Even if the environments are not very different, the populations may differentiate because different mutations and genetic combinations occur by chance in each. Thus, selection will have different raw material to act upon in each population. In short, physical isolation turns a single population into two, which, because of their lack of connectedness, may follow different evolutionary paths. The fate of the populations depends upon time and factors related to their different environments. If the two populations are soon rejoined, they may not differ very much, and likely will become a single population again.
Differentiation also depends upon the strength of selective pressures.
Strong selection can cause rapid change. Given time and selection, the two populations become two species. They may, at some later time, spread back into contact. The real test of the biological species concept is when two populations, on the threshold of becoming two species, come back into contact. They may simply merge.