norstogii complex probably emerged from an allopatric speciation
process resulting from recent and ongoing divergence processes due to drift stemming from founder effects, resulting in alleles from the original populations becoming fixed in most of the surviving populations.
Despite limited sampling, however, the inferred sequence data indicate an allopatric speciation
of the two sister taxa S.
Specifically, it suggests that (a) ecological conditions were not stable in tropical latitudes during the Cenozoic, alternating humid and dry climatic periods leading to continuous vegetational changes, (b) allopatric speciation
and subspeciation of forest and nonforest faunas took place during adverse climatic periods in isolated populations, and (c) particular regions acted as areas of differentiation ("refugia").
A causal analysis of stages in allopatric speciation
In line with the popular theory of allopatric speciation
, previous thinking had been that the confinement of T.
Parapatry is the geographic pattern that is assumed to result from parapatric speciation, allopatric speciation
with secondary contact, or peripheral isolated speciation (White 1973, Bush 1975, Endler 1977, Lynch 1989).
Other than recent secondary contact in Alaska, ranges of black guillemots and the Pacific species of guillemots are allopatric [ILLUSTRATION FOR FIGURE 1 OMITTED]; these distributions are inconsistent with parapatric, peripatric, or allopatric speciation
without major changes in breeding ranges of one or more species.
Geographic isolation, first recognized by Darwin (1859) as playing an important role in promoting evolutionary diversification, is the foundation upon which the theory of allopatric speciation
rests (Mayr 1942).
McPeek examines an unexplored way in which local species interactions in a community context could influence the potential for allopatric speciation
and thereby the generation of species diversity.
Evolutionary biologists generally acknowledge that allopatric speciation
via divergent selection in different regions of a species range is one of the most common and potentially most powerful mechanisms for generating new species, especially in animals (Mayr 1942, 1963, Lynch 1989, Allmon 1992, Rice and Hostert 1993, Coyne 1994).
Lynch (1989) concluded that vicariance events occur with greater frequency than other modes of allopatric speciation
or biogeographic patterning.
In the case of hosts and parasites, for example, a speciation event within a particular host lineage might be expected to isolate the parasite population associated with each incipient host species, and thus to produce an allopatric speciation
event among parasites.
Directional selection would be favored in a traditional model of allopatric speciation
whereby the extent of reproductive isolation reinforcement and character displacement would vary among lakes as a consequence of differential potential for competition among recolonizing populations.
via vicariance, may be the most common way in which new species originate (Lynch 1989).
A universally accepted mechanism of speciation is allopatric speciation
(White 1978; Littlejohn and Watson 1985) or adaptive divergence (sensu Templeton 1981).