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.
