Mutations of intermediate effect are responsible for adaptation in evolving Pseudomonas fluorescens populations

Abstract

The fixation of a beneficial mutation represents the first step in adaptation, and the average effect of such mutations is therefore a fundamental property of evolving populations. It is nevertheless poorly characterized because the rarity of beneficial mutations makes it difficult to obtain reliable estimates of fitness. We obtained 68 genotypes each containing a single fixed beneficial mutation from experimental populations of Pseudomonas fluorescens, evolving in medium with serine as the sole carbon source and estimated the selective advantage of each by competition with the ancestor. The distribution of selection coefficients is modal and closely resembles the Weibull distribution. The average selection coefficient (2.1) and beneficial mutation rate (3.8x10(-8)) are high relative to previous studies, possibly because the ancestral population grows poorly in serine-limited medium. Our experiment suggests that the initial stages of adaptation to stressful environments will involve the substitution of mutations with large effect on fitness.

Figure 1

Distribution of fitness effects for fixed beneficial mutations. The solid line shows the Weibull distribution with parameters that maximize the likelihood of observing our data (mean=2.09, coefficient of variation=0.51). The dashed lines show the Weibull distribution at the 95% confidence boundaries for maximum likelihood of the coefficient of variation (short dashed line: mean=2.03, coefficient of variation=0.43; long dashed line: mean=2.17, coefficient of variation=0.61).