Long-Term Dynamics of Adaptation in Asexual Populations

Science  13 Dec 2013:
Vol. 342, Issue 6164, pp. 1364-1367
DOI: 10.1126/science.1243357

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The Fit Get Fitter

Advances in modern biology have allowed us to measure evolutionary fitness and estimate the rate of fixation of beneficial mutations. Drawing on the Long-Term Evolution Experiment, studying the evolution of Escherichia coli in a constant environment, Wiser et al. (p. 1364, published online 14 November) demonstrate that even after 50,000 generations over 20 years, gains in fitness show no evidence of leveling off. Instead, fitness is following a power-law relationship that is dependent on epistasis and clonal interference.


Experimental studies of evolution have increased greatly in number in recent years, stimulated by the growing power of genomic tools. However, organismal fitness remains the ultimate metric for interpreting these experiments, and the dynamics of fitness remain poorly understood over long time scales. Here, we examine fitness trajectories for 12 Escherichia coli populations during 50,000 generations. Mean fitness appears to increase without bound, consistent with a power law. We also derive this power-law relation theoretically by incorporating clonal interference and diminishing-returns epistasis into a dynamical model of changes in mean fitness over time.

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