Global change is causing widespread shifts in species distributions. While the rapid spread of invasive species is causing great harm to natural communities, many native species will depend on their ability to migrate in order to track climate change. Recent work has shown that spreading populations may evolve rapidly in response to novel environmental conditions, greatly complicating predictions of their spread velocity. In order to forecast biological invasions and range shifts in response to climate change, a better understanding of evolution during spread will be essential.
We studied the evolution of phenology during the recent range expansion of Dittrichia graveolens, a Mediterranean annual plant species (Asteraceae) that is currently spreading north along highways. We collected seeds from the core of the range in southern and central France (3 and 6 populations, respectively), and at two expanding range edges in Switzerland and the Netherlands (3 populations each). After a generation in the greenhouse to reduce maternal effects, we grew plants of all origins in parallel common garden experiments in Switzerland and the Netherlands. We monitored the phenology of the plants twice a week and used the number of developed seed heads at the time of harvest as a measure of fitness.
In both common gardens, we found a latitudinal gradient in phenology that extended from the core of the range in France to the Netherlands, a latitude the species only reached over the last 20-50 years. Plants of northern origin flowered earlier than southern genotypes, and fitness of late-flowering plants strongly declined in Switzerland and in the Netherlands. This pattern can be explained by the late phenology of this species, which flowers in September-October and is therefore likely constrained in seed set by the earlier onset of winter in the north.
Population genetic analysis using RAD-sequencing revealed that Swiss and Dutch plants were most closely related to central French plants, which provided a background of their spread history. Swiss plants did not differ in flowering time from central French plants, suggesting they were pre-adapted to the climate in Switzerland. In contrast, the recent differentiation in phenology between the Dutch and central French plants may have played an important role in the northern range expansion of Dittrichia. We conclude that even on short time scales and in native range expansions, local adaptation should be taken into account in forecasts of species migration.