Blackwell Publishing

Adaptations in sexual reproduction - Does sex reduce the effects of deleterious mutations?


Such is the theory: but how plausible is it?

The theory has two requirements:

1. The mutation rate must be high enough.

A female gains the advantage whatever the deleterious mutation rate, but the advantage increases with mutation rate. The interesting theoretical question is what deleterious mutation rate is needed to outweigh the two-fold cost of sex. The answer depends on the details of the theoretical model, but a total mutation rate of about one per individual is probably enough. If the deleterious mutation rate is that high, a sizeable proportion of females in the population will potentially produce offspring with multiple mutations. How plausible is this number? We do not really know. A total mutation rate of one is higher than has traditionally been supposed. The traditional view had both factual and theoretical support.

In theory, a mutation rate of one was thought to imply an intolerably high genetic load. And in fact, if we take a standard per locus mutation rate of 10 - 6, and assume that practically all mutations are deleterious, then organisms with 104 - 105 loci will have total deleterious mutation rates that are 10 - 100 times less than 1. The measurements of mutational rates are uncertain, and the traditional view of them could easily be in error. There is no strong factual evidence to rule out mutation rates as high as are required for sex to be favored.

2. Multiple deleterious mutations must not have independent effects on fitness.

They must interact, such that an individual with two mutations suffers a greater reduction in fitness than twice the average of the two single mutations. There is no conclusive evidence but a reasonable theoretical case can be made for it.

Therefore, one important conclusion from this discussion is that we urgently need further experimental work of the kind done by Mukai to measure both total mutation rates and the form of fitness interaction between different mutations. Meanwhile, we cannot conclude with certainty whether deleterious mutations alone are the reason why sex exists; but it is one of the two most popular modern theories. In it, the advantage of sex is that it enables females to eliminate more deleterious mutations from their progeny than they could by asexual reproduction.

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