Local adaptation and reproductive isolation: when does speciation start?

The speciation process often takes a long time. The speciation continuum framework has been useful to reconstruct the evolutionary processes that result in the formation of new species but defining when this continuum starts is far from trivial. Although a panmictic population is often considered the initial condition of speciation, this is unrealistic for almost all species. Local or divergent adaptation are viewed by many researchers as processes that shape intraspecific diversity and thus are not part of speciation. We propose that speciation starts when reproductive isolation becomes greater than zero, arguing in favour of the alternative view that local adaptation necessarily involves some reproductive isolation, independently of whether it results in the completion of speciation. Given that local adaptation is widespread, the consequence is that most species are constantly in the process of speciating. The process of speciation is best represented as the formation of separate subnetworks, defined by reproductive isolation, within extended and fluid spatial networks of populations.