Most natural populations are characterized by more or less obvious polymorphism. Individuals of the same sex and the same age in the population differ from one another in a number of quantitative and qualitative traits.
Part of this polymorphism is nonhereditary in nature and evolves as a response of the individual to the effects of the external environment that it or its immediate ancestors encountered during their lives or ontogenesis. However, a large portion of polymorphism is determined genetically and is thus hereditary to various degrees. Genetic polymorphism is a result of the existence of two or more variants (alleles) of the individual genes.
Genes were previously seen as hypothetical factors determining the value of the individual biological traits, the properties of living organisms. In order for the existence of such a gene to be distinguished, the relevant trait for which the particular gene was responsible had to assume at least two values.Monomorphic genes, i.e. the genes occurring in the population in a single variant, could thus not, in principle, be distinguished and described or studied.
Only with the ascendance of molecular biology and following revelation of the material nature of genes, i.e. DNA sections coding proteins or RNA molecules, did it begin to become possible to identify the individual genes without first knowing their phenotype manifestations. This permitted identification of monomorphic genes. Modern methods of molecular genetics, specifically the methods of reverse genetics, permit determination of the biological function of a gene even when only a single variant is known. It is possible to either transfer the relevant gene to the genome of a suitable recipient (through a cell of the germinal line) and thus prepare a transgenic organism or, on the other hand, to employ the gene targeting (knockout) method to inactivate the particular gene in the zygote. Study of the phenotype of these genetically modified organisms then assists in determining which traits a particular gene determines.
While molecular biology has made it possible to also study monomorphic genes, it has simultaneously demonstrated that, strictly speaking, monomorphic genes do not exist. Almost all genes studied in detail occur in populations in a great many variants that differ, at the very least, in the presence of individual point mutations. Most of these mutations are apparently neutral in relation to the phenotype and selection, and occur, e.g., on the third positions of the nucleotide triplets, where their occurrence does not lead to substitution of an aminoacid in the protein chain. Some mutations lead to these substitutions; however, only some substitutions simultaneously lead to changes in the biological functioning of the relevant proteins. A large part of the polymorphism at the DNA level is thus not polymorphism in the true sense of the word and is not manifested externally in any way. This type of “pseudopolymorphism” is important for study of evolutionary and population phenomena, but is of relatively little importance from the viewpoint of biological evolution. It arises from mutation processes and, on the other hand, is being continuously removed by genetic drift, genetic draft andmolecular drive.
If pseudopolymorphism is not taken into consideration, i.e. the presence of neutral mutations, that do not in any way appear in the phenotype of the organisms (i.e. if the location of zones on an electrophoretogram is not considered to constitute part of the phenotype), it is found that two groups of polymorphic genes exist. The first group consists in genes that occur in the population with great frequency in one standard form and in much lower frequency, usually less than 1%, in minority forms. For these alleles, it is generally assumed that they occur only temporarily here or that they are maintained as a result of the dynamic equilibrium of two processes – mutation pressure, i.e. constant formation of new alleles from the majority standard allele during mutagenesis, and selection, i.e. constant disappearance of the mutated alleles from the population.Polymorphism, caused by the temporary presence of rare alleles in the population, will be termed Type I polymorphism and will be discussed primarily in Chapter IX, devoted to the evolution of the DNA sequence and proteins.
However, for the second group of genes, it is difficult to say which allele is standard and which is mutated as they all, or at least a great many of them, occur in the population in high frequency (Fig. VIII.1). This type of polymorphism is maintained over long periods in the population through the action of specific mechanisms and is of incomparably greater importance from the viewpoint of evolutionary and ecological processes. Here, it will be termed Type II polymorphism and will constitute the subject of this chapter.See also Origin of Rh-blood group polymorphism and also Sickle-cell anemia.
The existence of polymorphism within a population is of substantial importance both from the perspective of ecological processes and also from the microevolutionary and macroevolutionary viewpoints. From an ecological perspective, it is important that a polymorphic population is capable of utilizing very varied resources and consequently of utilizing its environment more economically. A polymorphic species is also less vulnerable to random fluctuations in the environmental conditions, as at least part of the population can survive even during drastic changes.
Polymorphism also changes the microevolutionary potential of populations and species, i.e. the ability of populations and species to respond to short-term selection pressures in the environment,as it provides selection with genetic material from which it can choose suitable variants that better correspond to the altered conditions in the environment (Fig.VIII.11).Thus, populations need not wait for the formation of new mutations and can use the already-present variability. As the fate of isolated new mutations immediately after their formation is determined primarily by accident (see V.3.1), it is far more advantageous if potentially suitable variants occur in the population with sufficient frequency right at the beginning. In addition, in case of cyclic changes in the environment, it is ensured that the genetic composition of the population can readily return to the original state after the environmental conditions go back to their original values (Fig. VIII.12). According to some theories, it is this reversibility of microevolutionary changes that is mainly responsible for the greater evolutionary success of sexually reproducing species (Williams 1975, Flegr 2008). In asexual species, the composition of the gene pool of the population follows even temporary environmental changes. Thus, all the alleles that are essential for their bearers under normal conditions can be very easily lost from the population during short-term changes. This can understandably be very disadvantageous for an asexual species in the medium term.
From the perspective of long-term or even permanent changes in the environment, and thus, for example, from the perspective of macroevolutionary processes, the impact of polymorphism on the progress and rate of evolution can be quite the opposite (Flegr 1998). Due to the phenomenon of genetic homeostasis andepistatic interactions, the effectiveness of natural selection is greatly limited in polymorphic sexually reproducing populations (see IV.9.2). Thus, populations and species react to the action of selection pressure only through a shift in the frequencies of individual alleles that are already present in the gene pool, while the fixation of new alleles through selection is greatly limited. Thus, a genetically polymorphic species remains selectionally frozen and anagenetic changes can occur only following a drastic reduction in the polymorphism, for example, in connection with a long-term drastic reduction in population size(see V.2.2).