In particular, allopatric speciation, parapatric speciation, and sympatric speciation all share similar processes. Note that any mode of speciation depends on the availability of appropriate genetic variation. Reproductive isolation is introduced as the essential process for understanding speciation. A newly evolving species could theoretically have an allopatric, parapatric, or sympatric geographical relation with its ancestor. The geography of speciation is introduced.
Geographic variation is widespread and exists in all species.
Existence of suitable variation is a precondition for any evolutionary process. Because extensive variation can be found in all sorts of traits in all sorts of populations, most discussions of speciation presume that appropriate variation will be available. The repeated mention, by Ridley, that variation within a species can be almost as great as that between species is rather curious, just as if there were a universal, valid, and generally accepted phenetic definition of species. Old ideas cast long shadows. Reproductive isolation is established by the barrier, prior to any evolutionary change in the populations.