How weedy Arabidopsis thaliana dominated the world: ancestral variation and polygenic adaptation

The composition of a species could change with demographic turnovers, where dominant populations quickly expanded and replaced others. However, whether such events have a genetic basis remains to be investigated. Previous studies showed that Arabidopsis thaliana experienced a significant demographic turnover, where "non-relicts" replaced "relicts" throughout Eurasia. Here, we showed that non-relicts have smaller seeds, more seeds per fruit, and a higher germination rate, making them more competitive over relicts. Using a unique population enriching relict alleles while minimizing population structure, we identified candidate loci and showed that such trait divergence was caused by the divergent sorting of multiple ancient haplotypes in a Mendelian gene and joint allele frequency change of polygenes affecting single-trait divergence and multi-trait covariance. This study is one of the few genetic investigations of species-wide demographic turnover, emphasizing the importance of processes different from the much-focused hard selective sweep.