Steve Jones - The Language of the Genes

To define sex is easy enough. It makes individuals who contain genes from more than one line of descent, so that inherited information from different ancestors is brought together. In an asexual lineage everyone has one mother, one grandmother, one great-grandmother and so on in an unbroken chain of direct descent from the ur-mother who began the lineage. Sexual organisms are different, because the number of ancestors doubles each generation. Everyone has two parents, four grandparents and so on. Each sperm or egg has half the number of genes present in body cells and in each the genes are scrambled into new arrangements by recombination. When they meet, the novel arrays come together to produce a new and unique individual. Reshuffling the genetic message is at the heart of sexual reproduction.

The nature of sex is illustrated by two eponymous heroes of British history, King Edward VII (who flourished in the years before the First World War) and the King Edward variety of potato (which has fed the British working class for almost as long). The potato, unlike the royals, reproduces asexually. Every King Edward potato is identical to every other and each has the same set of genes as the hoary ancestor of all potatoes that bear that name. This is convenient for the farmer and the grocer, which is why sex is not encouraged among potatoes. King Edward himself was a different kettle of fish. Malt his genes came from his mother, Queen Victoria, and half from his father, Prince Albert. He himself was a new and unique genetic mixture who combined some of the qualities of the two and of an ever-widening pool of more distant ancestors.

It is easier to define sex than to understand it. Men have made many attempts to justify their existence. Mutation may help to explain why life is not entirely female. A harmful change to the DNA in a sexless being will be carried by all her descendants. None can ever get rid of it, destructive though it might be, unless it is reversed by another change in the same gene (which is unlikely). In time, a second error will occur in another gene in the family line. A decay of the genetic message sets in as one generation succeeds another, just like the decay that takes place within an ageing body as its cells divide without benefit of sex.

In a sexual creature, by contrast, the new mutation can be purged as it passes to some descendants but not others. Quite often, one unfortunate will, by chance, inherit several damaged genes, and all are lost at the cost of a single death. Sex also has a more positive effect: as the environment changes some new combinations of genes may be better able to cope with the novel challenge. New mixtures of genes produce successful individuals who have been dealt a favourable hand and others who inherit a less advantageous set. George Bernard Shaw illustrated this in a hackneyed but accurate phrase. When an actress asked if she could bear his child, who might have her body and his brains, Shaw pointed out the risk of having an infant with her brains and his body. Sex reshuffles life's cards: it makes beautiful geniuses who survive and ugly fools who do not. It is a convenient way to bring together the best and purge the worst and to separate the fate of genes from that of those who carry them.

Recombination is a redemption, which, each generation, reverses biological decay. In some ways, it is the key to immortality; a fountain of eternal youth — not for those who indulge in it, but for the genes they carry. Sex speeds up evolution because each generation consists of now combinations of genes, rather than thousands of copies of the same one. Instead of always drawing the same hand in nature's card game (which might be successful in one encounter but will not be so in all), every fertilised egg has a new deal and a new chance to win. The chance may be a small one, but as so many hands are dealt sex becomes a worthwhile, albeit expensive, gamble against a hostile world.

To abandon males can cause problems. The majority of all-female plants can be used for only a few years. They become so loaded with genetic damage that they no longer thrive, or cannot keep up in the evolutionary race with their parasites who in time prevail. Their lineage has become old and tired. Potatoes show the risks of celibacy. The Irish famine happened because the plants used belonged to an old and sexless variety. In the mid-nineteenth century every tuber in European was descended from one or two introductions from the New World made three hundred years earlier. The new crop soon spread throughout Europe. Louis XVI of France, in an astute exploitation of the rustic mind, put guards on the first fields during the day but removed them at night. The peasants, impressed by the apparent value of the crop, were quick to steal examples and to grow them on their own land. In Ireland in 1840 every adult ate several pounds of potatoes a day (in part because their grain was exported to England to pay rent).

Famine struck with great speed and ruinous effect. In i 845, the Irish Freeman's journal wrote 'We regret to have to state that we have had communications from more than one correspondent announcing the fact of what is called 'cholera' in potatoes in Ireland, especially in the North. In one instance the party had been digging potatoes — the finest he had ever seen from a particular field, and a particular ridge in that field until Monday last; and digging in the same ridge on Tuesday he found the tubers all blasted and unfit for the use of man or beast.' In the next five years, a million and a half people starved. Their crop had been attacked by a fungus, the potato blight, which is sexual and has many generations to each one among its hosts. The parasites evolved at a greater rate than could the potato. Nowadays, plants with new sets of genes are tried every few years to counter this. Other asexual crops, such as bananas, have as yet escaped the fate of the Irish potato (although it cannot be indefinitely delayed). The potatoes were forced into an evolutionary dead end from which the sole escape is sex.

Because of the dangers, rather few animals have abandoned that pastime. They include the odd lizard or fish, but none of our close relatives. Even greenflies, which manage without it for most of the time, require a bout once a year or so. With occasional exceptions such as rotifers (tiny denizens of fresh water, among whom no male has ever been found), all-female lineages derive from recent ancestors with a normal sex life, as a hint that chastity is an evolutionary dead end. Just why abstinence is undesirable is still not certain. In spite of the attractions of the mutation theory the frank answer is that, although the reason for the existence of women is obvious enough, there is still plenty of room for argument about the point of being a man.

The perils of abstinence can be seen in men themselves, as they possess the Y, the only chromosome that has abandoned the hobby. When germ cells are formed, all the other chromosomes line up next to each other — chromosome 2 with chromosome 2 or X with X, for example — and indulge in recombination, the sexual orgy of genetic exchange. In a male, the Y does line up with the X, but its embrace of its fellow is less than enthusiastic. Only the tip of the Y exchanges material with its opposite. The rest of the chromosome is held in a kind of biological purdah, safe from the advances of other genes.

Chastity has had terrible effects on the Y. It has long sequences of meaningless DNA letters, many repeated thousands of times. Perhaps this is a hint of what might happen to asexual lineages if abstinence goes on for long enough. Mutations accumulate and cannot be shed and junk DNA may creep in and prove impossible to dislodge. Apart from its narrow role in ensuring the persistence of men, the Y chromosome is a warning of the dangers of continence.