Fig 2. Deleterious load and crop evolution.

Shown are 3 chromosomes sampled from populations of crop wild relatives, traditional varieties, and modern cultivars. Selection during crop evolution increases the frequency of domestication-related alleles, which are beneficial in agronomic settings, but not necessarily alleles for adaptation, which may only be beneficial in specific environments. Deleterious alleles are often concentrated in low recombination regions of the genome (white on the recombination scale bar) and preferentially removed by purifying selection, but some are fixed during the process of domestication and improvement. Adaptive alleles linked to deleterious alleles are difficult to introgress because of their negative impacts on fitness or agronomic traits (red dashed line labeled “hard”), but adaptive alleles far from deleterious alleles can be easily introgressed (red dashed arrow labeled “easy”). But the long-term combined action of introgression, recombination, and selection has allowed the historical introgression of “hard” adaptation alleles from crop wild relatives into traditional varieties (gray dashed arrow), where they could then be more easily incorporated into modern cultivars. Figure was created using BioRender.com.