Conclusions
Adaptation commonly occurs from standing genetic variation, but how this variation is maintained in ancestral populations is little explored. We have here presented observational evidence suggesting that overall, genetic variants important to adaptation to a highly derived habitat are maintained at moderate frequencies within the ancestral habitat. These variants do not appear to occur in higher frequencies in geographic regions where ancestral populations have a higher opportunity for gene flow from derived populations. We thus conclude that long-term gene flow-selection balance is an incomplete explanation for the maintenance of SGV. Instead, we propose that purifying selection of these variants in the ancestral habitat subsides as their frequency decreases, thus allowing their neutral persistence. This novel perspective on the maintenance of SGV should now be scrutinized by controlled experimental work quantifying the fitness consequences of individual genetic variants across different habitats and genomic backgrounds.