Speciation in black flies

Abstract

In many Simuliidae, patterns of spatial and temporal relationships among the most closely related species are more readily interpreted in terms of sympatric speciation than of allopatric speciation. Specific examples are (i) the allotriploid taxa in Gymnopais and other genera, (ii) the black fly faunas of geologically recent islands (Tahiti), and (iii) species in Prosimulium onychodactylum, a prototype of a continental multisibling species complex. A model of sympatric speciation is presented based on coadaptation of polymorphic sex chromosomes in pairs reinforced by progressive development of assortative mating. This model predicts that (i) populations should frequently exhibit sex-chromosome polymorphism, (ii) these sex-chromosome polymorphisms, and autosomal ones, should in some cases display linkage or association disequilibria, (iii) species pairs or complexes should be incurred that differ only in sex chromosomes and that share extensive ancestral autosomal polymorphisms, and (iv) such species should differ in their biology and perhaps their present-day distribution. Recent publications and observations are in accordance, in general, with predictions from the model. Genetic control, e.g., of diapause, larval developmental timing, and niche preference or ethology, could substitute as a basis of incipient cleavage. The evidence for sympatric speciation is purely inferential, but this is equally true for the allopatric interpretation, and in black flies the circumstantial evidence for prevalence of sympatric speciation appears more compelling. This is not to deny the efficacy of allopatry and founder effect in the origin of some species complexes.Key words: sympatric speciation, black fly, evolution.