title: Sustained fitness gains and variability in fitness trajectories in the long-term evolution experiment with Escherichia coli.
abstract: Many populations live in environments subject to frequent biotic and abiotic changes. Nonetheless, it is interesting to ask whether an evolving population's mean fitness can increase indefinitely, and potentially without any limit, even in a constant environment. A recent study showed that fitness trajectories of Escherichia coli populations over 50 000 generations were better described by a power-law model than by a hyperbolic model. According to the power-law model, the rate of fitness gain declines over time but fitness has no upper limit, whereas the hyperbolic model implies a hard limit. Here, we examine whether the previously estimated power-law model predicts the fitness trajectory for an additional 10 000 generations. To that end, we conducted more than 1100 new competitive fitness assays. Consistent with the previous study, the power-law model fits the new data better than the hyperbolic model. We also analysed the variability in fitness among populations, finding subtle, but significant, heterogeneity in mean fitness. Some, but not all, of this variation reflects differences in mutation rate that evolved over time. Taken together, our results imply that both adaptation and divergence can continue indefinitely--or at least for a long time--even in a constant environment.
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA Department of Integrative Biology, Michigan State University, East Lansing, MI, USA Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI, USA lenski@msu.edu.
authors.forename: Richard E
authors.initials: RE
authors.lastname: Lenski
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Integrative Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Wiser
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Noah
authors.initials: N
authors.lastname: Ribeck
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Zachary D
authors.initials: ZD
authors.lastname: Blount
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA.
authors.forename: Joshua R
authors.initials: JR
authors.lastname: Nahum
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Biology, University of Alabama at Birmingham, Birmingham, AL, USA.
authors.forename: J Jeffrey
authors.initials: JJ
authors.lastname: Morris
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Biology, University of Washington, Seattle, WA, USA.
authors.forename: Luis
authors.initials: L
authors.lastname: Zaman
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Integrative Biology, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA.
authors.forename: Caroline B
authors.initials: CB
authors.lastname: Turner
authors.affiliation: Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing, MI, USA.
authors.forename: Brian D
authors.initials: BD
authors.lastname: Wade
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Integrative Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Rohan
authors.initials: R
authors.lastname: Maddamsetti
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Alita R
authors.initials: AR
authors.lastname: Burmeister
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Elizabeth J
authors.initials: EJ
authors.lastname: Baird
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Integrative Biology, Michigan State University, East Lansing, MI, USA.
authors.forename: Jay
authors.initials: J
authors.lastname: Bundy
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Nkrumah A
authors.initials: NA
authors.lastname: Grant
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Kyle J
authors.initials: KJ
authors.lastname: Card
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA.
authors.forename: Maia
authors.initials: M
authors.lastname: Rowles
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA.
authors.forename: Kiyana
authors.initials: K
authors.lastname: Weatherspoon
authors.affiliation: BEACON Center for the Study of Evolution in Action, Michigan State University, East Lansing, MI, USA Department of Microbiology, University of Tennessee, Knoxville, TN, USA.
authors.forename: Spiridon E
authors.initials: SE
authors.lastname: Papoulis
authors.affiliation: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Rachel
authors.initials: R
authors.lastname: Sullivan
authors.affiliation: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Colleen
authors.initials: C
authors.lastname: Clark
authors.affiliation: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Mulka
authors.affiliation: Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI, USA.
authors.forename: Neerja
authors.initials: N
authors.lastname: Hajela
mesh_terms: D000222:Adaptation, Physiological / Q000235:genetics; D005075:Biological Evolution; D004777:Environment; D004926:Escherichia coli / Q000235:genetics*; D056084:Genetic Fitness; D005828:Genetics, Population; D008957:Models, Genetic; D059645:Mutation Rate
keywords: adaptation; divergence; epistasis; fitness; hypermutability; power-law model