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CHAPTER 9 THRESHOLDS FOR SURVIVAL: MAINTAINING FITNESS AND EVOLUTIONARY POTENTIAL Michael E. Soule The major goal in conservation genetics is the development of criteria for determining the population size (or minimum area) which will provide for the maintenance of fitness and adaptive potential. This is no academic exercise; economic and political forces relentlessly encroach on the land and the budgets given to conservation programs. Unless conservationists produce sound and defensible criteria for minimal population sizes there will be no rational way to counter these attacks, and all of our efforts to salvage samples of our magnificent large plant and animal species will be wasted. This chapter is an attempt to produce such guidelines. A useful device for considering the relevance of population and evolu- tionary genetics to conservation is the "time scale of survival." Employ- ing this scale, one can see, somewhat arbitrarily, three survival problems or issues: (1) the short-term issue is immediate fitness-the maintenance of vigor and fecundity during an interim holding operation, usually in an artificial environment; (2) the long-term issue is adaptation,.-the persis- tence of the vigor and evolutionary adaptation of a population in the face of a changing natural environment; (3) the third issue is evolution in the broade,st sense-the continuing creation of evolutionary novelty during and by the process of speciation. I will discuss the first and third of these issues (fitness and speciation). The issue of gradual evolutionary adaptation to a changing environment 151