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.