Sex as a way of combining advantageous alleles

Let us return and focus on the immediate benefits to a proportion of offspring of getting new combinations of advantageous genes rapidly together on the same genome.

Mathematical models suggest that the process could work to help explain the maintenance of sex, but it tends to depend, among other things, on having relatively small populations and relatively rare beneficial mutations. To see why we need these conditions, note that for sex to be of particular help in combining beneficial mutations, we need conditions where several beneficial mutations to genes are unlikely to rapidly accumulate in the same asexual lineage. Such an outcome will happen when the population is small and/or when beneficial mutations are rare (for the same set of reasons).

That said, sex can sometimes be particularly helpful compared to asexuality in combining advantageous alleles even in larger populations. For example, if there are a number of different genes that can have beneficial mutations and the population is large, then models show that asexual populations will have a really hard job of accumulating beneficial mutations when different clones with different beneficial mutations are continually battling it out (a phenomenon known as 'clonal interference'). In contrast, sex releases this 'speed limit' by allowing the cobbling together of each of these individual advantageous mutations.34

Before going any further, let us pause for a moment. One might reasonably point out that if sex were all about combining good parts of solutions, then parents would eventually run out of ways to improve offspring any further. One might even propose that it would pay an individual to become asexual once it has reached the 'pinnacle' of success. It turns out that the nature of what constitutes a 'beneficial mutation' may change in time and space, in part because of interactions between the effects of different genes, and in part because of interactions within and between species—ecology for short. In fact, thinking about ecology and environment has generated some of the most highly regarded explanations for the evolution of sex, and we will return to them shortly.

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