The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [26]
Matthew Meselson, a molecular biologist at Harvard University, has since come up with another explanation that expands on Kondrashov’s idea. Meselson suggests that ‘ordinary’ mutations that change one letter for another in the genetic code are fairly innocuous, because they can be repaired, but that insertions – whole chunks of DNA that jump into the middle of genes – cannot be reversed so easily. These ‘selfish’ insertions tend to spread like an infection, but sex defeats them for sex segregates them into certain individuals whose deaths purge them from the population.41
Kondrashov is prepared to stand by an empirical test of his idea. He says that if the rate of deleterious mutations turns out to be more than one per individual per generation, then he is happy; if it proves to be less than one, then his idea is in trouble. The evidence so far is that the deleterious mutation rate teeters on the edge: it is about one per individual per generation in most creatures. But even supposing it is high enough, all that proves is that sex can perhaps play a role in purging mutations. It does not say that is why sex persists.42
Meanwhile, there are defects in the theory. It fails to explain how bacteria, which have sex rarely in some species and not at all in others, none the less suffer from mutation at a low rate and make fewer proofreading mistakes when copying DNA. As one of Kondrashov’s critics put it, sex is ‘a cumbersome strange tool to have evolved for a housekeeping role’.43
And Kondrashov’s theory suffers from the same flaw as all genetic-repair theories and the Vicar of Bray himself: it works too slowly. Pitted against a clone of asexual individuals, a sexual population must inevitably be driven extinct by the clone’s greater productivity, unless the clone’s genetic drawbacks can appear in time. It is a race against time. For how long? Curtis Lively of the University of Indiana has calculated that for every tenfold increase in population size, the advantage of sex is granted six more generations to show its effects or sex will lose the game. If there are a million individuals, sex has forty generations before it becomes extinct; if a billion, it has eighty. Yet the genetic repair theories all require thousands of generations to do their work. Kondrashov’s is certainly the fastest theory, but it is probably not fast enough.44
There is still no purely genetic theory to explain sex that attracts wide support. An increasing number of students of evolution believe that the solution to the great enigma of sex lies in ecology, not genetics.
CHAPTER THREE
The Power of Parasites
The chess-board is the world; the pieces are the phenomena of the universe; the rules of the game are what we call the laws of Nature. The player on the other side is hidden from us. We know that his play is always fair, just, and patient. But also we know, to our cost, that he never overlooks a mistake, or makes the smallest allowance for ignorance.
Thomas Henry Huxley, ‘A Liberal Education’
Even for microscopic animals, the bdelloid rotifers are peculiar. They live in any kind of fresh water, from puddles in your gutter to hot springs by the Dead Sea and ephemeral ponds on the Antarctic continent. They look like animated commas driven by what appear to be small water-wheels at the front of the body, and when their watery home dries up or freezes, they adopt the shape of an apostrophe and go to sleep. This apostrophe is known as a tun and it is astonishingly resistant to abuse. You can boil it for an hour or freeze it to within one degree of absolute zero, that is to – 272°C, for a whole hour. Not only does it fail to disintegrate, it does