The Concept of Stasipatric Speciation

Abstract

Difficulties in the concept of stasipatric speciation are examined and a modified version of it proposed. It seems to consist of two components, an allopatric one and a “semigeographic,” or parapatric one. The different races of which many species of morabine grasshoppers are composed meet along very narrow zones of secondary intergradation, or “tension zones”; these appear to act like semipermeable membranes, allowing free passage to some genetic modifications, but holding back others. The accumulation of genetic differences along these zones is thought to lead eventually to full reproductive (but not necessarily mating) isolation between the two populations. This is the definitive, parapatric phase of the speciation. The races themselves are most simply regarded as originating allopatrically, by random fixation in very small, completely isolated colonies. When these expand and come into contact with the ancestral form, the tension zone is immediately created. This is the preparatory, allopatric phase. The tension zone moves across the country as a front, in the direction of the less adapted form.