Factors Affecting Gene Frequency

(Class 12 Biology – Chapter 24 Evolution)

Long/Short Note (4–6 Marks Question)

Gene frequency (allele frequency) is the proportion of a particular allele in the total gene pool of a population. According to Hardy-Weinberg principle, gene frequency remains constant if no evolutionary forces act. But in nature, it changes due to certain factors.

Here are the four most important factors that affect gene frequency:

1. Mutation

Definition: Sudden, permanent change in the DNA sequence that produces new alleles.
Effect: Introduces new alleles into the gene pool or changes the frequency of existing alleles.
Types: Gene mutation or chromosomal mutation.
How it affects: Even a small mutation rate can slowly change allele frequency over generations.
Example: Mutation in haemoglobin gene produces sickle cell allele (HbS), which increases in malaria-prone areas.

2. Natural Selection

Definition: Differential survival and reproduction of individuals due to differences in phenotype (Darwin’s idea).
Effect: Favours beneficial alleles and eliminates harmful ones → changes gene frequency rapidly.
Types:
- Directional selection
- Stabilizing selection
- Disruptive selection
Example: In industrial areas, dark-coloured moths (carbonaria) increased in frequency due to camouflage against pollution (industrial melanism).

3. Genetic Drift

Definition: Random fluctuation in allele frequency due to chance events (especially in small populations).
Effect: Can lead to loss or fixation of alleles; reduces genetic variation.
Two important types:
- Founder effect: Small group starts a new population (e.g., Amish population with polydactyly).
- Bottleneck effect: Population drastically reduced by disaster (e.g., cheetah population).
Example: A storm kills 90% of a small island population randomly → allele frequency changes by chance.

4. Gene Flow (Migration)

Definition: Movement of alleles between populations through migration of individuals or gametes.
Effect: Makes gene frequencies similar between populations (homogenization) or introduces new alleles.
How it affects: Prevents speciation by reducing differences between populations.
Example: Migration of people from one country to another brings new alleles (e.g., spread of sickle cell allele from Africa to other regions).

Summary Table (Draw in Exam)

Factor	Type of Change	Effect on Variation	Example
Mutation	New alleles	Increases variation	Sickle cell mutation
Natural Selection	Directional	Decreases harmful alleles	Industrial melanism
Genetic Drift	Random	Decreases variation	Founder effect in Amish
Gene Flow	Migration	Makes populations similar	Human migration

Important Exam Lines:

“These four factors disturb Hardy-Weinberg equilibrium and cause evolution.”
“Mutation is the ultimate source of variation, while natural selection is the most powerful directing force.”

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