Evolution - Notes | Class 12 | Part 4: Hardy-Weinberg Principle

Evolution: Hardy-Weinberg Principle

Hardy-Weinberg Principle

It states that allele frequencies in a population are stable and constant from generation to generation in the absence of disturbing factors.
The gene pool (total genes and their alleles in a population) remains 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.

Gene migration: Gene flow from one population to another. Here gene frequencies change in both populations. Gene flow occurs if migration happens multiple times.
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.
Mutation: It results in the formation of new phenotypes. Over a few generations, this leads to speciation.
Genetic recombination: Reshuffling of gene combinations during crossing over, resulting in genetic variation.
Natural selection: It is of 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 favored.
- Disruptive selection: Individuals of both extremes (peripheral character value at both ends of the distribution curve) are more favored.

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