The Effects of Heterozygosity and Recombination on the Relative Fitness of Experimental Populations of Drosophila Melanogaster.

I N nature, evolution is most likely taking place on the interpopulational level. Certain populations are better able to adapt to environmental changes than others. By comparing populations as a whole, one could attempt to find out what makes one population more successful in the evolutionary sense than another. BUZZATI-TRAVERSO ( 1952) emphasized this interpopulational problem when he stated that the most fit population is one which can best exploit the available space and energy sources. The method was developed by CARSON (1958) on experimental populations. He used population size as a measure of the relative populational fitness by comparing genetically different populations exposed to uniformly controlled environmental conditions. “The population which maintains the larger relative size is judged to be performing better from the over-all biological p i n t of view.” (CARSON 1961). The experiments of CARSON have shown that the introduction of a foreign genome into homozygous populations is followed by a very rapid enhancement of the relative population fitness; the populations trebled in size within three generations. Accompanying this change in population size was a change in the genetic composition of the population. A condition of balanced polymorphism ensued, apparently as a result of natural selection. The rapidity of the increase in fitness following the introduction of the foreign genome has led CARSON to the suggestion that the cause for the increase must be a simple heterosis requiring no coadaptive integration. The experiments reported here have utilized Drosophila melanogaster. They were designed to probe further into the populational phenomena observed by CARSON. The first experiment consists of five populations founded with individuals from a second-chromosome mutant stock. Previously, all the experiments of CARSON had been performed with a particular stock marked by third-chromosome mutants. Thus, this second-chromosome mutant stock was chosen in order to ascertain whether there was something unique about the third chromosome or the particular stock which CARSON had used, which produced the rapid increase in fitness. The experimental procedure was identical to that of CARSON so that the performance of these second-chromosome mutant populations with the addition