Phenotypic effect of mutations with Large & Small effect

    Fitness phenotypes for a particular genetic trait have a mean value b. A majority of variants have a sub-mean fitness, including one with fitness a. Mutations with small phenotypic effect may increase or decrease fitness, within the range of existing variance. Mutations with large phenotypic effect may be effectively lethal (displacement to left-hand tail) or of dis-advantageous effect  (displacement to the right-hand tail). This analysis suggests genotypic variance with populations results from small, incremental mutations, rather than large, discontinuous mutations