Hardy-Weinberg Principle

Hardy-Weinberg Principle:

According to this principle, the frequency of alleles & genotype in a population remains constant from generation to generation, if the population is stable & in genetic equilibrium.

GENE POOL: Total genes & their alleles in a population. Its gene pool remains constant.

This is applicable in case of:

  • Diploid organisms.
  • Sexually reproducing organisms.
  • Large populations: as this helps to ensure that chance alone does not disrupt genetic equilibrium.
  • Organisms showing random mating.
  • No mutation or if they occur the rate is same in both directions.
  • All members of a population survive and have equal reproductive potential.

Hardy-Weinberg equilibrium:

In a diploid organism, ‘p’ & ‘q’ represents the frequency of ‘A’ & ‘a’.


‘p’ represents the frequency of allele ‘A’  & where ‘AA’ is p

‘q’ represents the frequency of allele ‘a’  & where ‘aa’ is  q2 

2pq represents the frequency of 2Aa

According to him AA, Aa, aa forms the gene pool.

If conditions are fulfilled then p+q = 1

p+ 2pq + q2 = 1 (in a given population).


Constant gene frequencies indicate that evolution is not taking place. Changes in gene frequencies i.e. when the genetic equilibrium/ Hardy-Weinberg equilibrium is upset, leads to evolution.



Factors affecting genetic equilibrium are:

  1. MUTATION: It adds new genes & Alleles in a gene pool. It creates & maintains variation within a population.
  2. RECOMBINATION: It causes the reshuffling of genes of chromosomes, that leads to redistribution of different traits to different individuals in a population. This brings diversity in genotype & phenotype of different organisms.
  3. GENETIC DRIFT: It is a random change in frequency of alleles occurring due to chance fluctuations. It affects small populations. It may result in loss of some alleles or rise of other alleles. Founder’s effect: when a small group separates from a larger population and may not have all the alleles. A dramatic change in their allele frequency occurs as they colonize to a new habitat making them completely different from the parent population & forming a new species. They become founders.
  4. GENE MIGRATION or GENE FLOW: It is the movement of genes/alleles from a population changing gene frequency when migration of a section of the population occurs to another place. It occurs due to emigration & immigration.

Emigration is moving out of individuals from a population. Loss of alleles occurs from the gene pool.

Immigration is moving in of individuals into a population. New genes add to the gene pool.

New alleles get added to the gene pool when migratory individuals interbreed with the local population, This is Gene migration.

5. NATURAL SELECTION: It is the process by which better-adapted individuals are favored by nature from a heterogeneous population of less adapted individuals. Hence they will contribute a greater percentage of genes to the gene pool of next generation. Over generations, these genes will become more common in the population.

All these factors cause changes in gene frequency leading to EVOLUTION.

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