Hardy-Weinberg, natural selection and speciation

What changes allele frequencies

• Mutation creates new alleles.
• Natural selection: individuals with advantageous alleles are more likely to survive and reproduce, so those alleles become more frequent over generations.

Other factors include migration and chance (genetic drift) in small populations.

Hardy-Weinberg

For a gene with two alleles, dominant frequency p and recessive frequency q:

• p + q = 1
• p² + 2pq + q² = 1, where p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive.

The equation predicts unchanged frequencies if the population is large, mating is random, and there is no mutation, migration or selection. If observed frequencies differ from those predicted over generations, allele frequencies are changing: the population is evolving.

Usually start from q², the frequency of the recessive phenotype.

Speciation

If populations become reproductively isolated, gene flow between them stops. Isolation may be geographical (a river, mountain range or sea), ecological (different habitats), temporal (breeding at different times) or behavioural (different courtship). Each population then gains different mutations and faces different selection pressures, so allele frequencies diverge. Eventually they can no longer interbreed to produce fertile offspring: a new species has formed.

Worked example

4% of a population show a recessive phenotype. What percentage are heterozygous carriers?

1. q² = 0.04, so q = 0.2.
2. p = 1 - 0.2 = 0.8.
3. 2pq = 2 x 0.8 x 0.2 = 0.32

Answer: 32%

Check yourself

1. 9% of a population have a recessive phenotype. What percentage are homozygous dominant?
   Show answer

   49 %. q = 0.3, p = 0.7, p² = 0.49 = 49%.

2. In a population, 1 in 100 people has a recessive condition. What is the frequency of the recessive allele?
   Show answer

   0.1. q² = 0.01, so q = 0.1.

3. The frequency of the recessive allele is 0.4. What proportion of the population is heterozygous? Give your answer as a decimal.
   Show answer

   0.48. p = 0.6; 2pq = 2 x 0.6 x 0.4 = 0.48.

4. Which is a condition for Hardy-Weinberg equilibrium?
   • A small population
   • Strong natural selection
   • Random mating
   • Frequent migration
   Show answer

   Random mating. Large population, random mating, no mutation, migration or selection.

5. Observed genotype frequencies differ from those predicted by Hardy-Weinberg. What does this suggest?
   • Allele frequencies are changing: the population is evolving
   • The equation is wrong
   • There is no variation
   • The population is not reproducing
   Show answer

   Allele frequencies are changing: the population is evolving. A deviation means some factor (such as selection) is acting.

6. Why is reproductive isolation needed for speciation?
   • It increases interbreeding
   • It stops mutations happening
   • It stops gene flow, so allele frequencies in the populations can diverge
   • It makes populations identical
   Show answer

   It stops gene flow, so allele frequencies in the populations can diverge. Without gene flow, differences build up.

7. Two populations of frogs breed in different months. What type of isolation is this?
   • Temporal
   • Geographical
   • Behavioural
   • Mechanical
   Show answer

   Temporal. Different timing of breeding.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.