Population Genetics and Evolution:
A population is a group of all those individuals that live in a given geographical area at a particular time and interbreed freely. It is also called the Mendelian population. The total number of genes of all the individuals of a population constitutes the gene pool of that population. The members of a population share or contribute to the same gene pool by freely interbreeding with one another more preferentially than the members of neighbouring sister populations. It means that these have a “free gene flow“.
Evolutionary forces operate on the gene pool of the population causing changes in the relative frequency of different alleles of a gene. For example, say there are two alleles of a gene in a population and each one produces a different form of a specific enzyme. In case the relative frequency of individuals possessing each form of this enzyme changes, it means the relative frequency of associated alleles has changed changing the gene pool of the population. Such gradual changes in the gene pool of a population diversify it from other sister populations of the species and this is evolution.
The study of frequencies of different alleles of genes in populations is called “population genetics“. G. H. Hardy and W. Weinberg (1908) independently proposed “Hardy-Weinberg Law of Equilibrium” to define the genetic structure of the gene pool of a population and describe the gene and genotype frequencies in it. According to Hardy Weinberg Law “the relative frequencies of alleles of various genes in a large, randomly mating, nonevolving population tend to remain constant generation after generation in the absence of evolutionary force”.
According to this generalisation, if a population is large, its individuals have random mating, each parent produces a roughly equal number of gametes, these gametes combine at random and undergo no evolutionary changes, then the gene or allele frequency of different genes remains constant or unchanged and genetic equilibrium of genes is preserved through generations.
Hardy-Weinberg Principle provides a theoretical baseline for measuring evolutionary change. It is used to calculate allele frequency in a population. Comparison of allele frequency through generations provides clear-cut information on whether a population is evolving or not. The amount of deviation between observed allele frequencies through generations provides a degree of evolutionary change. Therefore, evolution occurs when genetic equilibrium in the population is disturbed or evolution is a departure from Hardy-Weinberg equilibrium.