Causes of Speciation

Test your knowledge with free interactive questions on Seneca — used by over 10 million students.

Allopatric Speciation

Allopatric speciation is a type of speciation where two (or more) populations are geographically isolated from each other.

Populations of a species can split into two or more populations.
This may be caused by environmental changes (e.g. emergence of a river) or random events (e.g. a storm causing a population of birds to be carried to another island).
When two populations are physically separated, they are geographically isolated.

Populations in different environments will be exposed to different environmental conditions (e.g. climate, food availability).
The different environments will place different selection pressures on the populations.
The different selection pressures will drive natural selection in different directions.

The different selection pressures on the populations cause the allele frequencies in the two populations to change.
If the populations continue to be geographically isolated for a long time, eventually the allele frequencies will change so much that if the populations are brought back together they can no longer interbreed.

Species are defined as a group of actually or potentially interbreeding individuals.
If two populations that have been geographically isolated can no longer interbreed when they are brought back together, they are considered to be two species.
This is allopatric speciation.

Sympatric speciation is a type of speciation where two (or more) populations are not geographically isolated from each other.

In sympatric speciation, a population of species is reproductively isolated without geographical separation.
Reproductive isolation may arise where a mutation emerges in a population that prevents some members of the population from interbreeding with others.

Sympatric speciation could be caused by mutations that influence many different factors:
- Polyploidy - where a cell or organism has an extra set, or sets, of chromosomes. Polyploidy organisms are reproductively isolated from diploid organisms.
- Mutations may influence the flowering times or mating times of individuals.
- Mutations may alter the reproductive organs.
- Mutations may alter mating behaviours.

The biological definition of a species is a group of individuals that can actually or potentially interbreed.
The emergence of a mutation that prevents two (or more) populations from interbreeding is called sympatric speciation.

Speciation can also be driven by genetic drift. Genetic drift is the random change in allele frequencies in a population.

When two populations are reproductively isolated, they are exposed to different selection pressures. These selection pressures drive the evolution of the two populations in different directions. This is called disruptive selection.
Genetic drift also contributes to the speciation by causing random changes in the allele frequencies of the two populations.
Because the two populations can no longer interbreed, the gene pools will become increasingly different. This causes speciation.

Genetic drift tends to have a larger influence on evolutionary change in smaller populations.
This is because the gene pool (the sum of all the genes of a population) is smaller.
If the gene pool is smaller, any change in allele frequencies has a larger impact on the overall gene pool.