Rapid speciation in small populations challenges the dominance of ecological speciation

Speciation - the process by which two lineages become reproductively isolated - plays a key role in the emergence and maintenance of biodiversity. Yet, our understanding of the time it takes for speciation to occur, and of the microevolutionary processes that influence this tempo, remains limited. Here, we thoroughly characterize how population size, mutation rate, local adaptation and migration are expected to influence the duration of speciation, as well as the shape of the "grey zone" of speciation. We show that the relationship between population size and speciation time is indicative of the speciation mode, as faster speciation in smaller populations only occurs in the case of non-ecological speciation. Leveraging genomic estimates of population size and speciation duration across 196 pairs of plant species, we uncover a positive association between population size and speciation duration. Taken together, these results challenge the view that ecological speciation is the source of much of species diversity.