Abstract
The fitness of spatially expanding species has been shown to decrease over time and space, but specialist species tracking their changing environment and shifting their range accordingly have been little studied. We use individual-based simulations and analytical modeling to compare the impact of range expansions and range shifts on genetic diversity and fitness loss, as well as the ability to recover fitness after either a shift or expansion. We find that the speed of a shift has a strong impact on fitness evolution. Fastest shifts show the strongest fitness loss per generation, but intermediate shift speeds lead to the strongest fitness loss per geographic distance. Range shifting species lose fitness more slowly through time than expanding species, however, their fitness measured at equal geographic distances from the source of expansion can be considerably lower. These counter-intuitive results arise from the combination of time over which selection acts and mutations enter the system. Range shifts also exhibit reduced fitness recovery after a geographic shift and may result in extinction, whereas range expansions can persist from the core of the species range. The complexity of range expansions and range shifts highlights the potential for severe consequences of environmental change on species survival.
Highlights
The rate of environmental change experienced by organisms plays a major role in driving evolution and determining species survival
This suggests that the rate of environmental change across the globe will play a large role in the survival of specialist species as compared to more generalist species
Expansion load is the consequence of genetic surfing of deleterious mutations at expanding range fronts [22,23], where inefficient selection due to small population size prevents the purging of deleterious variants, leading to severe fitness loss
Summary
The rate of environmental change experienced by organisms plays a major role in driving evolution and determining species survival. Expansion load is the consequence of genetic surfing of deleterious mutations at expanding range fronts [22,23], where inefficient selection due to small population size prevents the purging of deleterious variants, leading to severe fitness loss. This expansion load creates a gradient of fitness across species ranges, where high fitness individuals persist in the core of the species range and low fitness individuals exist at the edge. Other processes that slow expansion are expected to reduce fitness loss during a range shift, such as Allee effects which require a population to reach a given size before growing and expanding further [35]. The absence of migration from behind the expanding front is expected to reduce recovery after a shift
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