In animals some of the traits that vary racially are ones produced by sexual selection. For instance, lions' manes vary in length and in colour. Males of the astrapia birds of paradise in New Guinea have fancy tails to display to females, but different populations evolved tails of different shapes and colours. From west to east, the tails are broad and purple, short and white-based, very long and white, long and purple, and broad and purple again. Similarly, snow geese occur in two colour phases, a blue phase commoner in the western Arctic and a white phase commoner in the eastern Arctic. Birds of each phase prefer a mate of the same phase. Could human breast shape and skin colour similarly be the outcome of sexual preferences that vary arbitrarily from area to area? After 898 pages Darwin convinced himself that the answer to this question was a resounding 'yes'. He noted that we pay inordinate attention to breasts, hair, eyes, and skin colour in selecting our mates and sex partners. He noted also that people in different parts of the world define beautiful breasts, hair, eyes, and skin by what is familiar to them. Thus, Fijians, Hottentots, and Swedes each grow up with their own learned, arbitrary beauty standards, which tend to maintain each population in conformity with those standards, since individuals deviating too far from the standards would find it harder to obtain a mate. Darwin died before his theory could be tested against rigorous studies of how people actually do select their mates. Such studies have proliferated in recent decades, and I summarized the results in Chapter Five. There I showed that people tend to marry individuals who resemble themselves in every conceivable character, including hair and eye and skin colour. To explain that seeming narcissism of ours, I reasoned that we develop our beauty standards by imprinting on the people we see around us in childhood—especially on our parents and siblings, the people of which we see the most. But our parents and siblings are also the people to whom we bear the strongest physical resemblance, since we share their genes. Thus, if you are a fair-skinned, blue-eyed blonde who grew up in a family of fair-skinned, blue-eyed blondes, that is the sort of person whom you will consider most beautiful and will seek as a mate. In the meantime, my dark-skinned, dark-haired New Guinea friends were growing up with other New Guineans and learning to regard fair-skinned, blue-eyed blondes as grotesquely revolting.
To test that imprinting theory of human mate choice rigorously, one would have to do experiments like shipping some Swedish babies to adoptive parents in New Guinea, or painting some Swedish parents permanently black. Then, after waiting twenty years for the babies to grow up, one could study whether they preferred Swedes or New Guineans as sex partners. Alas, once again, the Search for Truth about humans founders on practical problems, but such tests can be performed with full experimental rigour on animals. Take snow geese, for example, with their blue or white colour phases. Do white geese learn or inherit their preference in the wild for white geese over blue ones? Canadian biologists hatched gosling eggs in an incubator, then put the goslings into a nest of goose 'foster-parents'. When those goslings grew up, they chose a mate with the colour of the foster-parents. When goslings were reared in a large mixed flock of both blue and white birds, they showed no preference between blue and white prospective mates on reaching adulthood. Finally, when the biologists dyed some white parents pink, their offspring came to prefer pink-dyed geese. Thus, geese do not inherit but learn a colour preference, by imprinting on their parents (and on their siblings and playmates). How, then, do I think that people in different parts of the world evolved their differences? Our insides remained invisible to us and were moulded only by natural selection, with results such as that of tropical Africans but not Swedes evolving the anti-malarial defence of a sickle-cell haemoglobin gene. Many visible features of our outsides also got moulded by natural selection. But, just as in animals, sexual selection had a big effect in moulding the external traits by which we pick our mates.
For us humans those traits are especially the skin, eyes, hair, breasts, and genitals. In each part of the world those traits evolved in tandem with our imprinted aesthetic preferences to reach different, somewhat arbitrary results. Which particular human population ended up with any given eye or hair colour may have been partly an accident of what biologists term the 'founder effect'. That is to say, if a few individuals colonize an-empty land and their descendants then multiply to fill the land, the genes of those few founding individuals may still dominate the resulting population many generations later. Just as some birds of paradise ended up with yellow plumes and others with black plumes, so some human populations ended up with yellow hair and others with black hair, some with blue eyes and others with green eyes, some with orange nipples and others with brown nipples.
I do not mean thereby to claim that climate has nothing whatsoever to do with skin colour. I acknowledge that tropical peoples tend on the average to have darker skins than temperate-zone peoples, though there are many exceptions, and that this is probably due to natural selection, though we are unsure of the exact mechanism. Instead, I am saying that sexual selection has been strong enough to render the correlation between skin colour and sun exposure quite imperfect.
If you are still sceptical about how traits and aesthetic preferences can evolve together to different and arbitrary end points, just think about our changing fashion preferences. When I was a schoolboy in the early 1950s, women rated men with crew-cuts and clean-shaven faces a's handsome. Since then, we have seen a parade of men's fashions, including beards, long hair, earrings, purple-dyed hair, and the Mohawk hair style. A man daring to flaunt any of those fashions in the 1950s would have revolted the girls and enjoyed zero mating success. That is not because crew-cuts were better adapted to atmospheric conditions of Stalin's last years, while a purple Mohawk has higher survival value in our post-Chernobyl era. Instead, men's appearances and women's tastes changed in tandem, and the changes occurred far more rapidly than evolutionary changes in skin colour, since no gene mutations were required. Either women came to like crew-cuts because good men had them, or men adopted crew-cuts because good women liked them, or something of both happened. The same goes for women's appearances and men's tastes.
To a zoologist, the visible geographic variability that sexual selection produced in humans is impressive. I have argued in this chapter that much of our variability is a by-product of a distinctive feature of the human life-cycle, our choosiness with respect to our spouses and sex partners. I do not know of any other wild animal species in which eye colour of different populations can be green, blue, grey, brown, or black, while skin colour varies geographically from pale to black and hair is either red, yellow, brown, black, grey, or white. There may be no limits, except those imposed by evolutionary time, on the colours with which sexual selection can adorn us. If humanity survives another 20,000 years, I predict that there will be women with naturally green hair and red eyes—plus men who think such women are the sexiest.