Fig. 3.

Proposal for experimental protocol to determine how the availability of beneficial mutation changes as fitness increases. a) The first step is to carry out multiple sets of long-term evolution experiments at different population sizes or mutation rates that span both the SSWM regime and the regime with weak clonal interference. For example, if the initial strain has estimated beneficial mutation rate ${\mu }_{b0}\approx {10}^{-5}$, one could carry out experiments with $N={10}^4,\hspace{0.17em}N={10}^5$, and $N={10}^6$. b) For each set of experiment (with a given N), one can then then obtain the average fitness trajectory by averaging over multiple replicates in each set (solid lines), and calculate the corresponding initial rate of fitness increase (dashed lines). c–e) By scaling time ($\tilde{t}=At$) such that the average trajectories have the same initial derivative, one can then infer how the beneficial mutation rate changes over time. c) If the scaled trajectories approximately overlap for all values of N, it would suggest that μ_b remains approximately constant. d) If the scaled trajectories speed up as N is increased, it would suggest that μ_b decreases as fitness increases over time. e) If the scaled trajectories slow down as N is increased, it would suggest that μ_b increases as fitness increases over time.