Morphological shifts in response to spatial sorting of dispersal behaviour in red flour beetles across multiple generations

Dispersal is a key component of life history that influences individual fitness, population dynamics and range expansion. Morphological traits that are functionally related to dispersal may change through the spatial sorting of phenotypes at population range edges and invasion fronts. Generally, since larger invertebrates are expected to have greater dispersal capacity, larger body size and mass is expected to evolve in dispersers. Yet, different study systems exhibit a range of trait-specific and sex-specific responses to spatial sorting. Furthermore, a persistent issue in this field is that spatial sorting can be challenging to separate from natural selection and population dynamics. To address this, here we investigated the responses of dispersal and morphological traits to simulated spatial sorting into disperser and non-disperser red flour beetles (Tribolium castaneum) using a controlled laboratory dispersal system that isolated spatial sorting from natural selection and allowed sex-specific dispersal traits to evolve independently. After seven generations of spatial sorting, the time taken to disperse decreased in dispersers and increased in non-dispersers, with males dispersing more quickly than females. In contrast to expectations that dispersers would increase in body mass and size, we found the opposite. Body mass and morphology diverged such that dispersers became smaller and narrower, while non-dispersers became larger relative to the randomly selected controls, but femur length did not change. The trait responses across generations were also sex specific. Divergence between male dispersers and non-dispersers was more substantial than females, both in dispersal and in some morphological traits. We hypothesize that small individuals were more biomechanically efficient at dispersing through the dispersal apparatus and were therefore the ones that dispersed more readily. We suggest that key differences in dispersal modes across biological systems may impact the morphological and phenotypic trait changes from spatial sorting processes in range expansions and biological invasions.