The Maintenance of Sex and David Lack’s Principle

Combining George C. Williams’ idea that evolutionary constraints prevent asexual mutants from arising more frequently in low fecundity organisms, like mammals and birds, with an earlier one by David Lack that the brood size of these organisms has an optimum, and producing larger broods reduces their fitness, leads to a novel hypothesis about the maintenance of sex in them. All else equal, the eggs of an asexual mutant female should simply start developing without fertilisation, and there is no reason to assume that they would stop doing so after the optimal number of offspring has been produced. Without a way to control their reproductive output, asexual mutants should over-reproduce and suffer a cost of doing so. Experimental studies suggest that the cost of enlarged broods could limit the advantage of asexual mutants considerably. Moreover, research discovered that increased reproductive effort reduces immune functions of low fecundity organisms. This offers a surprising synthesis between Williams’ constraint and Hamilton’s parasite hypothesis on maintaining sex in low fecundity organisms: Compromised immune functions of asexual hosts may render them susceptible rather than adaptation on the side of parasites to overcome host resistance.