- It states that allele frequencies in a population are stable and is constant from generation to generation in the absence of disturbing factors.
- The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium (Hardy-Weinberg equilibrium).
- Sum total of all the allelic frequencies = 1
- E.g. Consider, in a diploid, p & q are the frequencies of alleles A & a respectively.
Frequency of AA = p2
Frequency of aa = q2
Frequency of Aa = 2pq
Hence p2 + 2pq + q2 = 1 [binomial expansion of (p+q)2]
Change of frequency of alleles in a population disturbs Hardy-Weinberg equilibrium. This change is due to evolution.
Factors affecting Hardy-Weinberg equilibrium
a. Gene migration: Gene flow from one population to another. Here gene frequencies change in both populations. Gene flow occurs if migration happens multiple times.
b. Genetic drift: The gene flow by chance causing change in frequency. Sometimes, the change in frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founders and the effect is called founder effect.
c. Mutation: It results in formation of new phenotypes. Over few generations, this leads to speciation.
d. Genetic recombination: Reshuffling of gene combinations during crossing over resulting in genetic variation.
e. Natural selection: It is 3 types.
§ Stabilizing selection: Here, more individuals acquire mean character value and variation is reduced.
§ Directional selection: Individuals of one extreme (value other than mean character value) are more favoured.
§ Disruptive selection: Individuals of both extremes (peripheral character value at both ends of the distribution curve) are more favoured.