MUTATIONS DO NOT ACCUMULATE IN ASEXUAL ISOLATES CAPABLE OF GROWTH AND EXTINCTION-MULLER'S RATCHET RE-EXAMINED.

Abstract

The rarity of exclusively asexual species is often attributed to Muller's Ratchet. This supposes that because asexual populations cannot recreate individuals with fewer mutations than the currently least-loaded line, mutations will accumulate in such isolates. However, because the computer models that corroborate this theory have assumed isolate immortality, it is possible that mutations will accumulate only if there is "soft" selection acting on relative, rather than absolute, fitness. Here we, therefore, describe several models in which 200 asexual organisms randomly selected from an infinite population in genetic equilibrium under "hard" selection (acting through absolute fitness), were followed for 100 generations. When there were no limits to the fluctuations in population size, the deterministic distribution of mutations per individual was maintained for 100 (as well as for 200) generations. If population growth was limited by a proportional decrease in fertility of the whole isolate, then the isolates tended to become extinct. The rate of extinction was inversely related to maximum isolate size. When resource limitation at maximum population size had an extra deleterious effect on mutants, then isolates shed the mutant classes. Mutations accumulated (ad inifinitum) in immortal isolates whose population numbers were kept constant by proportionately increasing or decreasing each class's size whenever isolate size ≠ 200. Muller's Ratchet, therefore, operates only when mutations affect the outcome of intraspecific contests, but not the organisms' intrinsic ability to survive in the ecosystem.