Seminar Two
Evolution of sexuality
Why sex?

Why is sexual reproduction very common and asexual reproduction rare in eukaryotes? (Eukaryotes have cells with nuclei and multiple intracellular organelles, unlike the other two kingdoms of life, bacteria and archaea.)

The Red Queen hypothesis states that sex helps you to run as fast as you can just to stay in the same place (in the host-parasite arms race). In the host-parasite arms race, parasites and pathogens are evolving fast, and hosts infected by them must also evolve fast in order to survive. “You have to run faster just to stay in the same place,” the Red Queen tells Alice, in Lewis Carroll's Through the Looking Glass.

What is sex all about? Parasites and pathogens, according to the Red Queen hypothesis. Sex keeps them guessing. How? With the biological equivalent of lottery tickets.

Sexual reproduction, unlike cloning, produces offspring of different genotypes (different constellations of genes). One individual may survive an infectious or parasitic disease, whereas all may succumb if they are identical genotypes. Sameness kills – consider our disastrous loss of genetic diversity in food crops:

In the Irish Potato Famine of 1845–6, a potato crop blight triggered a famine in which almost one-eighth of the Irish population died. Today there is a serious risk of agroterrorism from the release of a crop pathogen, because we have deprived our crops of their earlier genetic diversity. In contrast, there are still over one hundred varieties of potatoes growing in Andean valleys.

To repeat: sexual reproduction may have advantages because it produces genotypic diversity in offspring. Genotypic diversity is a hedge against infection by pathogens and parasites.

The Red Queen is a very strong hypothesis. There is a second hypothesis, however, based on the discovery that sexual reproduction causes some offspring to carry a surplus of favorable mutations and others to carry a surplus of harmful mutations. Why would this favor sexual reproduction? Because favorable mutations would increase the probability of survival and reproduction. This would purge harmful mutations from the gene pool.

The advantage of this distribution is summed up by George Bernard Shaw's reply to the actress who suggested that they have a child together. The child, she said,would have her beauty and his brains. “Yes, madam,” he replied, “but what if it had my beauty and your brains?” (The wrong end of the bell curve.)

Do bacteria and viruses reproduce sexually? Surprisingly, yes! The way they do this is through horizontal gene transfer, which involves direct movement of genes from one bacterium or virus to another, often in an instant of time. This is one terrible way that antibiotic resistance evolves. Your could call this parasexual, if you insist on semantic distinctions. In any case it achieves genotypic diversity as effectively as true sexual reproduction.