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

7. EVOLUTION

HARDY-WEINBERG PRINCIPLE

-  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.

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