There are several evolutionary reasons why species distributions are locally and geographically limited, and these mostly revolve around gene flow and levels of genetic variation in populations. While there has been progress in developing models assessing the impact of gene flow, empirical data on hypotheses about factors limiting distributions remain rare. A few recent plant studies have highlighted that levels of additive genetic variance are not limiting, suggesting a role for gene flow. However, some animal studies, mostly on
Drosophilaspecies, have refocused attention on heritable variation as an evolutionary reason for distribution limits. There is a need for comparative studies that examine levels of heritable variation across related species groups with broad and narrow distributions, and also across a range of traits including those likely to be under selection at borders. These studies will help to test specific predictions about the relative importance of genetic variance and gene flow in limiting current borders and also help in predicting the impact of future environmental changes on distribution shifts.