Adaptability thresholds constrain range limits
Our results suggest that species’ ranges are determined by the maximum
possible span of environmental variation to which adaptive genetic
variants are suited. As a consequence, range expansions should be
constrained by adaptability thresholds. In our system, it seems that the
environmental range to which P. algirus is adapted is ultimately
linked to the amount of genetic variance under selection associated to a
specific bioclimatic gradient. If positive, a range expansion promoted
by adaptations towards more extreme environments should entail the
selection of new genetic variants. Moreover, such range expansion would
require that the effect of the new adaptive mutations is additive with
respect to the ones that define the adaptability threshold (Soberón and
Peterson 2005, Polechová and Barton 2015, Polechová 2018). Whilst our
results support this line of reasoning, further theoretical exploration
is needed to uncover the hypothesized positive relationship between the
magnitude of the increment in environmentally correlated additive
genetic variance, and the extent of range expansion that can be achieved
(Angert et al. 2008; Polechová et al. 2009).
Overall, we have shown that inferring species’ ranges from the
geographical distribution of SNPs under selection can be not only very
accurate, but also informative about the genetic dynamics that underlie
local adaptation all over a species’ range. Our results suggest that the
amount of genetic variability subject to selection is a determinant of
the location and shape of range boundaries. This conclusion sheds light
on the key processes that determine the configuration of distribution
ranges, putting forward the importance of inherent limits to adaptation
as an ultimate explanation for the evolution of their shape and
boundaries (Connallon and Sgrò 2018).