Alternative adaptations-different adaptive phenotypes maintained in the same life stage and the same population but not necessarily simultaneously expressed in the same individual-represent contrasting character sets produced by the same genome, in effect allowing a single species to occupy more than one sympatric niche. Such alternatives are particularly likely to give rise to novel adaptations because of selection for extreme dissimilarity between them and because established traits buffer populations against extinction while independently expressed alternatives evolve in new directions. Particular alternatives can be suddenly fixed in populations with little or no genetic change, leading to a period of rapid evolution (especially, of morphology) exaggerating the characteristics of the newly fixed form. This burst of change would facilitate rapid speciation and could produce "punctuated" patterns of evolution. Evidence from a wide variety of organisms shows that alternative phenotypes are exceedingly common in nature and that they are probably important in speciation and macroevolution. Although many of these ideas and observations have been noted piecemeal by previous authors, bringing them together demonstrates the probable importance of alternative adaptations in the origin of major evolutionary novelties and calls for a revision of current and traditional ideas about the role of behavior and ontogeny in the genesis of organic diversity.