Chapter The origin of sex

What use is sex?

John Maynard Smith, 'The origin and maintenance of sex' (1971)

It has often been noted that sex is a ludicrous and messy business. Simple organisms seem to be able to reproduce perfectly successfully by splitting or budding: amoebas go on feeding until they are quite large, and then one individual splits into two; a yeast or a sponge buds off side shoots that eventually break free as separate little organisms. So what's the point of sex?

In his book The evolution of sex, the noted British evolutionary thinker John Maynard Smith (1920-2004) wrote in 1978 about the twofold cost of sex. He pointed out that asexual organisms, those that have only one gender and that reproduce by splitting or budding, can increase their population sizes rapidly. Because each individual is effectively a female, each of the offspring is capable of reproducing independently. Sexual organisms, those that reproduce following exchange of genetic material, have two sexes, female and male, and it's the males (of course) that are the problem. So if each female produces two offspring, and there is 1: 1 sex ratio, then on average the two offspring will consist of one female and one male. The rate of doubling of the population size is half that of an equivalent asexual organism.

Technically speaking, the sexual female has half the fitness of the asexual female. Fitness, genetically speaking, is a measure of reproductive success. So the 'twofold cost of sex' is that a sexual organism has half the fitness of its asexual counterpart.

So what is it about sex that has made it such a worthwhile pursuit? Maynard Smith suggested that the advantage was a long-term one, that sex shuffles genes more effectively than parthenogenesis (the production of live young from unfertilized eggs), introducing more genetic variability, and hence adaptability, into a population. He showed that sexual populations can evolve more rapidly than asexual ones, an ability that makes species which reproduce sexually much more resilient when the population is attacked by disease or parasites. The balance of advantage can go both ways. Normally asexual organisms such as aphids may pass through occasional sexual generations. Equally, parthenogenesis has

5 evolved many times among lizards and snakes, groups that are o t typically sexual, of course.

ยก= Sex requires the transfer of genetic material between the male and female, and it is a feature unique to eukaryotes, the more complex organisms. So when, in the rather obscure history of Precambrian life, did eukaryotes arise, and then when did sex first happen? The evidence comes partly from the study of modern organisms, partly from geochemical studies of biomarkers, partly from investigations of ancient atmospheres, and partly from fossil specimens.

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