Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

Most quantitative traits such as, for example, height and weight, are affected by a large number of independent genes in the individual. In each of the relevant loci can be present alleles whose presence in the given individual cause an augmentation in the given trait as well as other alleles responsible for its reduction. Simultaneously, there exists a certain optimum for the value of the vast majority of traits and individuals that exceed this optimum or do not attain it are variously eliminated from the population, dependent on the degree of this deviation from the optimum, by stabilizing natural selection (see IV.6.2.1). From the standpoint of biological fitness of the individual it is advantageous if the maximum of its progeny has values of quantitative traits close to their optimum values. One of the mechanisms through which this can be achieved is based on the fact that alleles, that augment this trait and alleles that reduce this trait will alternate on the chromosomes of neighbouring loci. The carriers of recombined chromosomes on which, for example, there would be a predominance of alleles increasing the given trait, would attain the optimal magnitude in progeny only when they would reproduce with individuals that, to the contrary, bear chromosomes on which would predominate alleles reducing the given trait. The average biological fitness of the carriers of these chromosomes would thus be low. Individuals on whose chromosomes both types of alleles alternate will attain the optimal magnitude of the trait in their progeny when reproducing with the broadest possible spectrum of sexual partners. Through natural selection (and, of course, through the natural process of increasing entropy – there are more combinations with even distribution of both types of alleles on the chromosomes than combinations with uneven distribution), there will be a gradual increase in the population of chromosomes on which will alternate alleles augmenting and alleles reducing quantitative traits.

The degree to which this alternation of augmenting and reducing alleles is evenly distributed depends to a substantial degree on the intensity of inbreeding in the population, i.e. on the intensity of crossing between mutually related individuals. If this intensity is high, there is a substantial chance that an individual will inherit two identical copies of a certain chromosome; on the other hand, if it is low, this probability is substantially lower. Thus, during evolution, the distribution of the individual alleles on the chromosome is optimized, similar to their frequency in the framework of the individual loci, in relation to the degree of inbreeding in the given population and the related frequency of homozygotes in the population. If we artificially augment or reduce the frequency of close-relative crossing between individuals from a certain population, this can very substantially reduce the average biological fitness of the progeny. This phenomenon, which can, of course, have quite different causes, is called inbreeding or outbreeding depression.