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Mechanism of Evolution and Hardy-Weinberg Principle

The mechanism of evolution is the change in allele frequencies in a population over generations, and the Hardy-Weinberg principle states that allele and genotype frequencies remain constant when evolutionary forces are absent.

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Student-friendly explanation

Evolution is understood at the population level. Variations arise through mutation and recombination, while gene flow, genetic drift, and natural selection change allele frequencies. Hardy-Weinberg equilibrium gives a reference condition where no evolution is occurring. For a gene with two alleles, p and q represent allele frequencies, and p², 2pq, and q² represent genotype frequencies. The equation p² + 2pq + q² = 1 is used when mating is random and disturbing forces are absent. Any deviation from equilibrium indicates that factors such as mutation, migration, genetic drift, recombination, or natural selection are operating.

How to write this in exams

1
Start with the exact idea
The mechanism of evolution is the change in allele frequencies in a population over generations, and the Hardy-Weinberg principle states that allele and genotype frequencies remain constant when evolutionary forces are absent.
2
Then show how to use it
For numerical questions, identify whether the given value is p, q, p², 2pq, or q²; take square root if recessive phenotype is q²; use p + q = 1; calculate required genotype frequency; write the biological meaning. For theory questions, define allele frequency change and list disturbing factors with one-line effects.
3
Add one concrete example
If q = 0.2 for a recessive allele, then q² = 0.04 represents the expected frequency of homozygous recessive individuals under Hardy-Weinberg equilibrium.
4
Avoid this incomplete answer
If 9 percent show recessive phenotype, then q = 0.09 and p = 0.91.

Definition

Example

If q = 0.2 for a recessive allele, then q² = 0.04 represents the expected frequency of homozygous recessive individuals under Hardy-Weinberg equilibrium.

Rule to remember

Hardy-Weinberg equation: p² + 2pq + q² = 1, with p + q = 1. Here p and q are allele frequencies, p² and q² are homozygous genotype frequencies, and 2pq is the heterozygous genotype frequency.

Memory hook

Phenotype recessive is squared: recessive visible frequency is q², not q.

Examples and method

Worked example

Question: In a population, frequency of recessive phenotype is 0.09. Find q and p. Solution: recessive phenotype = q² = 0.09, so q = 0.3. Since p + q = 1, p = 0.7.

Method to apply

For numerical questions, identify whether the given value is p, q, p², 2pq, or q²; take square root if recessive phenotype is q²; use p + q = 1; calculate required genotype frequency; write the biological meaning. For theory questions, define allele frequency change and list disturbing factors with one-line effects.

Diagram support

A concept map is useful with central box Hardy-Weinberg equilibrium and branches for mutation, gene flow, genetic drift, recombination, and natural selection. Students should notice that these factors disturb equilibrium.

How CBSE asks it

Asked as numerical calculation using p² + 2pq + q² = 1, short answer on factors affecting equilibrium, assertion-reason on genetic drift or natural selection, and long answer on mechanisms of evolution.

Avoid common mistakes

Common confusion

Students often confuse allele frequency with genotype frequency, writing q = recessive individuals instead of q² for homozygous recessive individuals.

Common wrong answer

If 9 percent show recessive phenotype, then q = 0.09 and p = 0.91.

Exam tip

Always identify whether the question gives allele frequency or recessive phenotype frequency. If recessive phenotype is given, it usually represents q², not q.

Quick check

What does deviation from Hardy-Weinberg equilibrium indicate?

Deviation from Hardy-Weinberg equilibrium indicates that evolution is occurring in the population. It means one or more forces such as mutation, gene flow, genetic drift, recombination, or natural selection is changing allele frequencies.

Answer writing and exam use

1-mark answer

State that evolution is change in allele frequencies, write the Hardy-Weinberg equation with meanings of p and q, mention equilibrium conditions, list disturbing factors, and explain that deviation from equilibrium signals evolution.

2-mark answer

3-mark answer

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