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Character Change, Speciation, and the Higher Taxa

Sewall Wright
Evolution
Vol. 36, No. 3 (May, 1982), pp. 427-443
DOI: 10.2307/2408092
Stable URL: http://www.jstor.org/stable/2408092
Page Count: 17
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Character Change, Speciation, and the Higher Taxa
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Abstract

The implications of the shifting balance theory with respect to the course of evolution agree in the main with the pattern indicated by the fossil record, according to Simpson in 1944 (cf. Wright, 1945), reiterated by Gould and Eldredge (1977) in their statement quoted in our introduction: "Punctuated change dominates the history of life: evolution is concentrated in very rapid events of speciation (geologically instantaneous even if tolerably continuous in ecological time." I would, however, substitute the phrase "of character change" for "of speciation." Character change and speciation (in the sense of reproductive isolation) are wholly different phenomena genetically, even though closely correlated in occurrence. There is agreement only with the first but not the last part of the sentence also quoted in our introduction, from Gould (1980): "Evolution is a hierarchic process with complementary but different modes of change of its three leading varieties: within species, speciation and patterns of macroevolution." The shifting balance process is a two-level one (selection among individuals and among differentiated local populations), but no difference is assumed in the rates of minor and major mutation during the phases of near-stasis and rapid change. The interpretation of these phases under the shifting balance theory is in terms of differences in ecological opportunity. Speciation tends to accompany rapid change both because each of these processes tends to bring about the other and because speciation from chromosome rearrangement and peak-shifts is favored by the same population structure (numerous small colonies subject to frequent extinction and refounding by stray individuals from the more flourishing colonies). According to the shifting balance theory the determining factor for rapid change, and the origin of a new higher taxon that usually accompanies such change, is the presence of one or more vacant ecological niches, whether from entrance of the species into relatively unexploited territory or from its survival after a catastrophe has eliminated other species occupying related niches, or from gradual attainment of an adaptation that opens up a new way of life. We consider first the course of evolution of a species restricted to a single niche (because of occupancy of all related niches by other species) and living for a long time under relatively unchanging conditions. If its population density is great or there is a wide dispersion of offspring, a state of near-stasis should soon be reached as it comes to occupy the most available selective peak. It cannot move down from this against natural selection to reach a saddle leading to a higher selective peak. If on the other hand, the species occupies a wide range but in part at least only sparsely and with restricted dispersion, the operation of the shifting balance process leads to gradually improving adaptation of the species as a whole, by means of successive minor peak-shifts and selective diffusion from them. Still with only a single niche but under continuously changing conditions, there is continual readaptation largely of the treadmill sort, change without progress. A very gradual improvement is, however, to be expected as the species is shuffled into the higher general regions of the selective topography. This occurs even in populations that are effectively panmictic. In such a population, the rate of change is approximately according to Fisher's fundamental theorem except as qualified by frequency dependent selection or linkage disequilibrium. If, however, population structure permits significant operation of the shifting balance process, readaptation is facilitated by minor peak-shifts, not allowed for in Fisher's theory. In cases in which new ecological niches become available in any of the ways referred to, their occupation may require allelic substitutions with major effects. Such substitution may occur in spite of imperfect adaptation and the inevitable deleterious effects of any major change, because of the absence of competition, but is greatly facilitated if population structure is favorable to peak-shifts, involving the gene in question and one or more nearly neutral modifiers that tend to eliminate the more deleterious of the side effects. Such major peak-shifts are most likely if the major mutation is recurrent and thus becomes available, sooner or later, wherever a favorable modifier or a pair of such modifiers, reaches sufficiently high frequencies for crossing of a saddle that pulls the major mutation to occupancy of the higher peak. Such occupancy tends to give incipient reproductive isolation, followed by full speciation under selection against hybridization. On the other hand, speciation may come first, because of geographical isolation of a portion of the species or local establishment of a chromosome rearrangement or other cause, and be followed by occupancy of the new niche by means of a major peak-shift. In either case, the decisive peak-shift occurs within a species and is likely to be accompanied by minor peak-shifts that improve adaptation. The occupancy of new niches, accompanied by speciation, constitutes the origin of a new higher taxon. This may merely be a new genus but if there is extensive adaptive radiation into new niches, the array may constitute a new family, new order, or very rarely a new class or phylum. It may seem that mutations with impossibly drastic effects would be required for the origins of the higher of the taxa. Such origins, however, probably all occurred from species, the individuals of which were so small and simple in their anatomies that mutational changes, that would be complex in a large form, were not actually very complex. The final conclusion is that the evolutionary processes indicated by the fossil record can be interpreted by the shifting balance theory without invoking any causes unknown to genetics or ecology.

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