Hardy- Weinberg Principle

In a given population one can find out the frequency of occurrence of alleles of a gene or a locus. This frequency is supposed to remain fixed and even remain the same through generations. Hardy-Weinberg principle stated it using algebraic equations. 

This principle says that allele frequencies in a population are stable and remain constant from generation to generation. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium. Sum total of all the allelic frequencies is 1.

p + q = 1

Where: p - Frequency of dominant allele (A)

q - Frequency of recessive allele (a)

The binomial expansion of this equation is:

p² + 2pq + q² = 1

Where: p² – Frequency of individuals with genotype AA

q² – Frequency of individuals with genotype aa

2pq – Frequency of individuals with genotype Aa

When frequency measured, differs from expected values, then the difference (direction) indicates the extent of evolutionary change. Disturbance in genetic equilibrium, or Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a population would then be interpreted as resulting in evolution.

Five factors are known to affect Hardy-Weinberg equilibrium. These are-

1. Gene migration or gene flow

2. Genetic drift

3. Mutation

4. Genetic recombination

5. Natural selection

1.0NATURAL SELECTION AND ARTIFICIAL SELECTION 

Natural Selection

(1) Industrial Melanism - This phenomenon was studied by Bernard Kettlewell in England.

                                  Generation 0, 10% dark–colored phenotype

Several Generations Later, 80% dark–colored phenotype

Phenomenon of industrial melanism 

In a collection of moths (Biston betularia) made in 1850s, i.e., before industrialization set in, it was observed that there were more white-winged moths on trees than dark-winged or melanised moths.

However, in the collection carried out from the same area, but after industrialization, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.

The explanation put forth for this observation was that 'predators will spot a moth against a contrasting background'.

Before industrialization set in, thick growth of almost white-coloured lichen covered the trees - in that background the white winged moth survived but the dark-coloured moths were picked out by predators.

Lichens can be used as industrial pollution indicators. They will not grow in areas that are polluted.

During post industrialization period, the tree trunks became dark due to industrial smoke and soot. Under this condition the white-winged moth did not survive due to predators while dark-winged or melanised moth survived.

Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived.

This understanding is supported by the fact that in areas where industrialization did not occur e.g., in rural areas, the count of melanic moths was low.

This showed that in a mixed population, those that can better-adapt, survive and increase in population size. Remember that no variant is completely wiped out.

(2) Drug resistance: 

The drugs which eliminate pathogens become ineffective in the course of time because those individuals of pathogenic species which can tolerate them survive and flourish to produce tolerant/resistant population.

Excess use of herbicides, pesticides, etc., has only resulted in selection of resistant varieties in a much lesser time scale. This is also true for microbes against which we employ antibiotics or drugs against eukaryotic organisms/cell. Hence, resistant organisms/cells are appearing in a very less time scale of months or years and not centuries. These are examples of evolution by anthropogenic action.

This also tells us that evolution is not a directed process in the sense of determinism. It is a stochastic process (random/by chance process) based on chance events in nature and chance mutation in the organisms.

How does industrial melanism support natural selection?

On the dark background, the pale moths were no longer well-camouflaged and were easily caught by birds so black colour moth survives and becomes common. This is an example of natural selection. As the environment selects the black colour moth so their number increases and nature favours their survival.   

Artificial Selection 

• Artificial selection is similar to natural selection except that the role of nature is taken over by man and the characters selected are of human use.
• Man has been taking the advantages of genetic variations for improving the qualities of domesticated plants and animals. 
• By artificial selection animal breeders are able to produce improved varieties of domesticated animals like dogs, horses, pigeons, poultry, cows, goats, sheep and pigs from their wild ancestors. 
• Similarly, the plant breeders have obtained improved varieties of useful plants like wheat, rice, sugarcane, cotton, pulses, vegetable, fruits etc.

2.0Also Read