by Steve Jones
Einstein once said that 'God does not play dice.' He was wrong: for genes, God does. What number comes up has nothing to do with the DNA involved. That raises an almost theological issue. Is it their own fault that genes, and those who carry them, are damned — or do they perish at random because of simple bad luck? ".
Such evolution by accident is known as genetic drift. The process has been important in our own past. Homo sapiens was until not long ago a rare species that lived in small bands. Until a few tens of thousands of years before the present there were no more people worldwide than live in London today. The few tribal peoples to have survived hint at what society was like.
Until the 1970s, when their lives were destroyed by gold-miners and loggers, about ten thousand Yanomamo Indians lived in a hundred scattered villages in the rainforests of southern Venezuela and northern Brazil. They called themselves 'the fierce people', with good reason.
About a third of all male deaths were due to violence, often in battles between the villages and, the Yanomamo believed, many more to malevolent magic.
Their society was not robust enough to allow groups of more than eighty to a hundred people, including around a dozen young adult males, to stay together. Any larger band tended to split. The splinter group moved away to found a village somewhere else. As a result, the Y.inomamo existed for their whole history (which stretches back in some form to the earliest Americans twelve thousand and more years ago) as a series of small communities in constant conflict.
Most hunter-gatherers may have lived like this. The ancient Siberians who hunted mammoths made houses from their bones. The size of their bony villages suggests that each group, like today's Yanomamo, consisted of a few score people. One odd fact about modern society may also be a hint about the size of ancient groups. Most team efforts involve about the same number of individuals. There are nine members of the US Supreme Court, eleven on a football team, twelve on a jury — and Jesus had twelve disciples. Each Yanomamo band has, curiously enough, about a dozen healthy adult males. Is the difficulty of reaching consensus in a larger group a relic of earlier times? Most people can identify about twelve others whose death would cause them anguish. Aristotle himself pointed out that it is impossible to love more than a few. Could this be a clue (albeit a feeble one) about the size of ancient communities?
Just as for surnames, random genetic change takes place more easily in small populations, when few people bear a particular gene. Then, all or most of the carriers may, by chance, fail to transmit it. In a larger group, a variant may be rare but will be borne by enough people to ensure that at least one will pass it on.
Strange things befall genes in small populations. Again, surnames show what can happen. Their evolution is easy to study, as it needs no more than a telephone book, and names: ire preserved for centuries in marriage records. The world has about a million surnames. Those in China are the oldest and date back to the Han dynasty two thousand years ago. In contrast, Japanese surnames go back only a century or so, when they were ascribed by order of the authorities. Various complications face those who study them. For example, in many places the same name (like my own, Jones, which means 'son of John') appeared independently many times. In some societies, such as those of Spain and Russia, the system breaks down as children take the name of their father and the 'surname' changes each generation. The same was once true in Wales. A boy would take the name of his father and more distant ancestors, each prefixed by the term *ap or 'son of; and — the more the names, the more respected the family. Remnants of the system exist in modern Welsh surnames such as Pugh (son of Hugh), Price (son of Rhys) and Parry (son of Harry). In most places this practice has almost gone.
The telephone book in a long-settled part of the world (such as the mountainous country around Berne) shows that villages just a few miles apart each have a distinct set of names, with, in some villages, almost everyone a bearer of the same one. Within each isolated hamlet there has been an accidental loss of names as, by chance and over the years, some men have had no sons. Because the effect is random, different surnames have taken over in each place. The process may be helped by each village having been founded by a group which had, again by chance, its own characteristic set of last names. It is not the case that within a village one family label is somehow better than the others. Instead, its prevalence reflects the errors of history.
The genes of isolated populations are much the same.
Adjacent Yanomamo (and even Alpine) villages have rather different frequencies of blood groups and other variants. In Alpine villages, blood group frequencies diverge to just the extent predicted from what marriage records say about their size since they were founded. They have evolved by accident.
In large modern cities such as Berne the picture is quite different. The phone book contains thousands of names, none of which is particularly common. Again, the rules of chance and time are at work. Cities contain so many people that it is unlikely that any name, or any gene, will go extinct just because its few carriers fail to pass it on. Such places attract immigrants, so that new names (with their associated genes) come in all the time and the population becomes more diverse. A simple but effective way to measure how distinct a community might be is to count the number of surnames in relation to the number of people. If more or less everyone has a different label then the community is open to migration from many places and is, in effect, so big that accident is unimportant. A glance at the New York telephone directory compared to that of, say, Oslo shows at once that the two have had different histories. The USA has a higher proportion of all global names than anywhere else. That reflects its chronicle of immigration from all over the world.
Shared names mean shared ancestors which in turn means shared DNA. A population in which many people carry the same gene (or the same surname) because they have inherited it from a common ancestor is said to be inbred. To some extent we are all inbred as we are all to some degree related. Everyone has two parents, four grandparents and so on. If all were unrelated, the number of ancestors would double each generation to give an absurd number of ancestors within a few centuries. In fact, related people married, and the lines of descent have merged and blended. As a result we all have many ancestors in common.
Pcrh.ips tlu- most inbred individual ever recorded was,m aristocrat, I Jropjtra-Bcrenikc HI, aunt of the Cleopatra enamoured of Anthony. She may have had identical copies of lull her genes because they descended from a single ancestor. As the ancient Egyptians saw the pharaohs as their gods' posterity they were anxious to keep the deities' blood-line as pure as possible with mating among relatives (sometimes, even, between brother and sister). The story is confused by difficulties in reading the hieroglyphs showing degrees of pharaonic relatedness.
Levels of inbreeding vary greatly from place to place. The incidence of marriages between people with the same name is quite a good indicator. This was noticed by George Darwin, son of the more famous Charles (who married his own cousin). He estimated from surnames that the proportion of cousin marriages (the closest legal form of inbreeding) among British aristocrats, by definition a small and exclusive group, was about four and a half per cent — more than twice that in the general population of his time. The pattern of family names shows that the British population as a whole is, on the average, more outbred than much of the rest of Europe. Even in remote and rural East Anglia, just one in fifty of those present at the end of the eighteenth century had been there in the seventeenth, evidence how much movement there had been in comparison to Switzerland or Italy.
A small village offers little choice when it comes to picking a spouse. As a result, relatives marry and the population becomes inbred. Sometimes the married couple have each received a copy of a harmful recessive gene from their common ancestor. As a result their children are at increased risk of having two copies. George Darwin found that Oxford and Cambridge oarsmen, a healthy group, were less likely to have issued from a cousin marriage than were their more indolent pe
ers.
There are constraints on how close a relative one may marry. Brother with sister is forbidden everywhere but even first cousin marriages may be illegal (as in most US states in the nineteenth century and in Cyprus today). This social imperative may have arisen, in part at least, from a fear that the children might be less healthy. As childhood mortality was in any case so high when the taboos were formulated (so that a small increase because of genetic disease would not be noticed) perhaps they have no biological basis at all.
The death rate does increase and development slows in the children of close relatives. Cousins share a grandparent in common. If he or she carried a harmful recessive (as almost everyone does) their children and grandchildren are more likely than average to inherit two copies. In some Japanese villages before the Second World War, up to a third of all marriages were between cousins. The huge survey of the population of Hiroshima after the atom bombs showed that the children of cousins walked and talked later than others and did worse in school. Part of this was due to the relative poverty of their parents but part reflects their heritage. The same is true in India and in Pakistan, where up to half of all marriages are still between cousins or between uncle and niece. The picture is confused here because such marriages tend to retain wealth within the family and to increase the number of children the parents can afford. Nevertheless, these too survive less well than the children of unrelated parents. First-generation Pakistani immigrants to Britain are also rather inbred. Just one birth in fifty is to such parents, but about five per cent of all inborn disease among British children is to those with Pakistani parents.
It is important not to overstate the dangers of inbreeding. Parents who are cousins have rather more than a ninety per cent chance of a perfectly normal baby, compared to more than ninety-five per cent for unrelated parents. Inbreeding has an effect, but it is dwarfed by the improvements in child health which have taken place in the past few decades. The map of human genetic diversity, based as it is on thousands of points across the genome also gives an insight into inbreeding: a child of' a marriage between a couple with a common ancestor is likely to have double copies of long sections of identical sequence. In populations known from the records to share ancestors in common, this is often the case; but, quite often, children not otherwise known to be inbred also show the same decrease in variation in parts of their genome. The effects of forgotten inbreeding long ago can, it seems, persist for many generations.
In part because of this effect, isolated populations often show high frequencies of inherited abnormalities which are rare elsewhere. Most of the gypsies of South Wales belong to one extended kindred and half their marriages are between relatives (which makes them one of the most inbred peoples on earth). One Welsh gypsy in four carries a copy of the gene for phenylketonuria, which is four hundred times more frequent in this group than in Wales as a whole. A long history of social and sexual isolation has had an effect on their genetic health. Other close-knit family groups, such as the Bedouin of Israel, Jordan and neighbouring countries, may show high levels of inbreeding with, in some places, more than half of all marriages among cousins (and a concomitant local increase in diseases such as inborn deafness). Attempts by geneticists to promote outbreeding on health grounds have had limited success.
The effects of marriages of relatives may be subtle. A few women suffer from recurrent abortion. They often become pregnant, but the foetus is lost. The problem is found among the Hutterites, a religious group who originated in the Tyrol in the sixteenth century. In the 1770s, they moved to Russia, where they flourished and multiplied ten-fold from their original community of a hundred or so. A century later, bigotry was renewed, and the Hutterites migrated to America. All thirty thousand alive today, many of whom live in South Dakota, trace their descent from fewer than ninety founders and nearly all marry within the group. Over the years they have all become quire close relatives, and the more inbred a Hutterite woman might be the longer the interval between her children. Hutterite women who find it difficult to have children share, it transpires, a high fraction of their genes with their husbands. This may reflect the malign effects of inbreeding on the embryo.
In lower animals, genetic variation on the surface of cells determines whether a sperm is allowed to fertilise a particular egg. If the two cells are too similar, then fertilisation fails. Perhaps this is why the complicated system of genetic identification on the cell surface evolved in the first place. The repeated failure of pregnancy in genetically similar husbands and wives may be a remnant of a method of ending fertilisations which arise from the attentions of too close a relative. Spontaneous abortion, perhaps in the first few weeks of pregnancy, kills them off.
Mice have the mechanism in more dramatic form. Females can tell from scent how close a relative a male might be. Given the chance, they avoid mating with their brothers. What is more, if a mouse pregnant by a relative is offered an unrelated male (or even the scent of his urine) she aborts and mates with the new partner. The genes responsible for mouse scent are linked to those that control cell-surface variation.
Among the Hutterites, too, married couples are less similar to one another for certain genes in the immune system than are pairs who are just friends. The genes involved are related to those which drive sexual choices in mice. Perhaps, quite unconsciously, most Hutterites — and most people — fall for someone with a set of identity cues different from their own. What is more, they arc keenest to avoid a partner whose genes are too much like their mother's: the Hutterite mother is to he avoided as a role model in the choice of a wife. Just how the mechanism works, no one knows, but scent may be involved somewhere.
Accidental genetic change is close to how God might play dice. Statistics is needed to study it. Population genetics is infested with mathematics, much of which is incomprehensible even to population geneticists. It is, nevertheless, unavoidable. The importance of random change depends on the size of the population. It is not enough just to know the number of people around today. What is important is its average size since it began; after all, a large town may once have had just a few ancestral inhabitants. What is more, a special kind of average is needed. This pays particular attention to episodes of reduced numbers. Like so many ideas in evolution, the idea of the "harmonic mean' comes from economics. Think of a village in ancient times, with one rich squire and many hungry peasants. Perhaps the fifty poor peasants each had an average income of a hundred pounds a year, while the squire gloried in a million. The average income was nineteen thousand pounds, which is a rather pointless statistic for anyone interested in rural reality. The harmonic mean income, in contrast, was a hundred and two pounds, which is a better reflection of what society was actually like.
The same logic applies to populations which change in number. Thus, the average size of a population whose size in succeeding generations is 1000, 1000, 10, iooo, and iooo is 802 but its harmonic mean size is only 48. Any population bottleneck — ten individuals, in this case — has a dramatic effect that can persist for many generations.
To measure the real size of a population involves other subtleties. Variation in the number of children produced by each person means that its effective size may be less chan first appears. Many tribal populations (and perhaps most ancient societies) show big differences in reproductive success, most of all among males. A few (ias.mov.is monopolise the females, while lots of reluctant celibates do not get their fair share. Freud, in Totem and Tabim (di/light-fully subtitled Some Points of Agreement between the Mental Lives of Savages and Neurotics) built his theory of psychoanalysis on this: a supposed time of a primal horde led by a dominant father with sexual rights to all the women. His sons killed and ate him, inheriting the Oedipus complex which has been such a nuisance ever since.
Many societies do have a rather Freudian structure. In one Yanomamo village, four of the old men had 41, 42, 46 and 62 grandchildren respectively, while twenty-eight had only one grandchild and many more had none. Women, on the other
hand, each had about the same number of descendants. A simple count of the men would much overestimate the real population size. From evolution's point of view, many of them might just as well not be there.
All populations have a history. The iron rules of chance mean that any episode of reduced size — a population bottleneck — will have a prolonged effect. From earliest antiquity humans have been colonisers, first as they filled the world from their African home and later as economic pressure drove people to conquer new lands. The emigrants were a small group, a tiny sample of the people left behind. The new colony may grow into millions, but all its inhabitants carry only the genes of the founders. As there were so few pioneers, the new population may be, by accident, quite different from those who stayed at home.
This 'founder effect', as it is known, is important throughout evolution. Darwin's first port of call on the Heaglt' voyage w.is the island of Madeira. He commented on how ditlcmu its snails were from their European ancestors. This difference became even more conspicuous when he began to look at the birds and tortoises of the Galapagos. Perhaps, Darwin thought, the accidents of history, with chance colonisations of each island, helped to explain why archipelagos were natural laboratories for evolution.
The quirks of colonisation have been just as important in our own past. Ironically enough, the best example of evolutionary accident comes from not an escape but a return: the Afrikaners' journey back to their ancestral continent after an absence of more than a hundred thousand years. They began their migration from Europe in the 1650s. The pioneers brought with them a lasting legacy. It included more than Calvinism and bigotry. The surnames and the genes of their descendants are still a bequest from the first migrants. The three million Afrikaners in South Africa all derive from a small group of settlers, some of whom were so enthusiastic in their fecundity as to leave tens of thousands of descendants today. A million Afrikaners share just twenty names (Botha being one). This fits what history tells us about the number of immigrant families. Even today, half the surnames arrived before 1691 and the other half before 1717.