The relation of recombination to mutational advance.

TLDR: It is shown that this calculation does not apply for mutant genes that act advantageously only when in some special combinations with one or more other mutant genes, and that as far as these cases of special synergism are concerned recombining lines have no evolutionary advantage over non-recombining ones.

Abstract: The method of calculation is shown wherebt a formula has been derived that approximately the ratio of the rate of accumulation of advantageous mutant genes in a population that undergoes recombination to the rate in an otherwise non-recombining one. A table is given showing the ratios thus found for different frequencies of advantageous mutations and different degrees of their advantage. It is shown that this calculation does not apply for mutant genes that act advantageously only when in some special combinations with one or more other mutant genes, and that as far as these cases of special synergism are concerned recombining lines have no evolutionary advantage over non-recombining ones. Other limitations of the formula are pointed out and assessed. It is explained that most factors that retard the rate of recombination—for expample, linkage, rarity of outbreeding, intercalation of sexual reproduction between more frequent cycles of sexual propagation, and partial isolation between subpopulations—must usually cause little long-term retardation of the speed of advance that is fostered by recombination. Moreover, even where long-term evolutions has virtually ceased, recombination of mutant genes still confers upon a population the means of adopting short-term genetic “dodges”, that adjust it to ecological and “physical” changes in its circumstances, much more rapidly than would be possible for a comparable asexual population. Under conditions where only stability of type is needed, a non-recombining does not actually degenerate as a result of an excess of mutation over selection, after the usual equilibrium between these pressures is reached. However, a irreversible ratchet mechanism exists in the non-recombining species (unlike the recombining ones) that prevents selection, even if intensified, from reducing the mutational loads below the lightest that were in existence when the intensified selection started, whereas, contrariwise, “drift”, and what might be called “selective noise” must allow occasional slips of the lightest loads in the direction of increased weight.