, complex eggs.See . The Stephen Jay Gould quotation is from "Sociobiology: the art of storytelling," New Scientist (November 16, 1978), 530.

Not only does Evolution rely on just-so stories, but it also frames yet another scientific issue in religious terms. We have just been presented with two competing ideas: the idea that "God created ornate flowers and feathers to inspire man's wondering devotion," and the idea that peacocks' tails are a result of Darwinian sexual selection. Then we are told that the tendency of male peacocks with big, ornate tails to attract more mates and have longer-surviving offspring is "a logical consequence of the differing reproductive strategies" of males and females--as though that somehow refutes the idea that God created ornate flowers and feathers. Why not just present the evidence, and leave religion out of it? Once again, Evolution goes out of its way to speak to the religious realm.

C. Marriage and Family

"But the goal isn't just to have offspring," the narrator continues. "The young have to survive long enough to have their own offspring. Sometimes that requires paying as much attention to behavioral traits as to physical ones." In a scene from an old movie, Katherine Hepburn has difficulty choosing between sexiness and dependability in her man. This "mirrors a deep biological dilemma," the narrator tells us. "For some species, the chances of offspring surviving increase if a female chooses a mate who'll stick around over the one with the best genes."

We watch songbirds in which males and females share the job of parenting. The narrator tells us that the female needs the male's help, but the male will stay home only if he believes the chicks he's helping to raise are his own. "The result is monogamy--a social solution to a biological dilemma." The scene shifts to the human family we met earlier. The husband and wife affirm their commitment to their adopted child and to each other as the narrator says that "monogamy isn't easy to maintain. While some evolutionary forces encourage it, others threaten the family values that are at its core."

"Songbirds are unusually monogamous," the narrator continues. "But even as they pair off and set up nests, inevitably some of them are lusting after their neighbors." Cornell University behavioral ecologist Stephen T. Emlen explains how a female songbird returning from migration sometimes has to settle for "a fairly low-quality male" in comparison with her neighbors. The female is now torn, according to Emlen, between a desire to have a faithful mate who will help her raise her young, and a desire to have her chicks sired by a male of higher genetic quality. "Cheating, at least for certain female songbirds," says the narrator, "gives their chicks better genes, and therefore a better chance of surviving until they can reproduce."

For a species of bird in Panama, the narrator tells us, "survival of chicks is so uncertain it's led to an amazing gender role reversal." So many chicks are lost to crocodiles that females leave their eggs for the males to raise, and go off to reproduce again. "Now it's the females who care more about quantity than quality. Now it's the females who fight over mates. Over time, they've taken on traditionally male characteristics." Emlen explains that the females of this species are aggressively territorial, and try to attract "harems" of four or five males.

The narrator continues: "So here is an evolutionary revelation about gender: Male and female roles are not set in stone. They're largely determined by which sex competes for mates, and which invests in the young."

Wait a minute! Just a few scenes earlier, we were told that males and females behave as they do because the former have lots of small, fast sperm and the latter have a few large, complex eggs. Now we are told that the behavior of males and females depends on which sex competes for mates and which invests in the young. Yet the males of these Panamanian birds still produce sperm, and the females still lay eggs. Apparently, the reasons for male and female behavior are not as simple as Evolution would like us to think.

D. Chimpanzees and Bonobos

"Solving the problem of passing on genes can even trigger the emergence of new species," the narrator says. "Sometimes what separates species is more social than physical, as it is with our closest relatives, chimpanzees and bonobos." Chimpanzees and bonobos look alike, live in similar environments, and eat similar food. Chimpanzees are very pugnacious, however, while bonobos are essentially peaceful. "Bonobos are predisposed to make love, not war," the narrator tells us, while two of them copulate on screen.

We watch wild chimpanzees fighting--and occasionally stopping just long enough to have sex. Then we visit the San Diego Wild Animal Park, where we witness bonobos having heterosexual and homosexual intercourse "in every way imaginable"--with a running commentary to make sure we don't miss anything. No doubt many of the teenagers who watch the Evolution series in their public school science classrooms will be hugely entertained by these promiscuous primates.

So why do chimpanzees make war, and occasionally love, while bonobos seem to make only love? The answer, we are told, is female solidarity: Bonobo females are "able to form alliances with each other and cooperatively dominate males. And this changes the whole balance of power and the whole social dynamic in the group, and makes it radically different from chimpanzees."

Why have bonobos evolved this strategy, and chimpanzees haven't? "It looks as though a relatively simple change in the feeding ecology is responsible for this dramatic difference in sexual behavior," we are told. Bonobos live in forests where they can forage for food on the ground. Although there are chimpanzees that live in similar forests, those forests are also occupied by gorillas. The gorillas eat the food on the ground, leaving the chimpanzees dependent on fruit trees. So female chimpanzees have to forage intermittently and alone, without the opportunities for social interactions enjoyed by female bonobos.

The narrator concludes: "The simple fact that there was food available on the ground appears to have been the force that drove the evolution of bonobos." He speculates that some chimpanzees evolved into bonobos about two million years ago because they were able to forage on the ground after a drought killed the gorillas. We are told that if our own ancestors had experienced the same conditions that supposedly led to the evolution of bonobos, "we might have evolved to be a totally different, more peaceful, less violent, more sexual species."

But does "more sexual" really mean "less violent"? Violent chimpanzees seem to be just as sexual as peaceful bonobos. The slogan "make love, not war" sounds good, but sex and love are not synonymous--a lesson which many people have learned only after deep personal tragedies. What is Evolution trying to teach students here?

In any case, this account of the evolutionary origin of bonobos is another just-so story. We have two species with differing behavior patterns. But did differences in behavior lead to the origin of two species? Or did the two species originate in some other manner, with different behavior patterns from the start? Did opportunities for ground foraging produce female solidarity, which in turn established social peace? Or was it the other way around? How can we know? Where's the evidence?

As we watch actors in hairy costumes cross an African plain, the narrator acknowledges that this theory of the origin of bonobos "is little more than interesting speculation. But the idea behind it is consistent with a growing but controversial body of scientific thought that claims much of present-day human behavior is rooted in our distant past." That controversial body of thought is "evolutionary psychology," and it claims that modern human behavior patterns were formed under primitive conditions on the plains of Africa millions of years ago.

E. Evolutionary Psychology

"Evolutionary psychologists begin by pointing out," says the narrator, "that regardless of the culture in which we grow up, we all tend to respond the same way to a surprising variety of things. Most of us find spiders unpleasant, certain body types sexy, and particular smells disgusting. All, they say, are legacies of our evolutionary past."

Researchers conduct an experiment in which young men sleep in the same T-shirts night after night, and put the shirts in plastic bags during the day. Then a panel of young women smells the shirts and rates their sex appeal. According to the researchers, the women consistently prefer the shirts from those men who differ most from them in their immune genes. "From an evolutionary perspective, this makes sense," the narrator tells us. "Choosing a mate with different immune genes gives offspring a greater protection against viruses, parasites, and other pathogens."

Facial beauty, we are told, is simply a collection of subconscious biological cues that let us know whether a potential mate is genetically desirable. In one experiment women choose attractive male faces from a computer program, and they tend to prefer more masculine faces at those times of the month when they would most likely become pregnant.

Experiments such as these are conducted by evolutionary psychologists who think human behavior must be explained within a Darwinian framework. But of all the claims by evolutionary psychologists, the narrator tells us, none are more sweeping than those made by Geoffrey Miller: "He believes the human brain, like the peacock's magnificent tail, is an extravagance that evolved--at least in part--to help us attract a mate, and pass on genes."

"The human brain," Miller says, "is the most complex system in the known universe. It's wildly in excess of what it seems like we would need to survive on the plains of Africa. In fact, the human brain seems so excessive that a lot of people who believe in evolution applied to plants and animals have real trouble imagining how natural selection produced the human brain."

We watch ants scurrying along a log as Miller continues: "All the other species on the planet seem to get by with relatively small, simple nervous systems that seem tightly optimized just to do what the species needs to do to get by." The scene shifts to a yet another hunched-over actor in a hairy costume. "I think people are perfectly sensible in being skeptical about the ability of selection for survival to account for the human brain. I think there was a sort of guidance happening, there was a sort of decision-making process that was selecting our brains. But it wasn't God, it was our ancestors. They were choosing their sexual partners for their brains, for their behavior, during courtship." We see an apeman-like figure squatting on a narrow ledge. "And I think our semi-intelligent ancestors were the guiding force, they were the guiding hand, in human evolution."

We return to the husband and wife with their adopted child. "When choosing a mate, we still notice beauty," the narrator says, "but what really counts is how someone thinks, feels and acts. All of these are products of the brain." After watching an old film clip of long-nosed Cyrano de Bergerac professing his love to Roxanne, we are told: "It's brains, not beauty, that win her heart."

Miller continues: "There are all sorts of things that mess up brains. And paradoxically, for that reason, brains make really good indicators of how fit you are during courtship. In fact, they're probably better indicators of that even than, than a peacock's tail is about how fit a peacock is."

But Darwin formulated his theory of sexual selection to explain the striking differences we see between the males and females of some species. Sexual selection is supposed to explain why male peacocks have large, colorful tails--and females don't. But men's brains are not significantly larger or more colorful than women's brains. Miller is quite right when he says it is implausible to attribute the human brain to natural selection, because our brain is so much more than what creatures would have needed to survive on the plains of prehistoric Africa. But attributing the human brain to sexual selection is even more implausible

Nevertheless, we are told that Miller's hypothesis is "an intriguing idea," because "it's not the same old saw of tool use, language, culture--it's something entirely different." This is consistent with Stephen Jay Gould's claim, quoted above, that "virtuosity in invention" is "the criterion for acceptance" among evolutionary psychologists. Evolution is telling us just-so stories.

The scene shifts to a performance of the "Hallelujah Chorus" from Handel's Messiah, as the narrator says: "Miller is just getting started when he argues that the size of our brains can be attributed to our ancestors' sexual choices. He's also convinced that artistic expression, no matter how sublime, has its roots in our desire to impress the opposite sex. And that includes music, art, the poetic and storytelling uses of language--even a good sense of humor. According to Miller, they all stem from our instincts for sexual display."

"I think," says Miller, "when a lot of people produce cultural displays, what they're doing in a sense is exercising these, these sexual instincts for impressing the opposite sex. They're not doing it consciously, but what they're doing is investing their products with an awful lot of information about themselves." We watch part of a ballet, and Miller concludes: "I think the capacity for artistic creativity is there because our ancestors valued it when they were making their sexual choices."

So Miller sees all of human culture as a by-product of sexual urges--just as Freudian psychology did. But Freudian psychology is no longer considered good science. "Freud's views lost credibility," wrote University of Chicago evolutionary biologist Jerry A. Coyne in 2000, "when people realized that they were not at all based on science, but were really an ideological edifice, a myth about human life, that was utterly resistant to scientific refutation. By judicious manipulation, every possible observation of human behavior could be (and was) fitted into the Freudian framework. The same trick is now being perpetrated by the evolutionary psychologists. They, too, deal in their own dogmas, and not in propositions of science."

Coyne was criticizing evolutionary psychology in general. But many biologists also criticize Miller's specific ideas about the evolution of the human brain. "How does one actually test these ideas?" wrote University of Sheffield behavioral ecologist Tim Birkhead in a 2000 review of Miller's work. "Without a concerted effort to do this, evolutionary psychology will remain in the realms of armchair entertainment rather than real science." In another review of Miller's work, American Museum of Natural History paleoanthropologist Ian Tattersall wrote: "In the end we are looking here at a product of the storyteller's art, not of science."See . The Coyne quotation is from Jerry A. Coyne, "Of Vice and Men: The fairy tales of evolutionary psychology," a review of Randy Thornhill and Craig Palmer's A Natural History of Rape, in The New Republic (April 3, 2000), last page. The entire review is available at:

Although this episode acknowledges that Miller's ideas are controversial, it presents them uncritically, without mentioning the fact that many biologists don't even consider them scientific. Like the controversy over the biological reasons for sexual reproduction, the controversy over the scientific status of evolutionary psychology is completely ignored by Evolution.

This shallow and lopsided account of evolutionary psychology also has a religious--or rather anti-religious--component. Despite the lack of evidence for his hypothesis, Miller says he is confident that the reason we have our brains "wasn't God, it was our ancestors. They were choosing their sexual partners for their brains, for their behavior, during courtship." And to illustrate Miller's claim that artistic creativity is reducible to our ancestors' sexual choices, Evolution chooses--of all things!--the "Hallelujah Chorus."

God and the Messiah. More religion. Despite the assurance by Evolution's producers that they would avoid "the religious realm," they can't seem to stay away from it.

F. Into the Future?

As we watch a collage of scenes from throughout the episode, the narrator reminds us: "Sex is at the heart of evolution. The process of mixing and passing on genes produces variation, that helps species meet the challenge of life in a competitive world. Sexually selected variations are those that help individuals find mates, and successfully raise young. That's how, for humans, sex became fun, and parenting rewarding."

The scene shifts once more to actors wearing apeman costumes, and the narrator continues: "Those of our ancestors who took pleasure from sex, and satisfaction from parenting, had more surviving offspring than those who didn't. That was true generation after generation. These traits are now almost universal. Even if we choose not to have children, we still enjoy sex. And even when we adopt a child who doesn't carry our genes, we can still find parenting rewarding."

We look in once again on the husband and wife who adopted a baby. Humans, we are told, are the only species that will care for biologically unrelated children over the long term. "Humans are unique," the narrator says. "We are a product of evolution. But we've taken the first tentative steps towards controlling our evolutionary destiny. It's a brave new world we're entering. Only time will tell if we'll be as successful at guiding our future as evolution has been."

So parenting is a good thing--even when it is no longer connected to sex, when it no longer serves the evolutionary purpose of passing on genes. This episode leaves us with some strangely mixed messages. The reproductive behavior of males and females is due to the differences between sperm and eggs--except in certain Panamanian birds, when it isn't. Sexual selection produces striking differences between males and females--except in the evolution of the human brain, when it doesn't. And parenting must be understood in an evolutionary perspective--except in human families with adopted children, when it mustn't.

By ending with a husband and wife who are raising a child unrelated to them, this episode actually raises an issue that is even more of a problem for Darwin's theory than the existence of sex: altruism. Altruism is defined biologically as increasing the fitness of another at the expense of one's own fitness. But an altruist thereby reduces his or her own chances for survival; so in the context of evolutionary theory, altruism should not survive or evolve. Yet altruistic people exist--as we have seen in this episode. Harvard University sociobiologist Edward O. Wilson (founder of the discipline which gave rise to evolutionary psychology) has called this "the culminating mystery of all biology." Why doesn't Evolution even mention it?See . The E.O. Wilson quotation is from Sociobiology (Cambridge, MA: Harvard University Press, 1975), 382. The definition of altruism is taken from the abridged paperback edition (1980), 55.

So this episode completely ignores three major controversies raging beneath the surface of its topic: one over the biological reasons for sex, another over the scientific status of evolutionary psychology, and still another over the mystery of altruism. Rather than educate viewers about what's really going on in biology, Evolution emphasizes two simple-minded messages: God is out, and sex is in. Sex is more important than life itself. Sex is our immortality. Sex is why we have big brains. Instead of providing us with solid scientific evidence, or an honest treatment of serious scientific controversies, this episode relies on just-so stories and sex scenes.

Why Sex? It's a fascinating question. But viewers of Evolution may find themselves asking: Why so much sex--and so little science?

Notes

1. . For a review of the research on Mexican minnows, see Robert C. Vrijenhoek, "Animal Clones and Diversity," BioScience (August, 1998), at: