What is the relationship between evolution, allele frequencies, and populations?

According to Swell Wright’s Synthetic Theory, evolution of a population takes place when changes in their allele (gene) frequencies occur and the population moves to a higher adaptive level in that particular ecological area.

A simple way of explaining this relationship between populations, their alleles and evolution was described by Philip McClean in his publication ‘Population and Evolutionary Genetics’ in 1997 (see resource link).

McClean referred to a population of peppered moth which existed in pre-industrialized central England. At that time, the peppered moth existed in white and dark coloured alleles with the white coloured allele predominating.

The reason the white-coloured allele peppered moth were more predominant was that they were able to avoid bird predators by hiding on white-barked trees in that area, which enabled them to mask their presence.

With the advent of industrialization, pollution from industries stained the white-barked trees, making them darker. The white-coloured moth no longer had hiding places and they became vulnerable to attack by bird predators.

Their population gradually reduced while the dark-coloured allele moth increased in number, gradually becoming the more predominant allele in that area. Changes in allele frequencies occurred by the creation of more copies of an already existing allele in that population and this brought about the evolution of that population of peppered moth to a higher adaptive level.