Figure 1.

Predicted extinction times of the Amazon molly. Muller's ratchet might cause extinctions of the unisexual Amazon molly due to the accumulation of slightly deleterious mutations. (A) analytical results only, (B) analytical results and simulations combined. The upper bar denotes the assumed age of the line (70 Ky, min 40 Ky, max 100 Ky). The lower bar marks the border to neutrality for the effective population sizes (N_e) used by spanning the selection coefficients from N_es = 1 for the largest (10⁷ females) to the smallest conceivable N_e (10⁴ females). The lines represent the analytic predictions of the extinction time for different deleterious genomic mutation rates (U_sdm) with N_e = 316,000, generation time T_gen = 1 year and maximal reproductive capacity R_max = 500 offspring/generation. Our current best overall estimate for U_sdm is 0.4 critically deleterious mutations/diploid genome/generation. Our upper limit is close to U_sdm = 1. The dashed lines indicate the variability of the extinction time estimates for a value that is close to our lowest credible mutation rate estimate (U_sdm = 0.05, green diamantes) using the corresponding upper and lower limits of N_e, T_gen and R_max; variability in extinction time is similar for other U_sdm. Large symbols denote valid extinction time estimates from simulations with at least 2 observed clicks of Muller's ratchet (usually many more, up to 500). Small symbols denote lower limits for extinction times from simulations without observed clicks, based on the (usually wrong) assumption that the ratchet would have clicked just after stopping the simulation. Each symbol denotes an independent simulation with a different random seed and assumes the same mean T_gen and R_max as analytic predictions (different N_e have been plotted on top of each other to avoid a series of similar plots). This plot contains 24,251 simulations with a total of 14.78 years of computing time. See main text for a discussion of parameter combinations and Loewe [7] for an in-depth explanation of the U-shaped plot shown here. The location of the wall of background selection for a particular parameter combination is approximately given by the vertical part of the corresponding line: all mutations with effects larger than the location of this wall are removed deterministically.