The Concept of Stasipatric Speciation

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. In a recent study of the cytogenetics of the "coastal species" in the viatica group of morabine grasshoppers, White, Blacldth, Blackith, and Cheney (1967) have introduced the concept of "stasipatric speciation"' as a new mode of species differentiation contrasted with the allopatric and alleged sympatric types. The concept is intended to cover the process inferred for these insects whereby "direct conversion of an essentially continuous population into a number of contiguous taxa" is brought about "by the spread of chromosomal rearrangements around which isolating mechanisms develop. " A chromosomal rearrangement is postulated to arise at a single point within the range of the parent form and to spread from there by virtue of an adaptive superiority of the new homozygote, in spite of some reduction in the fecundity of the heterozygote. In the form in which it has been put forward by White et al., this concept raises certain difficulties, some associated with the interpretation placed upon certain of the data, others of a logical nature. The purpose of the present paper is to review these difficulties and to propose certain adjustments that avoid them. DIFFICULTIES IN THE CONCEPT AS PRESENTED The principal difficulties fall into two categories, as follows: 1. There is no evidence that speciation sensu stricto has occurred among the coastal forms studied. Speciation is the process whereby species, in the sense of reproductively isolated natural populations, are brought into existence. Various processes are known that lead to the differentiation of adaptively specialized races, which may even show some diminution of fecundity upon interbreeding, but which do not become reproductively isolated. Any process claimed to be a new mode of speciation must therefore be shown beyond reasonable doubt to have produced genuine species. The criterion for a (biological) species, which seemed clear enough at least for sympatric species when Mayr wrote in 1942, has been weakened since then, under the influence, for example, of Simpson (1961), to the extent that the definition of "species level" offered by Mayr (1963:26) is, by his own admission, non-operational. However, Mayr in general seems to use the criterion of reproductive isolation in a much more straightforward fashion, as, for example, when considering the case of the carrion and hooded crows of Europe, which he treats as subspecies. This more rigorous criterion will be adopted here. From this standpoint, it is significant that White et al. were able to produce viable and substantially fertile hybrids be-