Parapatric speciation is speciation without geographical isolation by disruptive selection across environmental gradients (Gentry, 1989); that is, a special type of sympatric speciation (cf.
Parapatric speciation involves two processes: genetic adaptation to environmental heterogeneity and the development of reproductive isolation among the resulting differentially adapted subpopulations (e.
Modeling further predicts that parapatric speciation requires that reproductive isolation be achieved by divergence in a relatively small number of mating trait loci even when environmental instability is not considered (Dieckmann & Doebeli, 1999; Kondrashov & Kondrashov, 1999).
For example, if a species has two important seed dispersers, one that prefers wet microsites and another that prefers hilltops, gene flow would be limited between the two subpopulations, subject to different selection pressures, and parapatric speciation could result.
Parapatric speciation thus appears to be a probable mode of speciation in neotropical rain-forest palms, but what is the actual evidence thereof?
Even more indicative of such parapatric speciation is the occurrence of clear edaphic specialization found even within a species complex (Polyalthia hypoleuca, Annonaceae) of paleotropical rain-forest trees (Rogstad, 1990).