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▪ This principle states that allelic frequencies in a population are stable and remains constant from generation to generation, i.e., gene pool (total number of genes and their alleles in a population) is constant. This is called genetic equilibrium or Hardy–Weinberg equilibrium. 

▪ It can be expressed as p² + 2pq + q² = 1 where p and q are frequencies of different alleles. 

▪ Disturbances in genetic equilibrium results in evolution.

Before industrial revolution the environment was unpolluted. The lichens on the barks of trees were pale. The white-winged moths could easily camouflage, while the dark-winged were spotted out by the birds for food. Hence, they could not survive. After industrial revolution the lichens became dark (due to soot deposit). This favoured the dark-winged moths while the white-winged were picked by birds. The population of the former which was naturally selected increased.

According to Darwin, fitness refers to reproductive fitness. A fit species will leave more progenies. This will result in greater chances of survival. Greater the number of progenies which will survive, more they will be selected by nature to continue the species. This is called natural selection.

Recessive traits: If the frequency of a recessive trait such as cystic fibrosis or PKU is known, it is possible to calculate allele frequencies and genotype frequencies using the Hardy Weinberg equation and its assumptions are as follows: 

(i) say 1 in 1, 2500 Indian newborns have cystic fibrosis which means that the frequency of homozygotes for this recessive trait is 

 q² = 1/2,500 = 0.0004 

(ii) The square root of the frequency of recessives is equal to the allele frequency of the cystic fibrosis allele q = (0.0004)0.5 = 0.02 

(iii) The frequency of the normal allele is equal to 1 - the frequency of the cystic fibrosis allele p = 1- q = 1 - 0.02 = 0.98105 

(iv) The frequency of carriers (heterozygotes) for the cystic fibrosis allele is 2pq = 2 (0.98)(0.02) = 0.04 or 1/25 69 v. The frequency of homozygotes for the normal allele is 

p² = (0.98)² = 0.96

(vi) Thus the population is composed of three genotypes at the calculated frequencies of homozygous normal = 0.96, heterozygous carriers = 0.04, homozygous affected = 0.0004 

Before industrialization there were mostly light moths (peppered moth) as they merged with the background of trees, having lichen on their bark & fewer black moths due to predation by birds as they were clearly visible against the grey background.

After Industrialisation-deposition of soot due to emission from factories made the tree barks black and the black moths got cammouflaged in the background & survived but as the grey moths were now visible, they were easily spotted out & predated upon by the birds and their number got reduced, thus black moths naturally selected over the grey ones because they got best adjusted to their environmental condition.

It indicates that predators will spot a moth against a contrasting background.

1. After industrialization, the tree trunks became dark due to deposition of soot and smoke; under such a condition, the white winged moths stood out conspicuously and easily detected by the predators and hence they reduced in number.

2. The dark-winged moths could merge with the black colour and they escaped the predators and hence increased in number.

Pre-industrialisation period had more white winged moth against grey lichens on tree trunks. During industrialisation large amount of soot and smoke deposited on tree trunks, making the bark dark. Against the dark background white moths could easily be preyed upon. Melanised moth could camouflage against dark bark. This natural selection increased their number.