Mutations can
result from
one change in
one nucleotide
of 6 billion in a
human cell.
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. It does not explain why sex persists.


Strawberries
reproduce asexually
by vegetative
propagation --
sending out runners.
The main defect in Kondrashov's hypothesis is that 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. Currently, a great deal of effort is going into the testing of this model by measuring the deleterious mutation rate, in a range of organisms from yeast to mouse. But the answer is still not entirely clear.

,
human style 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 gentric mutations 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.
Salamanders can
reproduce sexually
or asexually.

sex advantage

Why does sex -- that is, sexual reproduction -- exist? In many ways, asexual reproduction is a better evolutionary strategy: Only one parent is required, and all of that parent's genes are passed on to its progeny. In sexual reproduction, only half of each parent's genes are passed to the next generation. What's more, a mate must be found. Yet sex persists.

This essay offers possible explanations of this evolutionary paradox.



Matt Ridley is the author of The Red Queen: Sex and the Evolution of Human Nature (1995), The Origins of Virtue: Human Instincts and the Evolution of Cooperation (1998), and Genome: The Autobiography of a Species in 23 Chapters (2000). A former science editor and Washington correspondent of The Economist, he now lives in northeast England, where he is chairman of a science center called The International Centre for Life