Genetic and environmental constraints causing species’ range limits

Human-caused global change has led to shifts in the geographic distribution of many wild species. This has renewed the interest of understanding the factors that shape species’ contemporary range limits from both an ecological and evolutionary perspective. Recent evolutionary theory particularly emphasized the role of past demographic processes and neutral evolution in contributing to range limits. The aim of my thesis was to study these factors and their interaction with the environment experienced at range edges in an empirical system, the North American plant Arabidopsis lyrata. By crossing populations of varying range position and demographic history, and raising their offspring in gardens distributed across and beyond the species range, I found that populations with a history of small size due to past range expansion or rear-edge isolation suffered from increased expression of mutational load driven by genetic drift. This latter effect was even stronger under environmental stress, particularly under a warmer climate. Furthermore, populations at range edges with heightened past exposure to genetic drift had a reduced signature of climate adaptation. Finally, I compared A. lyrata and a novel species it gave rise to, A. arenicola, with a more northern distribution, in a climate chamber experiment. This new taxon diverged from A. lyrata in coping with a cool climate and strong reproductive isolation, most likely allowing it to to colonize subarctic regions and escape maladaptive gene flow. Results generally support the newer evolutionary theory about a predominant role of neutral evolution in contributing to geographic range limits, via genetic drift opposing purifying and directional selection. The study of sister taxa however shows that these constraints to evolution at range limits are not absolute, and can be broken.