Figure 2. Potent selection favors mutation with large phenotypic variation.
A: The fitness distribution of two classes of mutations. Class A (red) and Class B (blue) generate different amounts of phenotypic variation (shown as the different characteristic widths a and b). For simplicity, we assume that both mutations have the same mode of skewed normal distribution of fitness (shown as varied resistance) and only one side of the distribution is shown. Under stress level x, only the mutants with a resistance level in the shaded area (survival probability α for Class A, β for Class B) can survive. B: Severe stress exaggerates the β/α ratio, and favors the survival of Class B mutants with large phenotypic variations. The 3-dimensional plot demonstrates that the survival probability of Class B mutants (β) relative to Class A mutants (α) increases with either enhancement of stress (x) or increase in phenotypic variation of Class B mutants relative to Class A mutants; the phenotypic variation is represented by characteristic width a and b, respectively. The stress level is normalized to the characteristic width a. For Class A mutation with a fitness distribution that has a characteristic width a, the survival probability α under stress level x is calculated as
where erfc denotes complementary error function.
Class B mutation's probability of survival is calculated similarly.