Definition of a Species

A species is a group of individuals who are physically able to mate with each other. This is not an exact definition since certain species can interbreed if it is absolutely necessary in the given circumstances.
A new species can emerge when two populations of a particular species change separately from one another in completely different environments. If the environment changes drastically in a specific location, then the population of a species in that location will change and adapt to the new environment. Over time, the differences between that population and another of the same species that was in a completely different environment would become large enough that the former would become a different species.