Chapter 37 - 1966 Genetic Variation

Abstract

The magnitude of genetic variation in natural populations is open for debate. Prior to the molecular era that began in the mid-1960s, the magnitude of genetic variation in natural populations remained controversial, with many evolutionary biologists falling into either of two opposing camps: the classicists who maintained that genetic variability in most species was low, such that conspecifics were homozygous for the same “wild-type” allele at most genetic loci; and proponents of the balance view who argued that genetic variation was extensive such that most loci were polymorphic and individuals typically were heterozygous at a substantial fraction of their genes. A central tenet of the classical school was the concept of genetic load: the idea that genetic variation in a population produces a heavy burden of reduced genetic fitness. In support of the classical view was the observation that many de novo mutations are deleterious. In contrast, the balance school tended to view genetic polymorphisms as beneficial and maintained in populations by one or another form of balancing natural selection (such as environmental heterogeneity, heterosis, or frequency-dependent fitness advantage). Two empirical observations sometimes interpreted in favor of the balance view were as follows: phenotypic variation is often extensive in natural populations for many traits; and artificial (human-mediated) selection for various phenotypic attributes was usually successful in many species, thus suggesting that genetic variation must underlie such traits. Nevertheless, by hard criteria such observations were inconclusive for firmly resolving the classical/balance debate. Biologists needed direct information on genetic variation from molecular biology.