Selection for heterozygotes

It followed from Equations (8) and (9) in Chapter IV.6.1 that an equilibrium frequency of the two alleles is established in dependence on the ratio of the selection coefficients of individuals with the particular genotypes. Polymorphism maintained by selection for heterozygotes is sometimes also termed balanced polymorphism. This mechanism is valid under conditions where the fitness of the heteozygote is greater than the fitness of any of the homozygotes. This case is apparently quite frequent (Fig. VIII.3) Geneticists sometimes connect this phenomenon with the heterosis effect, the greater viability of individuals with a high degree of heterozygosity (Hawkins & Day 1999). However, it is always necessary to strictly differentiate when the heterozygote actually has greater fitness and when it only has greater weight, e.g. as a consequence of poor balancing of ontogenetic processes. It is possible that a great many cases of the heterosis effect utilized in agriculture fall in this category. However, comparative studies simultaneously indicate highly significant correlation between the average heterozygosity in populations and the average fitness of their members (Reed & Frankham 2003). It is, however, apparent that, within a single population, individuals that are heterozygote in a large number of genes frequently actually do have greater fitness, exhibit mutually lower individual variability and their ontogenesis is more regular and more resistant against the action of external interfering effects (Fig. VIII.4). The latter fact is manifested, e.g., in lower fluctuation asymmetry in the body structures of species with bilateral symmetry, i.e. that component of morphological asymmetry that is manifested in some individuals in the population in a greater size of a certain structure on the right-hand side of the body and in some individuals on the left-hand side.

The cause of the greater viability of heterozygotes is not currently completely clear. According to some theories, this lies in the relatively greater number of recessive harmful (negative) mutations in the homozygous state in the genotype of homozygous individuals. An individual that is a homozygote in a great many genes is apparently the progeny of two mutually related individuals so that an elevated probability of the occurrence of rare harmful mutations in the homozygous state can be expected (Fig. VIII.5). According to some authors, the importance of this mechanism is reflected, e.g., in the fact that haplodiploid species, in which recessive lethal and highly harmful mutations are eliminated in haploid males, exhibit relatively lower polymorphism (Edwards & Hoy 1993). According to this model, the elevated viability of heterozygotes tends to be a manifestation of the reduced viability of homozygotes occurring through inbreeding, i.e. reproduction amongst relatives. If this were actually true, then the phenomenon of elevated viability of heterozygotes and the long-term survival of polymorphism in the population would not be functionally interconnected. A high degree of heterozygosity would only be an indication of a low level of inbreeding and thus low probability of the occurrence of rare harmful recessive mutations in the homozygous state in the given individual.

Another possible explanation of the greater fitness of heterozygotes is based on the assumption that the products of the individual alleles of a single gene fulfill a different function in the cell to at least some degree.  A heterozygous individual with two different alleles of a given gene is thus necessarily at an advantage over any homozygous individual. For example, different alleles exist in the population for a large percentage of enzymes, differing in their isoelectric point and thus mobility in an electric field. The occurrence of these alleles forms the basis for alozyme analysis (see XXIV.3.6). It is quite possible that the cell both accelerates and regulates its physiological processes by forming a pH-gradient and simultaneously an electric field in its interior and thus both concentrates and, as required, relocates the molecules of the individual enzymes to various areas of its inner space through intracellular isoelectric focusing (see also XII.5) (Flegr 1990; Flegr 1996a). The presence of two forms of enzymes differing in their isoelectric points can substantially assist heterozygotes to increase the effectiveness of some cellular processes, as it facilitates the simultaneous presence of the same enzyme activity of a monomeric enzyme at two places and of a multimeric enzyme at several places in the inner space of the cell (Fig. VIII.6).

The correlation between the polymorphism in the population and the intensity of environmental stress to which the populations or species are exposed in their environment is an indirect proof for maintenance of polymorphism through selection for heterozygotes. Thus, populations occurring at places with extreme and very variable natural conditions, for example in warm areas on the side of a valley exposed to the sun (FIG. VIII.7) exhibit substantially greater polymorphism. On the other hand, species occurring in an unvarying, stable environment, such as underground species, exhibit low polymorphism (Nevo, Filippucci, & Beiles 1994; Nevo 2001). This general trend indicates that polymorphism apparently has functional importance and increases the resistance of the population and evidently also of individuals against the action of various stress factors occurring in the environment.

The main objection to the importance of selection for heterozygotes for maintenance of the more important part of polymorphism in the natural population is that, in this case, the fitness of inbred individuals would have to be unrealistically low compared with outbred individuals. The selection coefficients of the individual alleles in the homozygous state would have to have a certain minimal value for any of the polymorphic genes in order to maintain the relevant content of the two alleles in the population. However, for inbred individuals, all the genes would have to be in the homozygous state, so that the overall fitness of these individuals would basically have to equal zero.

The relatively high polymorphism of a number of haploid species, including bacterial species, provides further evidence for the lower importance of selection for heterozygotes(Kimura 1985). This mechanism can, of course, not be operative for haploid species.

Was this information useful for you?
The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more