Types of Speciation (AQA A Level Biology): Revision Note

Reproductive separation

• In order for evolution to occur the new species population must be genetically and reproductively separated from the pre-existing species population

  • When this happens, there can no longer be an exchange of genes between the two populations

• Reproductive isolation occurs when changes in alleles or phenotypes prevent certain individuals from breeding successfully with others in the same population.

• These changes reduce gene flow and can lead to speciation.

• Examples include:

  • Seasonal isolation: individuals reproduce at different times of year

  • Mechanical isolation: changes in genitalia prevent successful mating

  • Behavioural isolation: changes in courtship behaviour prevent the attraction of mates

Genetic isolation

• When two populations become reproductively isolated, they stop exchanging genes

• As a result, allele frequencies change independently in each group over time

• If these genetic differences accumulate to the point where individuals can no longer interbreed and produce fertile offspring

• This means that speciation has occurred

Allopatric & sympatric speciation

Allopatric speciation

• Allopatric speciation happens when a population is split by a geographical barrier

• Barriers can be natural (e.g. mountains, rivers) or man-made (e.g. roads)

• The separated groups are reproductively isolated, so no gene flow occurs

• Different selection pressures act on each group, changing allele frequencies through natural selection

• Over time, these changes lead to differences in phenotypes, including physical, behavioural and physiological traits

• Eventually, the two populations become genetically distinct and can no longer interbreed – they are now separate species

Example of allopatric speciation in trees

• A population of trees is split by the formation of a new mountain range

• The two groups can no longer interbreed

• Each group faces different environmental conditions

• Over many generations, they evolve into two separate species

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

Sympatric Speciation

• Sympatric speciation happens without a geographical barrier

• The population lives in the same area, but splits into two groups with no gene flow between them

• This separation is caused by:

  • Ecological separation – groups live in different environments within the same area (e.g. soil pH affects flowering time in plants)

  • Behavioural separation – groups develop different behaviours (e.g. courtship, feeding, or communication), preventing mating

Example of sympatric speciation in fish

• A single fish species lives in a lake

• Some fish feed at the bottom, others stay in the open water

• These groups live in different environments and show different feeding behaviours

• Over time, selection pressures favour different traits:

  • Long jaws for bottom-feeders

  • Short jaws for mid-water feeders

• The groups also evolve different courtship behaviours

• Eventually, they can no longer interbreed – they are now separate species

Genetic drift

• Genetic drift is when chance (instead of environmental selection pressures) affects which individuals in a population survive, breed and pass on their alleles

  • For example, when a population is significantly small, chance can affect which alleles get passed onto the next generation

  • Over time, some alleles can be lost or favoured purely by chance

  • When there is a gradual change in allele frequencies in a small population due to chance and not natural selection, genetic drift is occurring

• In large populations, genetic drift is less likely to have an effect because any chance variations in allele frequencies usually even out across the whole population

  • Natural selection has a much more influential role in the evolution of large populations

Example of Genetic Drift in Plants

• In a small population of plants growing near a playground with a rubber floor:

  • Half of the plants have alleles for blue flowers

  • Half of the plants have alleles for pink flowers

• By chance, most pink-flowered seeds land on rubber and fail to germinate

  Blue-flowered seeds land on fertile soil and grow

• Over generations, pink alleles may disappear due to this random event, not selection

An example of how genetic drift can occur