Modelling the Evolution of Species’ Ranges

Sammanfattning: The fact that species have limited ranges is often due to a limited ability to adapt to the environmental conditions that occur outside their geographic ranges. However, due to ongoing climate change, the environmental conditions within species’ geographic ranges may change in the near-future. To avoid extinction, many populations therefore need to migrate to new areas and/or adapt to the new conditions. Migration to new areas may be problematic, however, because adaptation to new environmental factors, such as predation/grazing, competition, or parasitism from new species, or new physical factors besides temperature, may be necessary even though the temperature is the same in the new area as in the native habitat. In addition, migration to new areas is associated with a considerable loss of genetic diversity, which may severely reduce the ability to adapt to new conditions. To understand if and how populations may adapt to new environments, or if their ranges will contract when the environmental conditions change, it is necessary to understand which evolutionary mechanisms underly the geographic range limits of species. In my dissertation, I am using mathematical and computer-based modelling to study the limits to evolution at range margins. I find, among other things, that the ability to self-fertilise often is favourable at range margins, despite the depletion of genetic diversity that is typically associated with self-fertilisation. Likewise, I find that it is often favourable for range expansions if combinations of genes that are under selection tend to be inherited together (rather than being mixed up under so-called genetic recombination), in part because locally adapted combinations of genes are partially protected from being mixed up with less well-adapted genes. It is known that another factor that facilitates range expansions is phenotypic plasticity: that is, the ability of an organism to change its characteristics (phenotype) as a response to the environment it is exposed to. I find that evolution favours increased plasticity only as long as the cost of plasticity is not too high. To interpret empirical experiments involving plasticity correctly it is important to know if the observed change in phenotype improves the local fitness or if it is just a consequence of physiological stress, which I illustrate with simulations. Finally, I find that the effects of multiple environmental gradients (gradual changes in the environmental conditions across geographical space) are added to each other in such a way that the total environmental gradient may become steep enough to prevent further range expansion, even when each individual gradient is shallow and easy to miss in field studies. To conclude, the new insights from my thesis contribute to improving the understanding of why limits to species’ ranges form.