Why We Get Sick (30 page)

Recently, some scientists have proposed that sexual reproduction is maintained by the selective force of the arms race with pathogens. An individual who is genetically identical to many others is vulnerable to any pathogen that discovers the key to exploiting this bonanza
of susceptible individuals. If a clone of ten thousand parthenogenetic women are all vulnerable to influenza, they might all be wiped out by the next epidemic, which would claim only some of their genetically diverse competitors. There is growing support for this hypothesis, including several studies that have found asexual reproduction more frequent in species and in habitats with fewer parasites.

I
magine a time hundreds of millions of years ago, when cells had begun to exchange genetic material to provide variation but before the development of recognizable eggs and sperm. Such haphazard exchange of genetic material is fraught with conflict. A gene that can get itself donated to many other cells has a major fitness advantage, while one that allows itself to be replaced by genes from other cells is at a substantial disadvantage. The successful gene must get itself into new cells, yet not be displaced by incoming genes. In all organisms above the bacterial level, genes from different individuals are rarely allowed to enter. Genetic recombination is instead accomplished by the production of specialized sex cells (gametes) that can be sent off with half the genes needed for the initiation of a new individual. When two such cells find each other, they unite to produce a new organism with equal genetic contributions from each parent.

Gametes face two difficulties. First, they must have sufficient energy stores both to endure until they merge with another gamete and to nourish a developing embryo. Second, they must find another gamete. Large gametes may have abundant energy stores, but they are expensive to make. Small gametes can be produced in enormous numbers at moderate cost, but they can’t survive for long and have nothing to spare for nourishing an embryo. Middle-size gametes sacrifice numbers for larger but still inadequate nutrient supplies and are eliminated by natural selection. Multicellular organisms thus produce only large gametes, which we call eggs, and small ones, which we call sperm.

The next difficulty in understanding human sexuality is why there should be not only two kinds of gametes but two sexes. In other words,
why should there be males that produce sperm and females that produce eggs, rather than hermaphrodites that produce both? Many animals and most plants are hermaphrodites, with both eggs and sperm produced by the same individual. The consensus among biologists is that hermaphroditism can be expected when the same adaptations can serve both sexual functions. Big, bright petals on a flower, for instance, may attract an insect that both brings pollen that fertilizes the plant’s eggs and picks up pollen to fertilize other plants’ eggs. As expected, most flowering plants are hermaphrodites. In mammals, there is a dearth of double-duty adaptations. A penis and secondary characteristics such as antlers serve male functions only. A uterus and milk glands serve only female functions. An individual that invested its limited resources in both male and female strategies would not be much good as either. No species of mammal is hermaphroditic.

The investment a female makes in an egg is many times what a male makes in a sperm. Even when the egg is microscopic, as it is in humans, it is still thousands of times bigger than a sperm, and two hundred million sperm cells are released in a single ejaculate to compete to fertilize a single egg. This initial difference in gamete expense is perpetuated and magnified. If most of the eggs produced are fertilized, most of the nutrients put into them will go to the resulting young. If most of the more numerous sperm die from not being able to fertilize an egg, nutrients put into them will seldom benefit an offspring. Extra nutrients in a sperm would be more likely to retard its swimming and be a handicap in competing for the limited number of eggs.

If an animal releases eggs into the water, it becomes advantageous for the female to postpone their release until conditions are ideal and abundant sperm are nearby. If she can wait to pick a specific male, so much the better. Genes from a robust, healthy male may give her offspring an advantage. If she can induce males to fight over her or otherwise display their prowess, she will better her odds of picking the best possible mate. By retaining the eggs inside her until they are fertilized, she maximizes control over who fertilizes them, wastes fewer eggs that are never fertilized, and can protect the eggs to a later stage of development after fertilization. People automatically think of internal fertilization as meaning internal to the female, but logically this need not be. When seahorses copulate, a female lays eggs into a male’s brood pouch, analogous to a mammalian uterus, where the young develop to an advanced stage. This sort of development inside
the male is exceptional in the animal kingdom. The small size and mobility of sperm cells make it easier for evolution to produce adaptations for getting sperm into a female rather than eggs into a male.

Since the fertilization of a human egg takes place inside the mother, this puts her in charge of the process. It also increases her control over which male will fertilize her eggs. As with females of other species, it is in her reproductive interest to look for males with demonstrable evidence of health and vigor. If females start selecting males with a particular characteristic, such as the huge, colorful feathers of the peacock or the large antlers of an Irish elk, a process of runaway selection may ensue. Males with the characteristic have an advantage simply because females choose them, so females prefer them in order to have sons that the next generation of females will prefer, thus selecting for still more of the characteristic and giving well-endowed males a still greater advantage and a still greater desirability to females. This positive feedback loop elaborates the trait to the point where it may be severely detrimental to the everyday functioning of the males. The poor peacock can hardly fly, and the Irish elk’s antlers became so heavy and unwieldy they have been thought responsible for the species’ extinction. This is a fine example of how natural selection may create traits that are by no means helpful to the individual or its species, only to the individual’s genes. Helena Cronin, in
The Ant and the Peacock
, gives an exquisite history of this idea and of the reluctance of male scientists to acknowledge the power of female choice and its burdensome effects on males.

If there is internal fertilization, the young can presumably be released at the optimal stage. Optimal for whom? Mother? Baby? Father? We’ll come to that soon. Exactly how long the young are retained is a life history feature very much subject to natural selection. With the nine-month human pregnancy, in which an offspring grows from a microscopic mite to an infant of several kilograms, a mother’s investment in each baby is vastly larger than that of the father. On the other hand, she is sure the baby is hers, while her mate may well be uncertain. This uncertainty means that male expenditures of time and energy caring for the offspring will generally have a more dubious payoff than similar investments by females. The initial tiny difference in the cost of sperm versus the cost of an egg is greatly amplified by human reproductive physiology and leads, as we will see, to different reproductive strategies for males and females.

Girls and boys are born in nearly equal numbers, as we explained in
Chapter 2
, because individuals of whichever sex is in excess will have lower reproductive success, on average. Selection therefore constantly shapes parents who have offspring of the scarcer sex, thereby equalizing the sex ratio in the long run. From the standpoint of maximizing collective reproduction, this is inefficient. It takes only a few men to keep a large number of women reproducing at whatever rate would maximize the women’s reproductive success. This is a clear illustration of the greater importance of lower levels of selection relative to higher (group) levels. If selection at the group level were at all important, the sex ratio would be biased toward females.

This is not a matter of merely academic interest. In India, a cultural preference for males has combined with a proliferation of ultra-sound imaging machines, which allow the determination of the sex of a fetus, to severely distort the sex ratio. More than 90 percent of abortions in India are now of female fetuses, and the sex ratio in the general population is beginning to show an imbalance. Similarly, in many areas of China, where population limitation campaigns restrict a couple to one child, that child is a boy more than 60 percent of the time. In the long run such imbalances will be tempered by natural selection, but in the coming generation they will have unpredictable political and social consequences. Our guess is that the excess men will compete vigorously and the scarce women will gain social power with remarkable speed.

C
onflict between the sexes is not continuous. Men and women
can
get along, sometimes for whole days at time, even weeks. This harmony is, however, inevitably disrupted by conflicts that originate in the differing reproductive interests and strategies of men and women. From the original difference between the tiny sperm and the larger egg, whole separate worlds of conflicting strategies have emerged to ensnarl our lives. Women can have a limited number of babies, usually four to six, rarely even as many as twenty according to the record books. Men
can, however, have hundreds of children and have done so in cultures where a combination of surplus resources and social stratification made it possible for some men to have harems of hundreds of women while many others lacked even a single mate. These exceptional cases are extreme examples of the principle that the number of offspring may vary more widely for men than women. This difference arises from a woman’s unavoidably high investment in both time and calories for a single baby, compared to a man’s minimal expense of a few minutes and a single ejaculate.

These differences mean that men and women can and do use different kinds of strategies to maximize their Darwinian fitness. A woman can maximize the number of her genes in future generations by finding and keeping a man who will care and provide well for her and her children and who is disinclined to invest in other women. Men can use a similar strategy by finding and keeping a woman who is fertile, inclined to take good care of her children, and disinclined to mate with other men. Men also have another strategy not available to women, that of inseminating many women while providing little or no support for them and their babies. None of this implies that men and women think through their options in order to arrive at conscious strategies to maximize their reproductive success, and it certainly implies nothing about how people ought to act. Nonetheless, natural selection has inevitably shaped our emotional machinery in ways that maximize our reproduction—or that would have in Stone Age circumstances.

T
he problems that result from these divergent strategies are evident in courtship choices. Females of all species do best if they can find a male who offers good genes and abundant resources. Thus, when females can choose, males compete to prove their abilities in contests that range from the familiar butting contests of deer and sheep to the deep braggadocio of the bullfrog. In other species the female mates with the male with the biggest nuptial gift, usually an insect or other source of protein, sometimes the male himself, as when the preying mantis male is eaten by the female even as he copulates with her. The male mantis might try harder to escape,
but since he is unlikely to find another mate, he probably maximizes his own reproductive success by donating his bodily protein to the female, who can use it to give more to their offspring.

Men, while notoriously less choosy than women, still have strong preferences. A man maximizes his reproductive success by mating with a woman who has been healthy and successful (indicating good genes) and who is maximally fertile (indicated mainly by being in the peak reproductive years), uncommitted (indicated by lack of prior offspring), and able and motivated for mothering. As University of Michigan psychologist David Buss puts it:

So both sexes can increase their fitness by choosing their mates carefully, but they choose different characteristics. Males are relatively more interested in fertility and sexual loyalty, females in good genes and resources. In a study of 10,047 people from diverse cultures and religions in thirty-seven countries, Buss has confirmed these generalizations. Earning capacity was significantly more important to women than to men in all but one of the thirty-seven samples. Youth and appearance were relatively more important to men, and in twenty-three of the thirty-seven samples, men valued chastity significantly more highly than women did, while there was no culture in which the reverse was true.

Mate choice is especially complicated in the human species, where parents mate repeatedly and both provide care for the young. These circumstances mean that a woman faces the risk of being deserted and so must not only assess the current status of her mate but must also try to predict his ability and willingness to stay and provide for her and their offspring. An enduring bond and continuing investment
by the man mean that he now also runs a new risk compared to most other primates, that of being cuckolded. He therefore must assess the likelihood that his prospective mate will mate with other men, thus exposing him to the possibility of unwittingly investing in a woman who may be carrying another man’s baby and, later, in the offspring of another man.