MODES OF SPECIATION IN BIRDS: A TEST OF LYNCH'S METHOD

Abstract

The view that allopatric speciation is the predominant mode of speciation in animals seems firmly established (Mayr 1942, 1963). Mayr suggested that many species evolve via the establishment of small demes, often peripherally isolated, by dispersal of a few founders from the main body of the range. The view that dispersal is the primary means of achieving allopatry was seriously challenged by the school of vicariance biogeography (Platnick and Nelson 1978). Proponents of this approach suggest that vicariant events fragment ancestral populations and are just as important as, if not more important than, dispersal. Vicariant events are not necessarily expected to cause the isolation of small peripheral populations. Thus, dispersalist and vicariance biogeography predict different frequencies of geographic modes of speciation (Bush 1975), especially in regard to the prevalence of peripheral isolation. Cracraft (1982) studied the geography of speciation among Australian birds and concluded that speciation by vicariance was more common than peripheral isolation. Nevertheless, the frequency of geographic modes of allopatric speciation in birds is not well established. Lynch (1989) hypothesized that the frequency of vicariant, peripheral isolates, and sympatric speciation could be discovered by examining the relative range size and range overlap of sister taxa, either sister species or sister groups. 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). In brief, Lynch (1989) suggested that if sister taxa are allopatric and occupy ranges of similar size, their speciation mode is most parsimoniously considered vicariance. If the range