This chapter asks: why does sex exist? why aren’t there more efficient ways to procreate?
The most obvious reason to borrow genes is because you can benefit from the ingenuity of others as well as yourself.
Matt Ridley, The Red Queen
Sex invites mutations. A giraffe’s ancestor invented a longer neck, while another invented longer legs. The two together were better than either alone. But this argument conflates consequence with cause. The advantages will take a while to manifest, they will only be appreciated after a few generations. By then, any asexual competitor would have outnumbered its sexual rivals.
If sex is good at combining genes, it is even better at breaking them up. Offspring is always different from parents.
Plant breeders much prefer varieties of wheat or corn that are male-sterile and produce seeds without sex, because it enables them to be sure their good varieties will breed true. It is almost the definition of sex that it breaks up combinations of genes.
Sex disobeys that injunction: ‘If it ain’t broke, don’t fix it.’ Sex increases randomness.
There are theories of “good mutations”, these are interested in how to produce the same thing twice rather than two separate inventions. Suppose that blue eyes double fertility (compared to brown). But suppose everyone has brown eyes.
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The first mutation in a brown-eyed person to blue eyes will have no effect because blue eyes are a recessive gene. Only when descendants of the original mutant person marry, and both of their blue-eye genes come together will we see the benefit of blue eyes.
Only sex can make this possible. This segregation theory of sex is logical and uncontroversial. It is one of the advantages of sex.
Unfortunately, it is far too weak an effect to be the main explanation for sex’s prevalence. Mathematical models reveal that it would take five thousand generations to do its good work and a sex would long ago have won the game by then.
Recently, geneticists have begun to think more about bad mutations than good ones. They suggest that sex is a way of getting rid of bad mutations. This idea has its origin in the 1960’s. ‘Muller’s ratchet’ is an example where there are ten water fleas in a tank, and only one is free of mutations, while the others one or many defects.
On average only five of the water fleas in each generation manage to breed before they are consumed by a fish. The defect-free flea has a fifty percent chance of not breeding. So does the flea with the most defects. But there is a difference.
Once the defect-free flea is dead the only way for it to be recreated is for another mutation to correct the mutation in a flea with a defect – a very unlikely possibility. The one with two defects can be recreated easily, by a single mutation in a water flea with one defect anywhere among its genes. In other words, the random loss of certain lines of descent will mean that the average number of defects gradually increases. Just as a ratchet turns easily one way, but cannot turn back, so genetic defects inevitably accumulate. The only way to prevent the ratchet turning is for the perfect flea to have sex and pass its defect-free genes to other fleas before it dies.
Matt Ridley, The Red Queen