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The Epistasis Cycle: A Theory of Marginal Populations

abstract1 (full description below): Annu. Rev. Ecol. Syst. 1973.4:165-187.
The Epistasis Cycle. PDF/Acrobat file     1973. Michael Soule

INTRODUCTION EXCERPT: The subject of this speculative essay-review is the genetic structure of marginal populations. Admittedly, it is difficult to define a marginal population. A somewhat intuitive definition would be a population exposed to an extreme of one or more relevant environmental variables. In practice, though, such a nonoperational definition is of little use, and it would be better if some parameter of population performance served as a measure of marginality. One such definition would hinge on population dynamics: a marginal population is one characterized by relatively great fluctuations in numbers and a relatively high probability of extinction. (Here, as elsewhere, I am using population in the sense of a local group or deme, more or less isolated from other local groups.) Another such definition might be based on "population statics": a marginal population is one in which the individuals are relatively sparsely distributed and show effects of physiological stress, e.g. starvation, dehydration, or stunting. I prefer the dynamic definition. Others (2, 26, 71) have discussed the distinctions between marginal and central populations, and I will not belabor them as such distinctions inevitably are arbitrary. Two points, however, should be made clear. First, not all marginal populations are peripherally located. Topographic relief can impose marginal conditions in the geographic center of a range by producing deserts, rain shadows, and various altitudinal effects. Second, not all peripheral populations are ecologically marginal.