Inbreeding depression in Mimulus guttatus is not mainly caused by genes of large effect like lethal or sterile alleles. The study created inbred lines to purge these alleles and found that inbreeding depression did not decrease significantly. This suggests that small-effect genes, not large-effect ones, are the main reason for inbreeding depression in this population. The increase in outbred fitness was likely due to adaptation to greenhouse conditions, not the removal of major deleterious alleles.