During geographical expansion of a species individual colonizers have to
confront different ecological challenges, and the capacity of the
species to broaden its range may depend on the total amount of adaptive
genetic variation supplied by evolution. We set out to test whether the
distribution of loci under selection along a contrasting environmental
gradient can be turned into a model that accurately predicts a species'
range. If positive, this may shed light on the genetic source of
adaptive limits that shape range boundaries. We sampled five populations
of the western Mediterranean lizard Psammodromus algirus that inhabit a
noticeable environmental gradient of temperature and precipitation. We
used 21 SNPs putatively under selection to correlate the genotypes of 95
individuals with environmental variation among their populations, using
1x1 km2 grid cells as sampling units. By extrapolating the resulting
model to all possible combinations of alleles, we inferred the locations
that were theoretically suitable for the species. The inferred
distribution range overlapped to a large extent with the realized range
of the species, including an accurate prediction of internal gaps and
range borders. Our results suggest an adaptability threshold determined
by the amount of genetic variation available that would be required to
warrant adaptation beyond a certain limit of environmental variation.
These results support the idea that the expansion of a species' range
may be ultimately linked to the arising of new variants under selection.