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of these variants from one of your parents and the more common version from the other, you will be able to get a moderate tan. Inheriting a less common variant from both of your parents is likely to put you at an increased risk of skin cancer.

Your parents’ chromosomes that carry these genes got chopped and recombined when they made the eggs and sperm that created you or other offspring. So the instructions that these eggs and sperm end up carrying is something of a mishmash of the instructions that your parents actually inherited. That is to say, the way in which chromosomes are divvied up creates eggs and sperm (or an embryo, should they combine) with that unique mixture of parental genes. The lining up of chromosomes, the recombining of matching DNA from two individuals, ensures that genes with minor variations are mixed around any given population – although for much of human history, and indeed for many communities today, the pool of possible reproductive partners is rather limited.

On an evolutionary time scale, however, all this mixing is pretty insignificant. Major evolutionary innovations, such as the formation of a new species, or even the creation of an animal that is able to reproduce without sex, depend on the appearance of random mutations. In order for the early eukaryotes to preserve their exciting new method for recombining DNA – sex – mistakes and damage to their DNA needed to be picked up and corrected.

Sex was born not for promoting change and diversity but for limiting them.

There is no doubt about it: sex is popular. The vast majority of species – 99.9 percent of higher animal species and about 92 percent of higher plants – reproduce sexually, at least on an occasional basis. A tremendous amount of research has been devoted to asking why this is the case.

The magnitude of the question becomes apparent if you consider that, for females especially, having sex is dangerous, expensive, and foolhardy. The act of mating can be harmful, with sexual partners sustaining physical injuries or contracting sexually transmitted diseases. Finding a suitable partner involves an investment of time and energy – the work of trying to dodge those physical dangers as well as the risk of picking a mate that, in the worst-case scenario, never produces a viable offspring.

Even if a woman meets her ‘soul mate’ (a very human way of describing the safety of a reproductive partner), mixing genes may be a disaster; sexual reproduction breaks apart combinations of genes that work, after all, and they may not work in their new formation.

Moreover, because a sexually active female allows her mate’s foreign genes to enter her body (and thus her offspring), her own genetic contribution is diluted by one-half. That seems like basic biology, but not all genes are created ‘equal’. Most mutations, those mistakes in the combined DNA, arise in sperm.

Recent estimates show that the rate of mutations in males compared to females is two times higher in rodents, six times higher in most primates, and ten times higher in our primate, humans – which makes you question any concept that human evolution is the ultimate step in some hierarchical process. This comes down to a basic fact regarding how cells are made. Most mutations arise from mistakes in copying the DNA in one cell when it divides to make two cells, and they divide to make four cells, and so on. In cells that divide more often than others, more mistakes are likely to happen, simply because more copying is going on. It’s a bit like a game of Chinese Whispers – the more times a message is passed around a circle of people, the more likely it is to get distorted. In men, cells divide to make ninety thousand sperm every minute – room for a lot of mistakes. And in the past century, geneticists have found that most of these mutations that occur in copying are bad news, and that the mutations frequently interact with each other, with bad results. This means that the prolific production of sperm is more likely to pass on harmful mutations than is the relatively more modest supply of eggs – and, of course, sperm pass their mutations on to offspring produced sexually.

So why, then, do females reproduce sexually, since it’s not optimal for them or their progeny? This long-standing puzzle in evolutionary biology is known as the paradox of sex.

One of the first to tackle an answer was the German biologist August Weismann, deemed by Ernst Mayr to be second only to Darwin among the pioneers of evolution. In 1885, Weismann delivered a lecture series on the ‘Significance of Sexual Selection’. Weismann believed that the reason sex had evolved and was retained was because it provided the variation upon which natural selection could act. Consider his example:

Let us take the case of an insect living among green leaves, and possessing a green colour as a protection against discovery by its enemies... Let us further suppose that the sudden extinction of its food plant compelled this species to seek another plant with a somewhat different shade of green. It is clear that such an insect would not be completely adapted to the new environment. It would therefore be compelled, metaphorically speaking, to endeavour to bring its colour into closer harmony with that of the new food plant, or else the increased chances of detection given to its enemies would lead to its slow and certain extinction. It is obvious that such a species would be altogether unable to produce the required adaptation, for ex hypothesi, its hereditary variations remain the same, one generation after another. If therefore the required shade of green was not previously present, as one of the original individual differences, it could not be produced at any time. If, however, we suppose that