Four major forces are usually listed for changing gene frequencies in populations, namely migration, mutation, selection and random genetic drift. These forces constitute the mechanisms underlying the evolutionary process.
1. Migration:
Migration occurs when a large influx of people moves into another population and interbreeds with the latter. The phenomenon called gene flow takes place if one population contributes an allele to the other population. Let us suppose that a migrating population m interbreeds with members of another population.
Then the descendants of the next generation will have m genes from the migrants and 1 – m genes from members of the original population. Consider an allele A occurring with frequency p in migrant population.
In the original population this allele has frequency q. In the next generation the frequency of A in the new population would be:
r = (1 – m)q + mp
= q – m (q – p)
That means the frequency of allele A in the new population now would be the original allelic frequency q multiplied by the genes (1 – m) present in the original population plus the product of reproducing migrant individuals and their gene frequency imp). Thus there will be a new gene frequency in the next generation.
Migration is a complex phenomenon in humans, influenced by many factors. It seems however, that it leads to make populations genetically more similar than they would be otherwise.
2. Mutation:
The ultimate source of all genetic variation is mutation. Both chromosomal rearrangements and point mutations are implied here as they follow the same rules of population dynamics. However, mutations occur with an extremely low frequency. In humans where there may be from 30-50 successive mitotic divisions in the germ cells in each generation, only one gene in a million or 10 million roughly undergoes a mutation.
Consider an allele A that is homozygous in many individuals in a population. Assume that in every generation one A allele in a million mutates to a. This will reduce the frequency of A allele over many generations, while a allele will gradually accumulate in the population. The change in frequencies of A and a occurs at an unimaginably slow rate.
The a allele however, can also back mutate to A; this event will take place as the frequency of a alleles increases. After a very long time the number of A alleles lost by forward mutation would be balanced by the number of A alleles arising from back mutation of a to A.
When this happens gene frequencies of A and a are said to be in mutation equilibrium. Thereafter, no further change in frequency of A and a will occur in subsequent generations. This applies however, only when the other evolutionary forces such as migration, selection and genetic drift are not operating to affect gene frequencies in the population.
Experimental work has shown that the rate of mutation can be affected by environmental factors like radiation and chemicals. In some instances mutations are under genetic control. There is a recessive mutator gene on the second chromosome of Drosophila melanogaster.
In stocks of flies homozygous for the mutator gene, sex-linked recessive lethals occur spontaneously with high frequency. In maize the recessive mutator gene Dt acts on an unlinked locus a which controls synthesis of the purple anthocyanin pigment.
The mutator gene Dt causes mutation of recessive a alleles to the dominant form A which leads to synthesis of anthocyanin appearing as purple spots on the stem, leaves and kernels of the maize plant. Mutator genes also occur in micro-organisms including E. coli. As the action of mutator genes is directed at a particular locus, they are said to produce directed mutations in contrast to random mutations which are not specific.
Normally all genes could mutate. In the case of a rare allele its mutation to other alleles is difficult to detect due to low frequency and slow rate of mutation. But when an allele occurs more frequently in the population it leads to higher mutation rate and increases the population’s potential for evolutionary change. If a rare deleterious allele accumulates in the population, it is a disadvantage and constitutes what is referred to as the mutational load.
3. Selection:
Selection is one of the forces that change gene frequencies in the population and a fundamental process of evolutionary change. The idea was first conceived by Charles Darwin in his Origin of Species published in 1859 and by Alfred Russel Wallace.
Selection is defined as differential survival or fertility of different genotypes. If individuals carrying gene A are more successful in reproduction than individuals carrying its allele a, then the frequency of gene A will tend to be greater than that of gene a.
The wide variety of mechanisms responsible for modifying the reproductive success of a genotype are collectively included under selection. It is the process that determines the contribution that people of different genotypes will make as parents of the next generation. Selection does not act on individual genes, but rather on the organism bearing the genes.
The reproductive efficiency of a genotype is measured in terms of the average number of offsprings born to the bearers of the genotype and is called Darwinian fitness or relative fitness. It is also referred to as the organism’s adaptive value. The fitness value of 1 is usually assigned to the genotype with highest reproductive efficiency.
However, fitness does not have an absolute value, and is expressed in relative terms as a ratio. Relative fitness (w) is obtained by dividing the fitness of all the genotypes by the fitness of any one genotype. Fitness simply describes the average number of progeny that survive and reproduce.
The related term selection coefficient s = 1 – w. Some aspects in the individual’s life are likely to affect the survival, growth and reproduction; consequently they affect the fitness of genotypes and are referred to as fitness components.
Basically fitness depends upon survival and fertility. Persons affected with Huntington’s chorea, a dominant condition may have 25% reproductive efficiency as compared to normal human beings. On the other hand children with Tay Sachs disease usually die before reproductive age. Thus the fitness of a person with Huntington’s chorea is 0.25 and of Tay Sachs patient is zero.
When fitness of two alleles at a locus differs then selection favours survival of alleles with greater fitness and elimination of the other alleles. Thus frequency of one allele increases and of the other will decrease in the subsequent generations. However, if a rare allele occurs with low frequency, then selection is not able to cause much change in gene frequency.
Specifically selection occurs against a recessive allele, or a dominant allele, resulting in its elimination; it could occur in favour of a heterozygote or against a heterozygote leading to polymorphism in a given trait.
When selection occurs in favour of a heterozygote over both homozygotes it is called over dominance or heterosis. It occurs when the fitness of the heterozygous genotype is greater than the fitness of both homozygotes. Assume that the relative fitness of the genotypes AA, Aa and aa are 0.9,1 and 0.8 respectively.
The greater fitness of the Aa- genotype will not allow either A or a alleles from homozygotes to become fixed. Ultimately equilibrium gene frequencies would be attained. In humans over dominance has led to polymorphism in sickle cell trait, thalassaemia and G6PD.
The effect of selection is also counterbalanced by mutation. While selection is eliminating some genes from the population, mutation is creating new ones. The two forces selection and mutation operate in opposite directions, and tend to compensate each other. After a long time, gene frequencies will reach equilibrium.
There could be partial selection against recessives. This is a less complete form of selection against homozygous recessive individuals. In this case selection coefficient s is less than one, and the relative fitness w of the homozygous recessive individual is 1 – s, having value greater than zero.
A popular example of selection is industrial melanism as exhibited by the pepper moth Biston betularia. In the mid 19th century the light coloured forms of the moth were abundant on the pale barks of trees growing in unpolluted, non-industrialised regions of England. The dark form of Biston was extremely rare. As industry developed in the area, the environment became polluted and the barks of trees turned dark grey with smoke and dust.
The light moths on the dark coloured bark were easily noticed by the predators and were preyed upon. Their number began to decrease. In the following decades, the population of dark moths was observed to gradually increase to more than 95%; the light moths were hardly seen. Industrial melanism is thus a clear cut example of selection disturbing gene frequencies in the population.
Artificial and Natural Selection:
Selection was being practiced by humans since antiquity. Plant and animal breeders have been attempting to modify hereditary transmission of traits by selecting most desirable individuals to serve as parents for the next generation. This is called artificial selection. By contrast, when organisms are selected by natural forces instead of by human choice, they are said to be subject to natural selection.
4. Random Genetic Drift:
These are unexpected random changes that occur in gene frequencies from generation to generation in all populations. They are particularly noticeable as sampling variation in small populations. In some generations the frequency of a certain allele will by chance increase, in others it will decrease, in still others it may remain the same. These fluctuations in gene frequency occur at random.
In small samples there is greater variation as compared to big samples. Drift however, does not depend upon the total size of the population, rather on the number of breeding individuals who would produce the next generation. It is unlikely that random drift alone will affect allelic frequencies at a gene locus over long periods of time. It is more likely that selection, mutation or migration would also take place at one time or another.