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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.

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

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Question: In a population, frequency of recessive phenotype is 0.09.

  1. 1Find q and p.
  2. 2Solution: recessive phenotype = q² = 0.09, so q = 0.3.
  3. 3Since p + q = 1, p = 0.7.

Method you can reuse

  1. 1For 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.
  2. 2For theory questions, define allele frequency change and list disturbing factors with one-line effects.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What does deviation from Hardy-Weinberg equilibrium indicate?

Think first, then show answer
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.

Common wrong answer

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

Don't mix this up

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

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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. 2

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    Evolution is understood at the population level.

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