If the cooling and drying stressed individual species to the point where they went extinct over parts of their range and only survived in areas that were optimal, a mechanism for allopatric speciation emerges.
Each of the models of allopatric speciation is probably relevant to a different degree for the speciation process in different faunal groups or during different geological periods.
In this paper I describe one way in which species interactions operating in local communities may influence the potential for speciation by influencing the potential for generating differentiation across the range of a species as hypothesized in the allopatric speciation model with regional differences in selection.
A number of authors have argued that the primary force preventing population differentiation that would lead to allopatric speciation is the similarity in selection pressures experienced by different populations across the range of a species (e.
This difference in selection regimes between the two regions of the species range should therefore be conducive to generating allopatric speciation (Rice and Hostert 1993).
Natural selection may constrain or enhance the potential for population differentiation that can lead to allopatric speciation.
Thus, the species that are most likely to demonstrate speciation by distance around a ring are also the species that are most susceptable to geographic or allopatric speciation
, leading in most cases to uncertainty about the ultimate cause of reproductive isolation.
King stresses that there has been an overemphasis on allopatric speciation
at the expense of the vast amount of new data supporting one or more chromosomal speciation hypotheses.
Secondary zones have been identified by multiple concordant clines (including those for presumably neutral markers), and the prevalence of such concordance is one of the traditional arguments for the ubiquity of allopatric speciation
The importance of allopatric speciation
could, for example, be ruled out if regional patterning of gene pools was absent.
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).
Lynch viewed cases of higher-level sympatry ([is less than] 100% overlap) as the result of either sympatric speciation or allopatric speciation with subsequent dispersal, and thus classified these as indeterminate.