OCR A Level Biology

Revision Notes

6.2.12 Isolation & Speciation

Isolation & Speciation

Genetic isolation

  • When two populations of the same species become reproductively isolated from each other, they can eventually become genetically isolated
    • If the two populations are no longer reproducing with each other, then they do not interchange genes with each other in the production of offspring
    • Changes that occur in the allele frequencies of each group are not shared, so they evolve independently of each other, which can lead to the formation of two populations that are no longer successfully able to interbreed
    • When the genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring, speciation has occurred
    • There are different mechanisms of speciation:
      • Allopatric and sympatric

Reproductive separation, downloadable AS & A Level Biology revision notes

Reproductive separation of two populations can lead to speciation

Allopatric and sympatric speciation

  • Evolution causes speciation: the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
  • Genetic isolation between the new population and the pre-existing species population is necessary for speciation
  • There are two different situations when speciation can take place:
    • Two populations of a species are separated by a geographical barrier and become genetically isolated from each other
    • Two populations of species are living in the same area (experiencing similar environmental selection pressures) but still become genetically isolated from each other

Allopatric speciation

  • Allopatric speciation occurs as a result of geographical isolation
  • It is the most common type of speciation
  • Allopatric speciation occurs when populations of a species become separated from each other by geographical barriers
    • The barrier could be natural like a body of water, or a mountain range
    • It can also be man-made, like a motorway
  • This creates two populations of the same species who are reproductively separated from each other, and as a result, no genetic exchange can occur between them
  • If there are sufficient selection pressures acting to change the gene pools (and allele frequencies) within both populations then eventually these populations will diverge and form separate species
    • The changes in the alleles/genes of each population will affect the phenotypes present in both populations
    • Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)

Example of allopatric speciation in trees

  • Imagine there is a population of trees that are all one species
  • A new mountain range forms that divides the population into two
  • The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them
  • The two populations experience different selection pressures
  • Over thousands of years, the divided populations form two distinct species that can no longer interbreed

Allopatric speciation in trees (1), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (2), downloadable AS & A Level Biology revision notesAllopatric speciation in trees (3), downloadable AS & A Level Biology revision notes

The natural geographical barrier of a mountain range can lead to allopatric speciation in trees

Sympatric Speciation

  • Sympatric speciation takes place with no geographical barrier
  • A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
  • Something has to happen that splits or separates the two populations:
  • This can occur via:
    •  Ecological separation
    • Behavioural separation
  • Ecological separation: Populations are separated because they live in different environments within the same area
    • For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering, so a population growing in soil with a slightly different pH may flower at a different time from another population, leading to reproductive separation (and eventually genetic isolation) of the two populations
  • Behavioural separation: Populations are separated because they have different behaviours
    • For example differences in feeding, communication or social behaviours, such as courting behaviours to attract a mate

Example of Sympatric Speciation in Fish

  • A species of fish lives in a lake
  • Some individuals within the population feed on the bottom while others remain higher up in the open water
  • The different feeding behaviours separates the population into different environments
    • Here, behavioural separation has led to ecological separation
  • The separated groups experience different selection pressures
    • Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding
  • Over time natural selection causes the populations to diverge and evolve different courtship displays
  • They can no longer interbreed; they are now considered separate species


  • Both allopatric and sympatric speciation are reliant on mutations occurring within individuals in populations
  • Without mutations, there are no new alleles of genes for selection to act on
  • The changes in genetic material caused by mutations are important as these changes are what produce the differences in physiology, behaviour and morphology between populations over many generations, eventually leading to speciation

Exam Tip

When answering exam questions on sympatric speciation, try not to confuse the factors that originally cause separation between the populations vs the factors that then prevent them from breeding after genetic isolation. For the example of the fish: the difference in feeding behaviour is what originally causes separation but it is a difference in courtship displays (which is caused by genetic isolation) that prevents them from breeding together.


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