On r- and K-Selection

Abstract

Dobzhansky (1950) proposed that natural selection in the tropics operates in a fundamentally different way than it does in temperate zones. He argued that much of the mortality in the temperate zones is relatively independent of the genotype (and phenotype) of the organism concerned, and has little to do with the size of the population. Traditional examples of mass winter kills of fish and sparrows are extremes of this sort. Dobzhansky reasoned that in the relatively constant tropics, most mortality is more directed, generally favoring those individuals with better competitive abilities. Thus, in the temperate zones selection often favors high fecundity and rapid development, wheras in the tropics lower fecundity and slower development could act to increase competitive ability. By putting more energy into each offspring and producing fewer total offspring, overall individual fitness is increased. The small clutch sizes characteristic of many tropical birds are consistent with Dobzhansky's hypothesis. Dobzhansky's ideas were framed in terms too specific to reach the general ecological audience and have gone more or less unnoticed until fairly recently