Steve Jones - The Language of the Genes

A few societies once encouraged matings with outsiders. In the Ottoman Empire, talented people were produced by promoting marriages between people from different nations. Their children were seen as 'the fruit of the union of two different species of tree; large and filled with liquid, like a princely pearl 1. In South America after the arrival of the Spaniards there was what the invaders described as 'the conquest of the women'. Paraguay — the site of Elisabeth Nietszche's failed genetical experiment — was known as the Paradise of Mohammed, and every Spaniard had twenty or thirty Indian women. The Governor excused this, saying that 'the homage rendered to God in producing mestizos [children of mixed race who were raised as Christians] is greater than the sin committed by the same act'.

Outbreeding is not usually due to deliberate policy. Most of it arises, like so many of the biological events that shape the human condition, as a by-product of social change. Cities and transport play a central part as each provides a larger pool of potential mates than was available in the days of rural solitude.

In the Aeolian Islands off the coast of Italy in the 1920s a quarter of marriages were between first or second cousins. The figure has dropped to one in fifty (and in Italy as a whole is now less than one per cent). Britain, with its lack of a peasant class settled on its own land, has always been more outbred than most of Europe, but increased outbreeding can be seen here, too, with a striking drop in cousin marriages since Victorian times.

Elsewhere, the picture is not so simple. Some societies promote marriages between relatives for economic reasons. They are still frequent in Indian villages, where up to half of all unions may be of cousins, or of uncle with niece. Indeed, among Pakistani immigrants to Britain the incidence of cousin marriage is greater than in their native land, perhaps because of social isolation. Almost half of British Pakistanis of reproductive age are married to a cousin, a proportion higher that among their own parents.

A crude but effective measure of how related one's own ancestors may have been is to ask how far apart they were bom. If they come from the same village they may well be relatives, but if they were born hundreds of miles apart this is much less likely. For almost everyone today the distance between the places where they and their own partner were born is greater than that separating their parents' birthplaces. In turn, modern fathers and mothers were almost certainly born further apart than were their own parents. In nineteenth-century Oxfordshire the average distance between birthplaces of marriage partners was less than ten miles. Now, it is more than fifty. In the United States it is several hundred, so that most American couples are almost unrelated. All this shows how much the world's populations are beginning to merge. The most important event in recent human evolution has been the invention of the bicycle.

It will take a long time before the mixing is complete: an estimated five hundred years to even out the genetic differences between England and Scotland — and perhaps even longer to get rid of their cultural contrasts. Although homogeneity is a long way away, movement is bound to have an evolutionary effect. No longer will large numbers of children have two copies of a defective piece of DNA because their parents are related. Think of a sexual encounter between an African slave and a white slave-owner in early America. Each has a chance of carrying one copy of certain damaged genes. The most common error in whites is cystic fibrosis, in blacks sickle-cell anaemia. Only children who inherit two copies of either will suffer from inborn disease. Because cystic fibrosis is unknown in Africans and sickle cell in whites the child of a black-white marriage is safe from both.

In many parts of the world immigrant communities are merging with the people already there. Imagine that a tenth of the population of Britain were to immigrate from West Africa (where one person in fifteen is a carrier of the gene for sickle-cell anaemia) and to mate freely with the locals. The number of sickle-cell carriers in the next generation in the new mixed British population would go up by seven times. The incidence of sickle-cell disease (which demands two copies of the damaged gene, one from each parent) would drop by ninety per cent compared to the previous situation in the two groups considered together. Most children would be born to parents from the two different peoples, one of whom — the British partner — does not carry the sickle-cell gene. The incidence of the indigenous British problem, cystic fibrosis, would drop by a sixth.

This model of race mixture is simplistic but not unreasonable. In Britain now, about one marriage in thirtyis between two people of non-European origin; but a third as many is between a non-European and someone whose ancestors were born in the British Isles. The genes of Black Americans are evidence that there has been mating between Americans or African and of European origin for several hundred years. The effect will be more rapid in Britain, where inter-racial marriage is much more acceptable than it is in the New World. Such shifts may mark the beginning of an age of genetic well-being. Increased outbreeding means that recessive genes will more and more be partnered by a normal copy that masks their effects. Social change will dwarf the efforts of scientists to improve genetic health. In time the mixed populations will reach a new equilibrium and many of the hidden genes will reappear, but this will take thousands of years.

Accidental evolution has shaped the genes of small and isolated populations such as the Boers in South Africa and the inhabitants of Tristan da Cunha. In this new mobile world the chances of a small bottleneck and evolution to happen by accident are small indeed. The third part of the Darwinian machine — random change — has, like the other two, lost most of its power.

The great evolutionary fact of the past three centuries has been the population explosion. By the time of the Pilgrim Fathers, the population of the world was twice that on the first Christmas Day. Since then numbers have increased to reach their present six billion. For all creatures, evolution is a slower process than are shifts in distribution or in abundance. Many species go extinct before they have a chance to react to an ecological challenge. For ourselves, some ecological disaster (probably an Einsteinian bang rather than a Malthusian whimper) may mean that speculation about any genetical future is irrelevant.

That future, if it does arrive, will be influenced by local variations in the rate of population growth. Improvements in health care — and a subsequent increase in population number — always precede a decrease in the number of children that parents choose to have. The delay explains the recent explosion. Claims that the world population will double within a century have proved too alarmist. In most places the shift to the new world — a few, healthy children — has taken place more rapidly than even the most optimistic projected.

In the Pilgrim Fathers' day, European genes gained from population growth. Whites filled the world, while black and Asian numbers stayed more or less the same. Now, the equation has shifted. Growth is at its most rapid in Africa. Most European nations, indeed, are not sustaining their own populations, with the mean number of children per family in Italy and Spain well below replacement level. More than ninety per cent of growth is in the developing world, most of all in Africa. The United Nations estimate that more than ninety per cent of the population rise will be in these regions. Africa shows little sign of a decline in birth-rate. The number of children per woman in East Asia decreased from 6.1 to 2.7 between i960 and 1990; but in Africa the figures for those years were 6.6 and 6.2 A third of the globe's population may be of African origin by 2050. The AIDS spectre casts some doubt on this figure, but any difference in the relative rate of growth of distinct groups itself means evolutionary change. In the past — during the agricultural revolution, for example — increased numbers led to mass migration. In spite of the political barriers to movement in the modern world, future Utopians may be brown.