What are the reasons for the difference in the gene pools of isolated populations of the same species?

Isolation – the emergence of any barriers that violate free crossing, which leads to an increase and consolidation of differences between populations and individual parts of the entire population. Distinguish between geographical, environmental, and ethological isolation.
Geographical (or spatial) isolation is associated with the rupture of a single range of a species into non-communicating parts. Mutations can occur randomly in each isolated population. Due to the drift of genes and the effect of natural selection, the genotypic composition of isolated populations varies more and more. The reasons leading to the emergence of geographical isolation are numerous: the presence of mountains and rivers, isthmuses or straits, the extermination of populations in certain areas, etc.
Ecological isolation is associated with preference for a particular habitat. Sevan trout is an example of such isolation. Different populations of trout spawn at the mouths of various streams and mountain rivers flowing into the lake, so free crossing between them is extremely difficult. Ecological isolation, thus, prevents the crossbreeding of individuals from different populations and serves, like geographic isolation, the initial stage of the divergence of populations.
The evolutionary essence of the various options for spatial and environmental isolation is the same – breaking a single gene pool of a species into two or more isolated gene pools (termination of the exchange of genetic material between them; independent occurrence of an evolutionary process in isolated parts of the species). Its end result, although with a small probability, is the formation of new species. That is why primary forms of isolation are considered as triggers of the speciation process.