Range expansion is slower and less predictable across a spatial gradient in temperature

Rapid evolution in colonizing populations can alter our ability to predict future range expansions. Recent studies suggest that how evolution influences the predictability of range expansion speed depends on the balance of genetic drift and selection at the range front, with factors increasing selection proposed to produce greater consistency, and hence increased predictability, in range expansion speed. Here, we test whether selection from environmental gradients across space produces more predictable range expansion speeds, using the experimental evolution of replicate, duckweed populations colonizing landscapes with and without a temperature gradient. While range expansion across a temperature gradient was slower, with range-front populations displaying higher population densities and genetic signatures consistent with greater selection, we found that range expansion speed was more variable and less predictable across replicate populations. Our results therefore challenge current theory, highlighting that selection and spatial priority effects could together govern the predictability of range expansion speeds.