WHAT IS THE DIFFERENCE BETWEEN MODELS OF ERROR THRESHOLDS AND MULLERSRATCHET

Two independently derived theories predict upper limits to the mutatio n rate beyond which evolution cannot be controlled by natural selectio n. One is the theory of Muller's ratchet, explaining the low phylogene tic age of parthenogenetic clones, the other one is the theory of erro r thresholds, predicting the maximal information content of selfreplic ating molecules in prebiotic evolution. Both theories are based on sim iliar mathematical models but reach qualitatively different conclusion s. Muller's ratchet only works in finite populations, while error thre sholds are a deterministic phenomenon. In this paper it is shown that this discrepancy is due to different assumptions about the fitness val ues the selfreplicative units are allowed to assume. If no lower limit for the fitness values is assumed then the deterministic equilibrium frequency of the currently best genotype is strictly positive, no matt er how strong mutation is, and random drift is required to cause its e xtinction (Muller's ratchet). On the other hand, positive lower limits for the fitness values lead to zero equilibrium frequencies in the de terministic description provided the mutation rate is high enough and no back mutations occur.