Studies of the Relative Fitnesses of Experimental Populations of Drosophila melanogaster

Abstract

The relative fitnesses of experimental populations of D. melanogaster have been estimated by determining the average number of offspring produced by single pairs of flies. Each experiment consisted of a study of 50 single pairs of flies from each of 2 "competing" populations; each experiment extended over 10 generations. Results indicate that 3 of the populations studied (a control population, a population exposed to a large dose of x-rays more than 150 generations before these studies, and a population exposed continuously to about 300r gamma-radiation per generation for 150 generations) had approximately equal fitnesses. Two other populations which were exposed to about 2000r gamma-radiation per generation for 150 generations gave substantially lower estimates of fitness. Finally, 3 populations which were obtained from the latter, heavily irradiated ones and which were not irradiated for the past 40 generations had not regained fitnesses equal to that of the control. The studies revealed that the estimation of fitness depends upon the component of fitness studied; that the estimate obtained from one component fluctuates with time; and that different samples of individuals from the same population yield different estimates. Thus, fitness is in no sense to be regarded as a biological constant.