Evolutionary constraints to the climate niche of species

Limits of species ranges are often associated with particular climatic conditions or isotherms. Further evidence that climate is indeed a main driver of species’ distributions is that many species have undergone range shifts towards higher latitudes or elevations under recent global warming. However, there are also species that have gained little terrain at the cool end of distributions and retracted strongly from the warm end. As evolutionary biologists we need to answer the pressing question: What constrains the evolution of the climate niche at distribution boundaries? The short answer is: We do not know.

Overall objectives. I argue that the breakthrough will come from connecting predictions of recent eco-evolutionary models with the genetic architecture of range-limiting traits. This novel framework will be used to test the suggested environmental, demographic and genetic determinants of elevational range limits within a set of plants in the Swiss Alps. My focus on elevational distribution limits is motivated by the fact that temperature is linked with both elevational and geographic range limits in many species worldwide, but elevational gradients are easier to study than latitudinal gradients because they are short.

Specific aims. The proposed research will assess the potential determinants of range limits in 6 plant species with different elevational ranges. Assessments will focus on two mountain areas extending from 600 m to 3000 m over a few km. The first 3 studies are aimed at parameter estimation:

(1) Fine-scale distribution modelling will reveal the range-limiting climatic variables and how rapidly they change along the gradient.

(2) Sequencing of individuals will allow the estimation of load due to dispersal, drift-effective population size, and signatures of past climate adaptation.

(3) Crossing experiments will be performed to (a) calculate the genetic architecture of traits involved in coping with the most limiting aspects of climate, and (b) estimate genotypic selection on these traits at four elevations.

In study 4, we will verify the importance of parameters estimated in studies 1-3 using spatially explicit eco-evolutionary simulations, comparing predicted with observed distributions.

Impact. Taken as a whole, this research should pinpoint the causes of distribution limits and resolve the evolutionary enigma of constraints to the climate niche.