overdominant loci are common, then a high level of heterozygosity would result from natural selection, and mutation-inducing radiation may in fact facilitate an increase in fitness due to overdominance. (This was also the view of Dobzhansky.)

Debate continued through 1950s, gradually becoming a central focus of population genetics. A 1958 study of Drosophila by Wallace suggested that radiation-induced mutations increased the viability of previously homozygous flies, providing evidence for heterozygote advantage and the balance position;