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 need not produce fitness greater than the
mean. Mutations with
large phenotypic effect may be effectively lethal
(displacement to left-hand tail) or of advantageous effect,
substantially greater than the mean (right-hand tail).