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.g., Dieckmann & Doebeli, 1999; Kondrashov & Kondrashov, 1999; Tregenza & Butlin, 1999).
Simple mechanisms, involving few loci, for achieving fast reproductive isolation would thus be highly conducive to parapatric speciation. 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).
Palms are dispersed mainly by birds and mammals (Zona & Henderson, 1989), and these seed-dispersal mechanisms may promote parapatric speciation. Seed dispersal by birds or other animals may cause speciation when important dispersers provide directed seed dispersal to certain (micro)habitats (cf.
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).
Although parapatric speciation by edaphic specialization may well be an important speciation mode in neotropical palms, parapatric speciation occurring in microenvironmental heterogeneity in light availability is probably even more important.