Third, reinforcement of isolation. Finally, complete isolation. Most of the modes of speciation described in this chapter can be summarized by the above steps. 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.