Speciation (SQA National 5 Biology): Revision Note

Isolation barriers

• Speciation can be defined as:

the development of new species from pre-existing species over time

• For speciation to occur, two populations of the same species must be isolated from each other

  • Isolation prevents gene flow between the two populations

• Isolation of populations occurs due to the presence of an isolation barrier; examples include:

  • geographical barriers: a physical separation, e.g. a mountain range, a wide river or an ocean, keeps populations apart so they cannot interbreed

  • ecological barriers: populations use different habitats or resources, e.g. one on dry heath, one in a wetland, so they rarely encounter each other to breed

  • behavioural barriers: populations have different courtship signals, e.g. calls or mating displays, so they do not recognise each other as mates

The process of speciation

• Once two populations of the same species have been isolated by a barrier, speciation may occur as follows:

  • Different mutations may arise in the isolated populations

  • Each population may face different selection pressures in their environment

  • Natural selection selects different mutations in the two populations, and genetic differences accumulate over time

• The two populations are said to have become different species at the point at which they can no longer breed together to produce fertile offspring