Dynamics of reproductive isolation in stick insects at the transition between populations and species

Speciation is often viewed as a continuum along which populations diverge until they become reproductively-isolated species. However, such divergence may be heterogeneous, proceeding in `fits and bursts', rather than being uniform and gradual. We show in Timema stick insects that one form of reproductive isolation indeed evolves non-uniformly across this continuum, whereas another does not. Specifically, we use thousands of host-preference and mating trials to study habitat and sexual isolation among 42 pairs of taxa spanning a range of genomic differentiation and divergence time. We find that the evolution of habitat isolation is uncoupled from genomic differentiation within species, but accumulates linearly with it between species. In contrast, sexual isolation accumulates linearly across the speciation continuum. The results show different evolutionary dynamics for different components of reproductive isolation, indicate sudden transitions between phases of speciation, and highlight ...