Hardy-Weinberg, Selection, Drift and Speciation: Quick Revision

A brief note on Hardy-Weinberg, Selection, Drift and Speciation. Read it before a test. Try each question at the end before you open its answer.

Hardy-Weinberg

• Allele frequencies: $p + q = 1$. Count gene copies, two for each individual.
• Genotypes at equilibrium: $p^2 + 2pq + q^2 = 1$. $AA$ is $p^2$, $Aa$ is $2pq$, $aa$ is $q^2$.
• Principle: allele frequencies stay constant from generation to generation. The gene pool is constant (genetic equilibrium).
• Five factors that disturb it: gene migration (gene flow), genetic drift, mutation, genetic recombination, natural selection.
• Evolution: a change in allele frequency from one generation to the next.
• Start from $q^2$: the recessive trait shows its genotype. Take the square root to get $q$. Carriers are $2pq$.

Types of natural selection

• Stabilising: more individuals acquire the mean value. The curve narrows.
• Directional: more individuals acquire a value on one side of the mean. The curve moves.
• Disruptive: more individuals acquire values at both ends. The curve gets two peaks.
• Lethal recessive (A-Level and IB): $q' = q \div (1 + q)$. Slow when the allele is rare, because carriers hide it.

Drift, flow and speciation

• Genetic drift: chance change of allele frequency, strongest in small populations. Alleles can be lost or fixed.
• Founder effect: a small group starts a new population with a chance sample of the alleles.
• Gene flow: $p' = (1 - m)p + mp_m$. It makes populations alike.
• Mutation: the source of new alleles. De Vries: mutation theory, large and random, saltation. Darwin: small and directional variations.
• Modern synthetic theory: variation, natural selection, drift, gene flow and reproductive isolation.
• Speciation (A-Level and IB): allopatric with a barrier, sympatric without one.

A worked example

A recessive condition affects 1 person in 400 in a population at equilibrium. What percentage of people are carriers?

1. $q^2 = \tfrac{1}{400}$, so $q = \tfrac{1}{20} = 0.05$ and $p = 0.95$.
2. Carriers: $2pq = 2 \times 0.95 \times 0.05 = 0.095$, which is $9.5$ per cent.

Practice with new numbers

A program to try

The program prints the three genotype frequencies for a few values of $q$.

Mistakes to avoid

• Counting individuals instead of gene copies. Each individual has two copies.
• Using $q^2$ as $q$. Take the square root.
• Forgetting the 2 in $2pq$.
• Saying that drift favours the fittest. It is random.

Check yourself

1. What are the five factors that disturb the Hardy-Weinberg equilibrium?

2. Which type of selection moves the whole curve to one side?

3. What is the founder effect?

4. If $q = 0.3$, what is $p$?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 19.