A definition of a species is said to be two individuals that can interbreed to produce fertile offspring or exchange genes. Speciation is the process by which two individuals of the same species can become so evolutionary divergent they can no longer reproduce to produce these fertile offspring.
Geographical isolation is a simple example of a situation in which speciation might occur due to the changing of land formations that physically prevent two groups of the same species from interbreeding. These isolated groups of species are exposed to different environments and two completely different phenotypes are selected for in each new area. Thus the process of selection drives the differentiation of each species until they can no longer interbreed to produce fertile offspring.
- Initially, one group of the same species exist in the same environment each indivudual being exposed to the same environmental features (climate, pH ect...)
- The species become seperation by a physical means, this could be any number of geographical changes that would prevent two groups of species being reintroduced to each other such as a river. For clarity, the two new areas containing the same species will be known as Area A and Area B.
- A change is climate is observed in both of the areas however the changes are not similar. Area A is now a hot and dry climate whereas Area B is cold and wet.
- The new environmental features cause directional selection to occur, thus more extreme phenotypes are now selected for. Individuals in area A will be more likely to survive and reproduce if they have characteristics such as shorter fur and a large surface area: volume ratio in order to lose heat. Individuals in area B however will be more likely to survive and reproduce if they have adaptations which are focused around the conservation of heat.
- Over many generations these differing characteristics will become so prominant that the genotype of the original organism will have changed and eventually the two species will become so genetically different they can no longer reproduce to produce fertile offspring or exchange genes when they are re-introduced.