Mutagen-Mutation Equilibria in Evolution

Abstract

Mutations are double-faced like Janus. They are most often detrimental to their carriers — they also represent the ultimate source of genetic variability and, hence, of evolution. This antagonism requires a delicate balance. It may be achieved by one or more of three different strategies:

• 1. Mutations do not entirely occur at random in the genome. This is evident for specific types of somatic mutations in mammals (Baltimore 1981; Golub 1987; Meyer et al. 1986) and for transposon-induction of mutations in the germ line (Engels and Preston 1984; O’Hare and Rubin 1983; Rubin et al. 1982; see also Campbell 1983, and Syvanen 1984).

• 2. Mutation rates are adjusted to evolutionary requirements. This has since long been discussed controversially (Benado et al. 1976; Biémont et al. 1987; Chao et al. 1983; Cox and Gibson 1974; Dobzhansky et al. 1952; Gillespie 1981; Holsinger and Feldman 1983; Ives 1950; Nöthel 1983, 1987; Sturtevant 1937) but seems more likely in view of the surprising mixture of error-proof and error-prone (i.e. mutagenic) mechanisms of DNA repair (Kimball 1987).

• 3. In diploids, the genetic load due to recessive variants is not a mutational load that has to be avoided in order to gain optimal population fitness, but mainly is a tolerable “segregational noise” produced by selectively favoured heterozygotes according to the “balance-hypothesis”.