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Direct observation is central to our understanding of adaptation, but evolution is rarely documented in a large, multicellular organism for more than a few generations. In this study, we observed evolution across a century-scale competition experiment, barley composite cross II (CCII). CCII was founded in 1929 in Davis, California, with thousands of genotypes, but we found that natural selection has massively reduced genetic diversity, leading to a single lineage constituting most of the population by generation 50. Selection favored alleles originating from climates similar to that of Davis and targeted loci contributing to reproductive development, including the barley diversification lociVrs1,HvCEN,Ppd-H1, andVrn-H2. Our findings point to selection as the predominant force shaping genomic variation in one of the world’s oldest biological experiments. 

Many SNPs are predicted to encode deleterious amino acid variants. These slightly deleterious mutations can provide unique insights into population history, the dynamics of selection, and the genetic bases of phenotypes. This is especially true for domesticated species, where a history of bottlenecks and selection may affect the frequency of deleterious variants and signal a “cost of domestication”. Here, we investigated the numbers and frequencies of deleterious variants in Asian rice (Oryza sativa), focusing on two varieties (japonica and indica) and their wild relative (O. rufipogon). We investigated three signals of a potential cost of domestication in Asian rice relative to O. rufipogon: an increase in the frequency of deleterious SNPs (dSNPs), an enrichment of dSNPs compared with synonymous SNPs (sSNPs), and an increased number of deleterious variants. We found evidence for all three signals, and domesticated individuals con- tained 􏰚3–4% more deleterious alleles than wild individuals. Deleterious variants were enriched within low recombin- ation regions of the genome and experienced frequency increases similar to sSNPs within regions of putative selective sweeps. A characteristic feature of rice domestication was a shift in mating system from outcrossing to predominantly selfing. Forward simulations suggest that this shift in mating system may have been the dominant factor in shaping both deleterious and neutral diversity in rice.