Only the Gradient hypothesis is based on the model of
parapatric speciation; all other hypotheses are based on the generally accepted theory of geographic (allopatric) speciation.
Here I shall advance the argument that habitat and microenvironmental gradients in edaphic and canopy conditions through the process of
parapatric speciation may be important for the generation of much of the high species richness in neotropical palms.
Pleiotropy and
parapatric speciation. Evolution 36:263-270.
Under allopatric and
parapatric speciation models only a subset of the mt-haplotype variation would be captured by the incipient species; an even smaller subset would be captured under stasipatric and sympatric speciation models - although in the latter two models the effective population size for nuclear autosomes would also be reduced.
1983); (2) reproductive isolation as a by-product of selection along the altitudinal gradient is a mechanism for
parapatric speciation (Endler 1977; Barton 1983); and (3) the role of selection versus drift in the evolution of reproductive isolation is poorly understood (Coyne 1992).
He advocates
parapatric speciation, using as a model the Mullerian mimic races of Heliconius butterflies, in which positive frequency-dependent selection creates narrow hybrid belts.
Parapatric speciation, a third form of allopatric speciation (Bush 1975; Endler 1977), was not considered by Lynch because it appears to be phylogenetically indistinguishable from other forms of allopatric speciation (Wiley 1981: Cracraft 1982).