More Evolutionary Terms

The geographic relationships of two or more given related populations of organisms have long been characterized as sympatric or allopatric (Mayr, 1942:148149), sympatric populations overlapping in range ("coexisting at the same locality"), whereas ranges of allopatric populations do not overlap ("with mutually exclusive geographic ranges"). Genic interchange is geographically possible in some allopatric situations, wherein the ranges are contiguous, as well as in all sympatric situations, but not in fully disjunct allopatric situations. Recognizing that the distinction of two sorts of allopatry justifies a discriminative terminology, Smith (1955:75-76) proposed that the term allopatric be redefined to apply to situations wherein the ranges are not even in contact and genic interchange is therefore geographically precluded, whereas the term parapatric was proposed for situations wherein ranges are in contact and genic interchange is geographically possible even without sympatry. This proposal has however met with little favor (e.g., Mayr, 1963), appropriately enough if integrity of original definition is important to maintain, as indeed it is. Returning then to the original definition of allopatry, it would still be useful to distinguish between those allopatric situations that are conducive geographically to genic interchange, and those that are not. I now propose that these two conditions of allopatry be distinguished by the terms parapatry (as originally proposed) and dichopatry, with the latter term defined as separation of geographic range to the extent that individuals of the involved species never meet. As pointed out repeatedly, by Mayr (1942, 1947, 1963) and others, gross sympatry is not necessarily accompanied by physical contact, or at least by reproductive contact, of the populations involved; they may remain completely separated by different habitat preferences, or by different habits. In other words, microgeographic relationships are equally as important as macrogeographic relationships. Mayr (1942: 196) used the term "microgeographic," and extrapolating from this suggestion Smith (1955:75-76) proposed that the microgeographic relationship be designated microsympatry, and the macrogeographic relationship, macrosympatry. Sympatry is not the only geographic relationship that should be considered at the "micro" as well as the "macro" level. Microallopatry (Smith, 1955:76) certainly exists in some cases of sympatry (i.e., of macrosympatry), and such allopatry may be either microparapatric (Smith, 1955:76) or microdichopatric. Geographic relationships of populations may thus be summarized by the following hierarchy: