Estimates of minimum haploid second chromosome mutation rate, Umin, and maximum average effect of new mutations, . (A) Compared with the unloaded treatment, UL, average Umin was approximately 2.8 times greater for the loaded treatment, L (∼2.7 times for those carrying one treatment allele, L1, and ∼2.8 times for those carrying two alleles, L2). (B) Compared with the unloaded treatment, average was only approximately 1.3 times greater for loaded treatment (∼1.4 times for those carrying one treatment allele and ∼1.3 times for those carrying two treatment alleles). Error bars represent ± 1 SE among different backgrounds within the loaded treatments.