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

A far more frequent phenomenon than the disintegration of the population into a number of smaller reproductionally isolated subpopulations is either a combination of this phenomenon with a reduction in the number of individuals in the population, or only a reduction in the number of individuals in the population.A reduction in the size of the population is mostly drastically reflected in the composition of the gene pool of the population.

            It is frequently stated that the main manifestation of genetic drift consists in a reduction in the genetic polymorphism of the population as a consequence of fixation of the individual alleles.However, the situation is somewhat more complicated.A reduction in polymorphism is actually related to only certain components.Genetic driftis active in a population of any size.However, the individual processes that accompany it are more apparent in small populations as all processes, including fixation of alleles, then occur more rapidly.A large part of the genetic polymorphism present in the population at the instant of a reduction in its numbers thus rapidly disappears from the gene pool of the smaller population.The disappearance of a large part of the originally present alleles, specifically older alleles, thus differing in a large number of mutations (Fig. V.5), occurs in a population that underwent a substantial reduction in numbers.However, the polymorphism of the population is decided not only by the loss of older alleles, but also by the increase in the number of new alleles.This is a parameter in which a small population can outdistance a large population thanks to genetic drift.In large populations, selection has a far greater role than genetic drift in deciding on the fate of mutations (see V.4).Consequently, a large portion of selectionally negative (i.e. harmful) mutations are then very rapidly eliminated from the gene pool following their formation.These mutations have a far greater chance of lasting longer or even of becoming fixed in a small population.As a major fraction of mutations are probably selectionally negative (see V.5), a small population can even contain more polymorphism per individual than a large population.The relative increase in polymorphism is also promoted by the fact that elimination of polymorphism by selective sweeping  accompanying fixation of selectionally important positive (useful) mutations cannot occur so readily in a small population (se IX.4.4.1).

Fig. V.5. The bottle-neck effect and genetic composition of the population. The degree of polymorphism in the population is characterized not only by the number of alleles present and their frequencies, but also by the number of nucleotides in which the individual alleles present in the population differ. From the viewpoint of study of the history of the demographic structure of the population, the average number of nucleotides in which alleles derived from two individuals in the given population differ is a frequently studied parameter (the actual population diversity in the given gene). The history of demographic structure can be advantageously studied on the mitochondrial DNA. At equilibrium, the population diversity (m) should be twice the effective population size (N_e) multiplied by the mutation rate in the studied allele (u), most frequently expressed as the number of mutations formed in a certain section of the studied gene. If there is a rapid reduction in the size of the population, the population diversity decreases to its new equilibrium value very rapidly. If there is subsequently a rapid increase in the population to the original value, the diversity of the population will return to its original value only very slowly. Thus, on the basis of the magnitude of the average population diversity and the shape of the histogram of the distribution of the number of mutations in which two individuals in the population differ, it is possible, even after a long time, to demonstrate and approximately date the existence of a period when the given population passed through a “bottleneck”. Graphs (a) illustrate a situation when the reduction in the size of the population lasted 33 generations, while graphs (b) depict a situation when the reduction in the population was merely short-term and lasted only 4 generations. From Rogers and Jorde (1995).

           Polymorphism that accumulates as a consequence of the action of drift in small populations, i.e. preferentially accumulation of slightly negative or almost neutral mutations is, however, not full-value polymorphism.As the effectiveness of all types of selection is substantially reduced in small populations (see V.4), a substantial part of polymorphism disappears from the gene pool of a smaller population, while it would be maintained in a larger population through disruptive selection, selection in favour of heterozygotes and frequency-dependent selection.This type of polymorphism will be designated as type 2 polymorphism and will be the subject of a separate chapter (VIII).From the standpoint of the population and of the individual organisms, a reduction in type 2 polymorphism is a conspicuous and, in a great many respect, undesirable phenomenon.For example, from the standpoint of the population, it reduces its microevolutionary potential, i.e. ability to immediately react to alternating selection pressure from the environment.From the standpoint of macroevolution, which is dependent primarily on the formation of new mutations and not on the existing genetic polymorphism, a reduction in the polymorphism can, to the contrary, increase the evolutionary potential (see IV.9.2).A population with low type 2 polymorphism is more endangered by parasites and can be readily completely decimated by the action of a parasitic organism.In a sufficiently polymorphous population, at least a few individuals resistant to the action of parasites always survive.From the standpoint of an individual, it is advantageous if it differs from the other members of the population.In this case, it is not exposed to such intense competition within the population or to such a high risk that it will be attacked by a parasite or predator adapted to the commonest type of host or prey (see XIII.3.2.1 and XIII.3.2.2.3)

            The negative consequences of both the action of genetic drift  and of a reduction in the size of the population should be taken into account in the protection of endangered species and especially in programs to rescue these species in captivity.It follows from the mechanism of the action of genetic drift that a reduction in type 2 polymorphism cannot be prevented simply by crossing genetically remote individuals, on which most rescue programs concentrate, but only through maintenance of a sufficiently large population of the particular species.