How can geographic isolation lead to speciation?

if there's a physical barrier that prevents two populations of the same species from reproducing, then speciation will occur. Beneficial mutations, which will result in new alleles, will occur independently in each population and because of the physical barrier that prevents reproduction between these two populations, these changes won't be shared. And over time, the aculation of these mutations occurring independently in each population will eventually result in two different species. So a river that flows into two different lakes dries up, leaving the lakes geographically isolated and the fish populations (initially the same species) unable to reproduce with each other. Mutations in the fish that occur in lake A will be unable to be shared with the fish in lake B as they are reproductively isolated (due to geographic reasons). The fish in Lake A will therefore be slightly different then the fish of lake B, and eventually enough time will p for more changes to occur until they're completely different.