Species’ range dynamics depend on the balance between environmental change and genetic drift.

<p>Specifically, range limit evolution depends on two compound parameters: (1) the ‘cost of migration’, which is proportional to the mean dispersal distance of individuals of the species, as well as the steepness of the environmental gradient (i.e., the rate at which a trait optimum changes across space), and (2) ‘neighbourhood size’, which is inversely proportional to the local intensity of genetic drift. As Polechová [<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2006735#pbio.2006735.ref008" target="_blank">8</a>] demonstrates, species’ range dynamics, including the location and stability of range limits, are predicted by the relative magnitude of neighbourhood size versus the steepness of the environmental gradient. For example, with a shallow environmental gradient relative to neighbourhood size, the species’ range expands to fill all of the available habitat (blue plots). With a steep environmental gradient relative to neighbourhood size, the species’ range contracts, leading to extinction or a restricted or fragmented range (red plots). With a steepening environmental gradient (black plots), the neighbourhood size can be sufficiently large to maintain local adaptation and high population density through most of the range (see the regions between the vertical arrows). However, as the environmental gradient steepens, the threshold neighbourhood size required to maintain local adaptation increases. Eventually, the neighbourhood size falls below the threshold for local adaptation (black arrows note the threshold), and abrupt range limits arise.