Speciation is a constant theme in evolutionary biology. The continuity of population evolution makes it hard to define species. Here, we revisited three geographical patterns of speciation (sympatric, parapatric and allopatric speciation) and their recent empirical evidence, based on the classical concept of biological species. A survey of the literature reveals the molecular genetics basis of postzygotic barriers and the supporting evidence derived from analyses of population genomic data, including BDMI (Bateson-Dobzhansky-Muller incompatibility), QTL (quantitative trait locus) analysis, Haldane’s rule and the large X-chromosome effects. One prezygotic barrier to speciation, which is often not emphasized, is the pattern of mating system. Given the occurrence of a more common transition to selfing from outcrossing than the reverse direction in plant species, the so-called selfing syndromes for both flower characters and genomic structure could amplify population genetic structure and enhance speciation. It also increases the occurrence of asymmetric or absent gene introgression between incipient species with different mating systems. The role of driving or reinforcing speciation that mating system plays awaits identification with appropriate data collections.