iThe essence of speciation lies in the origin of genetic isolating mechanisms. Argument has raged as to whether the primary isolating mechanisms which initiate specific divergence operate at the pre‐mating or post‐mating level and whether complete geographic isolation necessarily precedes divergence. Speciation is essentially a cytogenetic process and the role of chromosomal rearrangements as primary isolating factors has probably been underestimated in the past. But speciation depends on many factors such as population size, vagility and type of life cycle, so it is unlikely that any universal model is applicable.