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But by mentioning ecologists we are getting ahead of ourselves. The point at issue is gene repair. If diploid creatures were to indulge in a little recombination between chromosomes every time their cells divided as the body grew there would be plenty of opportunity for repair. But they do not. They recombine their genes only at the final peculiar division called meiosis that leads to the formation of an egg or a sperm. Bernstein has an answer for this. He says that there is another, more economical way to repair damage to genes during ordinary cell division, which is to allow the fittest cells to survive. There is no need for repair at that stage, because the undamaged cells will soon outgrow the damaged ones. Only when producing germ cells, which go out to face the world alone, need you check for errors.31

So, what’s the verdict on the repair theory? I’d say unproven. Certainly the tools of sex seem to be derived from the tools of repair and certainly recombination achieves some gene repair. But is it the purpose of sex? Probably not.

Cameras and Ratchets

The geneticists, too, are obsessed with damaged DNA. But whereas Bernstein concentrates on the damage that is repaired, the geneticists talk about the damage that cannot be repaired. They call this mutation.

Scientists used to think of mutations as rare events. But in recent years they have gradually come to realize how many mutations happen. They are accumulated at the rate of about one hundred per genome per generation in mammals. That is, your children will have one hundred differences from you and your spouse in their genes as a result of random copying errors by your enzymes, or as a result of mutations in your ovaries or testicles caused by cosmic rays. Of those hundred, about ninety-nine will not matter: they will be so-called silent or neutral mutations that do not affect the sense of genes. That may not seem many given that you have 75,000 pairs of genes, and many of the changes will be tiny and harmless or will happen in silent DNA between genes. But it is enough to lead to a steady accumulation of defects and, of course, a steady rate of invention of new ideas.32

The received wisdom on mutations is that most of them are bad news and a good proportion kill their owners or inheritors (cancer starts as one or more mutations), but that occasionally, among the bad there is a good mutation, a genuine improvement. The sickle-cell-anaemia mutation, for example, can be fatal to those who have two copies of it, but the mutation has actually increased in some parts of Africa, because it gives immunity to malaria.

For many years geneticists concentrated on good mutations and viewed sex as a way of distributing them among the population, like the ‘cross-fertilization’ of good ideas in universities and industries. Just as technology needs ‘sex’ to bring in new innovations from outside, so an animal or plant that relies on only its own inventions will be slow to innovate. The solution is to beg, steal and borrow the inventions of other animals and plants: to get hold of their genes in the way that companies copy each other’s inventions. Plant breeders who try to combine high yield, short stems and disease resistance in rice plants are acting like manufacturers with access to many different inventors. Breeders of asexual plants must wait for the inventions to accumulate slowly within the same lineage. One of the reasons the common mushroom has changed very little over the three centuries that it has been in cultivation is that mushrooms are asexual and so no selective breeding has been possible.33

The most obvious reason to borrow genes is because you can benefit from the ingenuity of others as well as yourself. Sex brings together mutations, constantly rearranging genes into new combinations until fortuitous synergy results. One ancestor of a giraffe, for example, might have invented a longer neck while another invented longer legs. The two together were better than either alone.

But this argument confuses consequence