The Red Queen
THE RED QUEEN
A variety of theories have been proposed over the years to explain why sexual reproduction may be more advantageous than asexual reproduction, and, for that matter, why sexual reproduction even exists at all. For years everyone accepted the general proposition that sex is good for evolution because it creates genetic variety, which, in turn, is useful in adapting to constantly changing and challenging environments. But it may give organisms a very different kind of edge.
By the late 1980s, in the contest to explain sex, only two hypotheses remained in contention.
One, the deleterious mutation hypothesis, was the idea that sex exists to purge a species of damaging genetic mutations; Alexey Kondrashov, now at the National Center for Biotechnology Information, has been its principal champion. He argues that in an asexual population, every time a creature dies because of a mutation, that mutation dies with it. In a sexual population, some of the creatures born have lots of mutations and some have few. If the ones with lots of mutations die, then sex purges the species of mutations. Since most mutations are harmful, this gives sex a great advantage.
Can sex earn its keep?
But why eliminate mutations in this way, rather than correcting more of them by better proofreading? Kondrashov has an ingenious explanation of why this makes sense: It may be cheaper to allow some mistakes through and remove them later. The cost of perfecting proofreading mechanisms escalates as you near perfection.
According to Kondrashov's calculations, the rate of deleterious mutations must exceed one per individual per generation if sex is to earn its keep eliminating them; if 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 if the rate is high enough, all that proves is that sex can perhaps play a role in purging mutations.
A variety of theories have been proposed over the years to explain why sexual reproduction may be more advantageous than asexual reproduction, and, for that matter, why sexual reproduction even exists at all. For years everyone accepted the general proposition that sex is good for evolution because it creates genetic variety, which, in turn, is useful in adapting to constantly changing and challenging environments. But it may give organisms a very different kind of edge.
By the late 1980s, in the contest to explain sex, only two hypotheses remained in contention.
One, the deleterious mutation hypothesis, was the idea that sex exists to purge a species of damaging genetic mutations; Alexey Kondrashov, now at the National Center for Biotechnology Information, has been its principal champion. He argues that in an asexual population, every time a creature dies because of a mutation, that mutation dies with it. In a sexual population, some of the creatures born have lots of mutations and some have few. If the ones with lots of mutations die, then sex purges the species of mutations. Since most mutations are harmful, this gives sex a great advantage.
Can sex earn its keep?
But why eliminate mutations in this way, rather than correcting more of them by better proofreading? Kondrashov has an ingenious explanation of why this makes sense: It may be cheaper to allow some mistakes through and remove them later. The cost of perfecting proofreading mechanisms escalates as you near perfection.
According to Kondrashov's calculations, the rate of deleterious mutations must exceed one per individual per generation if sex is to earn its keep eliminating them; if 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 if the rate is high enough, all that proves is that sex can perhaps play a role in purging mutations.