Similar Habitat Gradients Across Native and Invasive Ranges Elicit Divergent Evolutionary Responses

Knowledge about an invasive species' biology within its native range illuminates the underlying evolutionary processes that occur following introduction. Cane toads (Rhinella marina) were introduced to coastal Queensland in 1935 and continue to expand their range across Australia. Despite low genetic diversity, they have accelerated their expansion across novel, arid environments as they travelled westward, differing from their eastern Australian conspecifics in morphology, physiology, and behaviour. The recent discovery of two ecotypes (from wetter 'rainforest' and drier 'coastal' areas) in the native range raises the question of how much of this geographical variation in Australia is due to (a) selection following introduction as toads encountered environmental variation, or (b) sorting of native range alleles into the environments similar to those in the native range. We used morphological data of 296 native range toads to demonstrate body-size differences between ecotypes. Then we used genetic data of 93 native and invasive toads to investigate whether native-range ecotypes are genetically distinct and whether patterns of selection for wetter versus drier environments in native and invasive populations were similar. We found moderate population structure within the native range across space and environments. When contrasting environmental gradients in the native versus invasive ranges, we identified distinct, non-overlapping sets of outlier SNPs. Nearly 80% of outlier SNPs detected in the invasive range were polymorphic in the native range, suggesting that genetic patterns in invasive toads have been primarily driven by standing variation from the native range. This study provides insights into evolutionary patterns occurring during biological invasion, demonstrating distinct evolutionary responses to similar environments.