Regarding the genetic structure of the population, the following two hypotheses have been proposed:- 1. Classical Hypothesis 2. Balance Hypothesis.
Model # 1. Classical Hypothesis:
It was developed by T.H. Morgan (1932) and supported by H.J. Muller and Kaplan (1966). The classical hypothesis proposes that the gene pool of a population consists at each gene locus of a wild-type allele with a frequency approaching one.
Mutant alleles in very low frequencies may also exist at each locus. A typical individual would be homozygous for the wild-type allele at most gene loci; at a very small proportion of its loci, the individual would be heterozygous for a wild and a mutant allele.
Except in the progenies of consanguineous matings, individuals homozygous for a mutant allele would be extremely rare. The “normal” ideal genotype would be an individual homozygous for the wild-type allele.
According to classical hypothesis, mutant alleles are continuously introduced in the population by mutation pressure, but are generally deleterious and, thus, are more or less gradually removed from the population by natural selection. Periodically, a beneficial mutant allele might arise, conferring higher fitness upon its carriers than the pre-existing wild-type allele.
This beneficial allele would gradually increase in frequency by natural selection to become the new wild-type allele, while the former wild-type allele would be eliminated. Evolution, thus, consists of the replacement at an occasional locus of the pre-existing wild-type allele by a new wild-type allele.
Model # 2. Balance Hypothesis:
This hypothesis was proposed by Dobzhansky (1970) and E.B. Ford (1971). This hypothesis was derived by direct study of natural populations. According to the balance model, there is generally no single wild-type or ‘normal’ allele. Rather, the gene pool of a population is envisioned as consisting at most loci of an array of alleles in moderate frequencies.
A typical individual is heterozygous at a large proportion of its gene loci. There is no ‘normal’ or ideal genotype, only an adaptive norm consisting of an array of genotypes that yield a satisfactory fitness in most environments encountered by the population.
The proponents of the balance hypothesis argue that the common allelic polymorphisms are maintained in populations by various forms of balancing natural selection. The fitness granted on its carriers by an allele depends on what other alleles exist in the genotype at that and other gene loci. It also depends, of course, on the environment.
Gene pools are co-adapted systems; the sets of alleles favoured at one locus depend on the sets of alleles that exist at other loci. Evolution occurs by gradual change in the frequencies and kinds of alleles at many gene loci. As the configuration of the set of alleles changes at one locus, it also changes at many other loci.
The balance model of genetic structure of populations has now become definitely established, although some controversy remains regarding the process maintaining the common polymorphisms.