The Good Book points out, in Ecclesiastes, that 'The race is not to the swift, nor the battle to the strong. hut time and chance happeneth to them all.' Evolution is all about change with time, but how things evolve is often a matter of chance. The nature of inheritance means that random events are bound to direct our genes as the generations succeed one another. As a result, much of the human condition is shaped by accident.
The importance of chance in evolution was noticed by the English cleric Thomas Malthus. He became interested in the history of the burghers of Berne and followed the records of their names over several centuries. To his surprise, many of the surnames present at the beginning of the period had gone by the end, although the number of burghers stayed about the same. Francis Galton showed why.
A surname is rather like a gene; it passes from father to son. Every generation there is a chance that a particular father does not have a male child. Perhaps he has just daughters (who lose their names on marriage) or no children at all. His name is then lost from his family line. If he has no children the name will go at once. It is also more likely to become extinct if he has a smafl family, as a limited brood will quite often consist of daughters alone. If, in a closed community like that of the Bernese bourgeoisie, this process goes on for long enough, more and more names will disappear as the years pass. Given enough time, just one surname will survive. Everyone will carry the same inherited message (at least as far as their name is concerned). In addition, the community will be more inbred than before, as rhc only mates available will be people who share the same name, all of whom descend from a common ancestor.
Just the same happens to genes. Perhaps, among the mediaeval Bernese bourgeoisie, there was a rare gene — an unusual blood group, for example. Because Berne was a small town, few people carried it. If none passed it on (because they had no children, or because by chance the gene did not get into the sperm or egg that made the next generation) then it was lost. On the other hand, the carriers might, again by chance, have had more children than the rest, in which case the variant became more common. In either case, its frequency altered (which means that the population evolved) in a manner that depends only on the accidents of time.
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.