Eco-evolutionary feedback during range expansions

Background/Question/Methods
Species expand their geographical ranges following an environmental change, long range dispersal, or a new adaptation. Range expansions provide a unique window into the coupling of ecological and evolutionary processes because evolution affects invasion and invasion affects evolution. Indeed, the evolution of faster dispersal accelerates the invasion of new territories, as was recently observed during the invasion of cane toads in Australia. At the same time, expansions lead to a reduction of genetic diversity due to the founder effect and changes in the fixation probabilities of beneficial and deleterious mutations. The coupling of these phenomena remains poorly understood especially in the case of frequency-dependent selection, which is common for social traits such as cooperation. Since the mathematical description of the interactions between cooperators and defectors (or producers and non-producers) closely mimics that of hosts and parasites as well as other ecological interactions, the study of social traits during an invasion can also elucidate how range expansions affect host-parasite interactions.

Results/Conclusions
We used both mathematical modeling and experiments to uncover the effects that social traits and range expansions have on each other. First, we developed a mathematical model that described population dynamics of cooperators, e.g. produces of a public good that benefits other individuals, and defectors (i.e. non-producers). We found that the coupling of ecological and evolutionary dynamics leads to an increase in the frequency of cooperators at the expansion front, and, under certain conditions, the entire front can be taken over by cooperators. Thus, a mixed population wave can split into an expansion wave of only cooperators followed by an invasion wave of defectors. After the splitting, cooperators increase in abundance by expanding into new territories faster than they are invaded by defectors. We tested this result in an experimental metapopulation of genetically engineered yeast cells. The experiments confirmed that range expansion can occur in two modalities: either as a mixed population wave of cooperators and defectors or as a wave of cooperators followed by a wave of defectors. More generally, our results show how range expansions and genetic or species diversity affect each other, predict novel temporal dynamics following the invasion, and suggest new approacher to managing population expansions.