The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature (16 page)

Runaway is especially good at explaining the evolutionarily unpredictable—why extreme traits can arise in one species but not in closely related species. Many of the human mind's most interesting capacities do not appear in other apes, and those of most hominids are not discernible from the archeological record. Runaway requires polygyny, and almost every human culture throughout history has been overtly polygynous to some extent. Runaway is extremely fast once it gets going. The fossil record reveals a few rapid increases in brain size punctuated by long periods of relative stasis, which could mark a series of runaway events.

The two major problems with the runaway brain theory are the multi-step progressiveness of brain size evolution, and the minimal sex differences in human mental ability. Pure runaway is not biased in any particular direction, yet for the last two million years human brain evolution has shown a consistent trend towards larger size and higher intelligence. Runaway should not be so consistent. Moreover, pure runaway should have produced large-brained, hyper-intelligent males, and small-brained, ape-minded females. That has not happened. I have reviewed some factors that may have minimized sex differences: genetic correlation between the sexes, the overlap of mental capacities for courtship behavior and for sexual choice, and mutual mate choice. But the most compelling of these factors, mutual mate choice, is not consistent with a pure runaway process.

I think that mutual mate choice in humans is so important that the pure runaway brain theory just cannot be right. This chapter started by praising it, but has ended by burying it. I do not think that female creative intelligence is a genetic side-effect of male creative intelligence, or arose simply as a way of assessing male courtship displays. I think that female creative intelligence evolved through male mate choice as much as male creative intelligence evolved through female mate choice. I shall turn next to a model of sexual selection that works better with mutual mate choice. It emphasizes how sexual ornaments advertise each sex's fitness to the other sex—a function of mate choice that may stretch back to the origins of sexual reproduction itself

4

A Mind Fit for Mating

Before sexual reproduction evolved, there were several ways for organisms to accomplish the evolutionary task of spreading their DNA around. There was the divide-and-conquer strategy: wrap DNA in single cells that busily eat nutrients until they grow large enough to split in half, leaving each half to grow and split in turn. Bacteria are the masters of this technique, capable of doubling their populations every few minutes, but vulnerable to mass extermination through perils such as toothbrushes and soap.

There was also the cloning-factory strategy: grow a body with billions of cells, and then assign the task of DNA-spreading to a

privileged minority of those cells, which bud off to make new,

genetically identical bodies. Many fungi reproduce this way, epitomizing the rustic virtues of simplicity and fecundity. Yet this

strategy, though successful in the short term, stores up trouble for the long term. Once a harmful mutation arises, as it sooner or later will, there is no means of expunging it. This propensity to accumulate damaging mutations makes such asexual species quite unsuited to evolving much sophistication. This is because bodily and mental sophistication require a great deal of DNA, and the more DNA one has, the more trouble mutations cause.

In the last few hundred million years, an increasing number of species have turned to a third way of spreading their DNA around—the fashionable new method called sexual reproduction, with improved mutation-cleansing powers. One grows a trillion-celled body to produce packets of DNA, makes sure those DNA packets find complementary DNA packets from suitable others, and permits the DNA to combine with that of another individual to

produce offspring that bear traits from both parents. Of the 1.7 million known species on our planet, most engage in sexual reproduction. Sexual species include almost all plants larger than a buttercup and almost all animals larger than your thumb. It includes most insects, all birds, and all mammals, including all primates.

Copying Errors

At the beginning, this DNA-combining called sex was probably not very selective. It was simply the most convenient way to make sure that not all of your offspring inherited your mutations. In evolution, mutations are generally a bad thing. Since almost all mutations are harmful, organisms evolve sophisticated DNA repair machinery to correct mutations. Of course, in the long term, mutations are necessary for evolutionary progress, because a tiny minority prove helpful when a species faces new challenges. But organisms don't plan for the long term. To the organism, mutations are simply copying errors—mistakes made when trying to spread DNA by producing offspring.

If you have only one copy of each gene, it is hard to know when certain kinds of copying error have been made. Some errors just won't look right to the DNA repair machinery. They are chemical nonsense, and easily fixed. But other errors look just like ordinary working DNA. These' pseudo-normal mutations are the problem. They look like good DNA to the repair machinery, but they do not act like good DNA when you try to grow an organism using them. They undermine the biological efficiency called fitness. Unless there is some way of eliminating them, they will accumulate, generation after generation, gradually eroding the fitness of offspring.

In very recent work, biologists Adam Eyre-Walker and Peter Keightley calculated that the average human has 1.6 harmful new mutations that neither parent had. Our ancestors would have accumulated mutations at the same rate. Geneticist James Crow thinks this estimate too conservative by half, and suggests that we have 3 new harmful mutations per individual every generation.

That doesn't sound too bad, given that we have about 80,000 genes, yet this mutation rate is near the theoretical limit of what selection can cope with. For a species to avoid going extinct as a result of accumulating too many harmful mutations, selection must be able to eliminate mutations at the same average rate that mutations arise, otherwise the species would suffer a "mutational meltdown." For technical reasons, it is very hard to avoid a mutational meltdown when more than one harmful new mutation arises per individual. In fact, it may be impossible without sexual reproduction.

Sexual reproduction probably arose as a way to contain the damage caused by mutations. By mixing up your DNA with that of another individual to make offspring, you make sure that any mutations you have will end up in only half of your offspring. Your sexual partner will have mutations of their own, but they are almost certain to be different mutations on different genes. Because offspring have two copies of each gene, the normal version inherited from one parent often masks the failures of the mutated version inherited from the other parents. Incest is a bad idea because blood relatives often inherit the same mutations, which are not masked by normal genes when close relatives produce offspring. For example, you may need just a little bit of the protein produced by a gene, so one copy of the gene may suffice. The mutated gene's inability to produce a working protein may not matter very much. This masking effect is called genetic dominance. Dominance makes sex very powerful in limiting the damage

caused by mutations.

However, dominance is often not perfect, and it is really only a short-term solution. Two normal genes are sometimes still better than one. And hiding the effects of mutations allows them to accumulate over evolutionary time. To keep mutations from accumulating over the longer term, sexual reproduction takes some chances. Consider two parents with average numbers of mutations. Each contributes half of their genes to each offspring. Most of the offspring will inherit nearly the same number of mutations as their parents had. But some may be lucky: they may

inherit a below-average number of mutations from their father, and a below-average number from their mother too. They will have much better genes than average, and should survive and reproduce very well. Their relatively mutation-free genes will spread through future generations. Other offspring may be very unlucky: they may inherit an above-average load of mutations from both parents, and may fail to develop at all, or may die in infancy. When they die, they take a large number of mutations with them into evolutionary oblivion.

This effect is extremely important. By endowing the next generation with unequal numbers of mutations, sexual reproduction ensures that at least some offspring will have very good genes. They will preserve the genetic information that keeps the species working. From a selfish gene's point of view, it does not matter that some offspring have very bad genes full of mutations, because those mutations would have died out sooner or later anyway. Better to concentrate them in as few bodies as possible so they do the least damage over the long term. Investment analysts will recognize that sexual reproduction is a way of implementing a risk-seeking strategy. Since evolution over the long term is a winner-takes-all contest, it is more important to produce a few offspring that have a chance to do very well, than a larger number of mediocre offspring.

Mutations, Fitness, and Sexual Attractiveness

Now, if the goal of sexual reproduction is to keep at least some of your offspring safe from your harmful mutations, it would be foolish to pick your sexual partners at random. Any sex partner will carry his or her own load of mutations. You should pick the partner with the lowest number of harmful mutations: that will give your offspring the highest expected fitness, which means the best chance of surviving and reproducing. If your choice of sexual partner is very good indeed, your genes may hitch a ride to evolutionary stardom on the genetic quality of your mate. Many biologists are coming to the view that mate choice is a strategy for getting the best genes you can for your offspring.

Because of genetic dominance, many mutations are hidden from view. They do not affect body or behavior, so they cannot be used in mate choice. However, dominance is often incomplete, and a lot of genetic variation between individuals does show up in body and behavior. Some traits reveal more genetic information than others. Complex traits such as peacock tails that vary conspicuously between individuals may be especially informative.

Their complexity means that their development depends on many genes interacting efficiently. They summarize more genetic

information by being more complicated. And their variation at the visible level of body and behavior means that genetic variation can be perceived during mate choice. With sexual selection there is a big incentive to pay very close attention to traits like these.

Such traits are called "fitness indicators." A fitness indicator is a biological trait that evolved specifically to advertise an animal's fitness. Fitness means the propensity to survive and reproduce successfully. It is determined mainly by an individual's genetic quality, which boils down to their mutation load.

There is a close connection between mutations and fitness. If a species has been living in its present environment for many generations, its average genes are probably very well adapted to that environment. Because they have already been tested again and again by natural selection, the average genes in the species are already optimal. If they weren't, they would already have been replaced by different genes. This suggests that any deviation from the genetic norm is a deviation from optimality. Mutations are deviations from the genetic norm. If a set of mutations makes an individual unable to grow an optimal body and unable to produce optimal behavior, then they impair that individual's ability to survive and reproduce. Since fitness means the ability to survive and reproduce, mutations almost always lower fitness; conversely, high fitness implies freedom from harmful mutations. If fitness indicators advertise high fitness, they are also advertising freedom from mutations, which is what mate choice wants. Normal genes are tried and tested, whereas mutations are shots in the dark.

Sexual selection needs some way to connect the sensory abilities

of animals to the mutation levels of the potential mates they are choosing between. Fitness indicators are the connection, for they are the traits that make fitness visible. What they make visible can be favored by mate choice, and what is favored by mate choice can evolve through sexual selection. Fitness indicators are the genetic sieve that lets sexual selection sift out harmful mutations. In this mutation-centered view of sex, sexual ornaments and courtship behaviors evolve as fitness indicators.

The Human Mind as a Set of Fitness Indicators

In the previous chapter we met the runaway brain theory. It has problems: it does not explain the trend of hominid brain evolution toward the big and the bright, and it does not work very well with mutual mate choice. However, there is another possible solution. Perhaps the human mind's most distinctive capacities evolved through sexual selection as fitness indicators.

We could call this the "healthy brain theory," in contrast to the runaway brain theory The healthy brain theory suggests that our brains are different from those of other apes not because extravagantly large brains helped us to survive or to raise offspring, but because such brains are simply better advertisements of how good our genes are. The more complicated the brain, the easier it is to mess up. The human brain's great complexity makes it vulnerable to impairment through mutations, and its great size makes it physiologically costly. By producing behaviors such as language and art that only a costly, complex brain could produce, we may be advertising our fitness to potential mates. If sexual selection favored the minds that seemed fit for mating, our creative intelligence could have evolved not because it gives us any survival advantage, but because it makes us especially vulnerable to revealing our mutations in our behavior.