Speciation

The evolution of a new species from excisting species.

Two populations of the same species are seperated in some way.

This stops the gene flow between them

The enviromental conditions which the two populations experience may differ causing different selection pressures between them. 

The type and frequency of alleles change 

Eventually, the two gene pools will change that much, they are no longer able to breed to form fertile offspring.

Therefore, they have became two different species.

This is one way in which the populations may be seperated.

Barriers such as oceans, rivers, mounatis and deserts provide a physical barrier which prevent the two populations from breeding.

The barriers which effect one species may not effect other species, e.g. oceans would be a barrier to mammals which live on the land but not to fish,