title: 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.
authors.affiliation: McGill University, Department of Biology, 1205 Avenue Doctor Penfield, Montreal, Quebec H3A 1B1, Canada. rbarrett@zoology.ubc.ca
authors.forename: Rowan D H
authors.initials: RD
authors.lastname: Barrett
authors.forename: R Craig
authors.initials: RC
authors.lastname: MacLean
authors.forename: Graham
authors.initials: G
authors.lastname: Bell
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics*; D005075:Biological Evolution; D009154:Mutation; D011551:Pseudomonas fluorescens / Q000235:genetics / Q000254:growth & development*; D012641:Selection, Genetic