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Distribution of fitness effects among beneficial mutations before selection in experimental populations of bacteria

Nature Genetics volume 38, pages 484488 (2006) | Download Citation



The extent to which a population diverges from its ancestor through adaptive evolution depends on variation supplied by novel beneficial mutations. Extending earlier work1,2, recent theory makes two predictions that seem to be robust to biological details: the distribution of fitness effects among beneficial mutations before selection should be (i) exponential and (ii) invariant, meaning it is always exponential regardless of the fitness rank of the wild-type allele3,4. Here we test these predictions by assaying the fitness of 665 independently derived single-step mutations in the bacterium Pseudomonas fluorescens across a range of environments. We show that the distribution of fitness effects among beneficial mutations is indistinguishable from an exponential despite marked variation in the fitness rank of the wild type across environments. These results suggest that the initial step in adaptive evolution—the production of novel beneficial mutants from which selection sorts—is very general, being characterized by an approximately exponential distribution with many mutations of small effect and few of large effect. We also document substantial variation in the pleiotropic costs of antibiotic resistance, a result that may have implications for strategies aimed at eliminating resistant pathogens in animal and human populations.

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Thanks to M. Al-Azzabi and E. Drummond for technical assistance in the lab. S. Otto, S. Aris-Brossou, C. Zeyl, F.B. Christiansen and O.F. Christiansen provided comments. This work was supported by a Discovery Grant to R.K. from the Natural Sciences and Education Research Council of Canada.

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  1. Department of Biology and Center for Advanced Research in Environmental Genomics, University of Ottawa, Ottawa K1N 6N5, Canada.

    • Rees Kassen
  2. Bioinformatics Research Center, University of Aarhus, Denmark.

    • Thomas Bataillon
  3. Unité Mixte de Recherche 1097, Diversité & Génomes des Plantes Cultivées, Montpellier, France.

    • Thomas Bataillon


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The authors declare no competing financial interests.

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Correspondence to Rees Kassen.

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