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).