• In a given population one can find out the frequency of occurrence of alleles of a gene on a locus.
  • This frequency is supposed to remain fixed and even remain the same through generations.
  • Hardy-Weinberg principle stated it using algebraic equations.
  • The principle states 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.
  • (p + q)2 = p2 + 2pq + q2 = 1.
  • When frequency measured, differs from expected values, the difference (direction) indicates the extent of evolutionary change.
  • Disturbance in genetic equilibrium, or 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:
    • Gene migration or gene flow.
    • Genetic drift.
    • Mutation.
    • Genetic recombination.
    • Natural selection.
  • Gene migration: When migrations of a section of population to another place occur, gene frequencies change in the original as well as in the new population. New genes /alleles are added to the new population and these are lost from the old population.
  • Gene flow: Gene migration occurs many time is termed as gene flow.
  • Genetic drift: change in gene frequency takes place by chance.
  • Founder effect: sometimes the change in allelic frequency is so different in the new sample of population that they became a different species. The original drifted population becomes founder species and the effect is called founder effect.

Operation of natural selection on different trait:

  • Natural selection can lead to :
    • Stabilization: in which more individuals acquire mean character value.
    • Directional changes i.e. more individuals acquire value other than the mean character value.
    • Disruption: more individuals acquire peripheral character value at both ends of the distribution curve.

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