The most important issue on which H. J. Muller and Th. Dobzhansky differed was a question that had puzzled plant and animal breeders during most of the first half of the twentieth century. Breeders were concerned with maximizing performance. Dobzhansky was concerned with the potential of the species for further evolution and so was Muller, but his immediate concern was the impact on future generations of a radiation-induced increase in the mutation rate. All three concerns centered on how much of the genetic variability in the population depends on overdominance, that is, gene loci where the phenotype of a heterozygote is outside the range of the phenotypes of the two corresponding homozygotes. Heterosis, the greater vigor and performance of heterozygotes, attracted so much attention that in the summer of 1950 Iowa State College held a five-week symposium on the subject.' It will be helpful to make the vocabulary clear at the outset with an illustrative example: