May the (selective) force be with you: Spatial sorting and natural selection exert opposing forces on limb length in...

Spatial sorting on invasion fronts drives the evolution of dispersive phenotypes, and in doing so can push phenotypes in the opposite direction to natural selection. The invasion of cane toads (Rhinella marina) through tropical Australia has accelerated over recent decades because of the accumulation of dispersal-enhancing traits at the invasion front, driven by spatial sorting. One such trait is the length of the forelimbs: invasion-front toads have longer arms (relative to body length) in comparison with populations 10-20 years after invasion. Such a shift likely has fitness consequences: an increase of forearm length would decrease the strength with which a male could cling to a female during amplexus and so render such a male less competitive in competition for mates, compared to short-armed conspecifics. Our laboratory trials of attachment strength confirmed that males with relatively longer arms were easier to displace, and competition trials show higher duration of amplexus for males with shorter arms. Together with the sharp cline in limb length observed behind the invasion front, these results imply an opposition of selective forces: spatial sorting optimizes dispersal, but as this force wanes behind the invasion front, we see the primacy of natural selection reassert itself.