Fig. 1.

Change in the mean and variance in fitness for 11 treatment groups over 46 generations of MA. (A) Mean fitness relative to controls over time for second chromosomes that accumulated mutations in different genetic backgrounds with respect to the third chromosome (solid line, unloaded third chromosome; dashed lines, one deleterious treatment allele; dotted lines, two deleterious treatment alleles). (B) The rate of decline in mean fitness, ΔM, was significantly greater in the loaded treatment (L) than in the unloaded treatment (UL; randomization test, P < 0.03). Error bars represent ± 1 SE among different backgrounds within the loaded treatment. Compared with the unloaded treatment, ΔM was approximately 2.8 times greater in the loaded treatment (∼2.7 times for those carrying one treatment allele, L1, and ∼3.2 times for those carrying two treatment alleles, L2). (C) Variance in fitness among lines within each genetic background over time. (D) The rate of increase in variance, ΔV, was significantly greater in the loaded treatments than in the unloaded treatment (P < 0.0002, based on simulation results; Materials and Methods provides further details). Compared with the unloaded treatment ΔV was approximately 3.6 times greater for loaded treatments (∼3.3 times for those carrying one treatment allele and ∼4 times for those carrying two treatment alleles).