Hardy-Weinberg principle: This principle says that allele frequencies in a population are stable and is 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.

Factors affecting it:- Five factors are known to affect Hardy-Weinberg equilibrium. These are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.

1) Gene migration or gene flow: When individuals migrate to another place or population, new genes or alleles are added to new population and are lost from old population. When gene migration occurs many times, it is called gene flow.

2) Genetic drift: Changes occurring in frequencies by chance is called genetic drift. Sometimes it forms a completely new population of species. Origin of new species from existing one are called founders. And this effect is known as founder’s effect.

3) Mutation: Beneficial mutations makes new phenotypes and after a few generations causes speciation.

4) Genetic recombinations: Variations due to recombinations also makes new phenotypes.

5) Natural selection: It says that the fittest who is able to survive under unfavourable conditions in better ways, will increases its progenies and naturally selected by nature. It shows three effects:

            i)_Stablizing selection:- Peak get narrow and high. Average value is adapted so, average phenotype is selected instead of extreme one. For e.g. ⟶

(1) Average height people are more than tall or small.

(2) New born are 2 to 10.8 pounds, and chances of better survival are for 7-8 pounds.

             ii)  Directional selection ⟶ Peak gets shifts to one side supporting a single extreme phenotype & shifts towards it.

ex- Industrial melanism, pesticides resistance in mosquito.

               iii)  Disruptive selection ⟶ Opposite of stabilizing selection, where extreme one is more adopted than average one.

ex- Black seed cracker. Those with large bills & small bills survived more than others.