Genetic signatures of adaptation to sudden, extreme, and unprecedented environmental changes

Small natural populations are increasingly exposed to environmental shifts that are unprecedented in speed and intensity on population-genetics timescales. If populations persist, adaptation to these abrupt changes is expected to leave localized footprints of selection around the loci contributing to the adaptive response, in contrast to genome-wide signals produced by demography alone. Here we provide an explicit theoretical description of genetic footprints expected under rapid adaptation from standing genetic variation. We derive exact analytical results for how single-locus sweeps and polygenic shifts driven by sudden environmental changes distort the site-frequency spectrum (SFS) of linked mutations, and we link these distortions to the expected changes in standard diversity measures and neutrality statistics. Our results provide a guideline for detecting and interpreting genetic signatures of adaptation to extreme events, including those associated with climate change, particularly in small populations where adaptation or evolutionary rescue often proceed quickly and from preexisting variation.