Data from: Evolution at two time frames: polymorphisms from an ancient singular divergence event fuel contemporary parallel evolution

When environments change, populations may adapt surprisingly fast,
repeatedly and even at microgeographic scales. There is increasing
evidence that such cases of rapid parallel evolution are fueled by
standing genetic variation, but the source of this genetic variation
remains poorly understood. In the saltmarsh beetle Pogonus chalceus,
short-winged ‘tidal’ and long-winged ‘seasonal’ ecotypes have diverged in
response to contrasting hydrological regimes and can be repeatedly found
along the Atlantic European coast. By analyzing genomic variation across
the beetles’ distribution, we reveal that alleles selected in the tidal
ecotype are spread across the genome and evolved during a singular and,
likely, geographically isolated divergence event, within the last 190 Kya.
Due to subsequent admixture, the ancient and differentially selected
alleles are currently polymorphic in most populations across its range,
which could potentially allow for the fast evolution of one ecotype from a
small number of random individuals, as low as 5 to 15, from a population
of the other ecotype. Our results suggest that cases of fast parallel
ecological divergence can be the result of evolution at two different time
frames: divergence in the past, followed by repeated selection on the same
divergently evolved alleles after admixture. These findings highlight the
importance of an ancient and, likely, allopatric divergence event for
driving the rate and direction of contemporary fast evolution under gene
flow. This mechanism is potentially driven by periods of geographic
isolation imposed by large-scale environmental changes such as glacial
cycles.