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cornerstone of the theory of evolution by gradual change is that the rate of A evolution is absolutely limited by the amount of genetic variation in the evolving population. FISHER’S “Fundamental Theorem of Natural Selection” ( 1930) is a mathematical statement of this generalization, but even without mathematics it is clear that genetic change caused by natural selection presupposes genetic differences already existing, on which natural selection can operate. In a sense, a description of the genetic variation in a population is the fundamental datum of evolutionary studies; and it is necessary to explain the origin and maintenance of this variation and to predict its evolutionary consequences. It is not surprising, then, that a major effort of genetics in the last 50 years has been to characterize the amounts and kinds of genetic variation existing in natural or laboratory populations of various organisms. The results so far have told us a great deal about cytological variation such as polymorphisms for inversions and translocations, about frequencies of rare visible mutations at many loci, and about frequencies of chromosomes that are deleterious when homozygous together with the degree of that deleterious effect. In addition, we know of some striking singlelocus polymorphisms. These results are familiar to all students of population genetics and evolution, and have been well reviewed by DOBZHANSKY (195 1 ) and more recently by MAYR (1 963). Yet, for all the wealth of observation and experiment, the techniques of population genetics have not allowed us to ask directly the most elementary question about the genetic structure of a population: At w h t proportion of his loci can we expect a diploid individual to be heterozygous? Put in another way, this is the question of how much genetic variation there is in any given population. That this question remains unanswered is best shown by a statement of MAYR (1963) at the end of more than 100 pages of review of our present knowledge.

genetics

A MOLECULAR APPROACH TO THE STUDY OF GENIC HETEROZYGOSITY IN NATURAL POPULATIONS. I. THE NUMBER OF ALLELES AT DIFFERENT LOCI IN <i>DROSOPHILA PSEUDOOBSCURA</i>

A MOLECULAR APPROACH TO THE STUDY OF GENIC HETEROZYGOSITY IN NATURAL POPULATIONS. I. THE NUMBER OF ALLELES AT DIFFERENT LOCI IN DROSOPHILA PSEUDOOBSCURA