Phenotypic effect of large & small mutations

Fitness phenotypes for a particular genetic trait have a mean value b, noting that a majority of variants have a sub-mean fitness, including one with fitness a. Mutations with small phenotypic effect may increase or decrease fitness, however, even a mutation with positive effect does not produce fitness greater than the mean. Mutations with large phenotypic effect may be effectively lethal (displacement to left-hand tail) or of beneficial effect, substantially greater than the mean (right-hand tail).