Evolution and speciation

Allele frequencies and natural selection

The gene pool is all the alleles in a population. Evolution is a change in allele frequency in a population over time.

• Mutation creates new alleles at random.
• Natural selection: a selection pressure (such as a change in climate, a new predator or disease) means individuals with an advantageous allele are more likely to survive and reproduce, passing the allele on. Its frequency rises over generations.

Isolation and gene flow

A species is a group of organisms that can interbreed to produce fertile offspring.

Speciation needs reproductive isolation, which reduces or stops gene flow between populations. Each population then gains different mutations and experiences different selection pressures (and chance), so their gene pools diverge.

• Prezygotic barriers: different habitats, breeding seasons (temporal), courtship behaviours (behavioural), incompatible genitals (mechanical) or gametes that cannot fuse.
• Postzygotic barriers: hybrids die early or are sterile (a mule, from a horse and a donkey, is sterile).

Allopatric and sympatric speciation

• Allopatric speciation: populations are separated geographically, for example by a mountain range, river, sea or a change in land use. Gene flow stops because they cannot meet.
• Sympatric speciation: populations live in the same area but become reproductively isolated, for example by using different food plants, breeding at different times, behavioural differences, or a mutation such as polyploidy in plants that stops them breeding with the parent population.

Worked example

In a population of 100 snails, 50 are AA, 40 are Aa and 10 are aa. Calculate the frequency of allele a.

1. Total alleles = 100 x 2 = 200
2. a alleles = 40 (from Aa) + 2 x 10 (from aa) = 60
3. Frequency = 60 / 200 = 0.3

Answer: 0.3

Check yourself

1. In 200 beetles, 72 are BB, 96 are Bb and 32 are bb. Calculate the frequency of allele b.
   Show answer

   0.40. b alleles = 96 + 64 = 160; total 400; 160 / 400 = 0.4.

2. What is evolution, in terms of populations?
   • A change in one individual during its life
   • The production of more offspring than can survive
   • Any mutation in a single cell
   • A change in allele frequency in a population over time
   Show answer

   A change in allele frequency in a population over time. Evolution acts on populations through changing allele frequencies.

3. A river changes course and splits a population of mice into two. What type of speciation could follow?
   • Sympatric speciation
   • Allopatric speciation
   • Artificial selection
   • Polyploid speciation in the same area
   Show answer

   Allopatric speciation. Geographical separation means allopatric speciation.

4. Two groups of insects in the same meadow begin feeding and mating on different plant species and stop interbreeding. This is an example of
   • allopatric speciation
   • geographical isolation
   • sympatric speciation
   • hybrid sterility
   Show answer

   sympatric speciation. They share an area but are isolated by behaviour and ecology.

5. Why does reproductive isolation lead to speciation?
   • It stops gene flow, so the gene pools of the populations diverge
   • It makes all individuals identical
   • It increases interbreeding
   • It stops mutations happening
   Show answer

   It stops gene flow, so the gene pools of the populations diverge. Without gene flow, differences accumulate.

6. Horses and donkeys produce mules, which are sterile. What does this show?
   • Horses and donkeys are the same species
   • Mules are a new species
   • Horses and donkeys are separate species
   • Hybrids are always fertile
   Show answer

   Horses and donkeys are separate species. A species produces fertile offspring; sterile hybrids show separate species.

7. Which is a selection pressure that could result from climate change?
   • Higher average temperatures in a habitat
   • A random mutation
   • Random mating
   • A large population size
   Show answer

   Higher average temperatures in a habitat. A changed environment favours different alleles.

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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.