What are the three isolation barriers by which speciation can occur?

Speciation is a complex topic, and indeed what defines a species is still debated today. However, one factor is key to the process of speciation; gene flow. Gene flow between populations of a species ensures that alleles are shared among the sub populations and thus homogenises the meta population. Speciation occurs when barriers are in place to prevent gene flow. The three barriers are:

- Geographic isolation: separation of population by natural features, such as a mountain, river etc.

- Ecological isolation: separation by ecological niche, i.e. pH levels, salinity and breeding locations can differ between populations even though there is no physical (geographic) barrier preventing gene flow.  

- Reproductive isoltation: The two forms are post and pre zygotic barriers. Pre zygotic barriers prevent the physical mating of two individuals, e.g. snails have two form left and right spiral shells, meaning mating between the two is not possible. Post zygotic barriers act after mating to prevent development of the fetus or hinder the fitness of the offspring, e.g. The Mule is a cross between a horse and a donkey, and is itself sterile (unable to mate).