Completed Speciation

Abstract The ultimate step in the process of geographic speciation consists of one allospecies invading the range of a sister allospecies. The result is geographical overlap (sympatry) of two species, both of which descended directly from the same ancestral species. We can identify 19 Northern Melanesian species pairs that appear to exemplify stages in this process. Collectively, these and other species pairs constitute a series of snapshots permitting one to reconstruct the completion of speciation. The series begins with the 35 superspecies, already discussed in the preceding chapter, that are represented in Northern Melanesia by two or more morphologically similar allospecies with completely allopatric distributions. The next stage consists of pairs of two former allospecies whose ranges are still largely allopatric but which now overlap in a small zone of sympatry. In further stages, the zone of sympatry becomes extensive, although each species still retains some zone of allopatry. In the final stage, the geographic range of one species has been completely overrun by its sister species, so that the former no longer retains any zone of allopatry. This progression from allopatric to sympatric distributions is accompanied by evolution of progressively greater morphological differences between the sister species.