Geographic isolation of two populations of a species tends to increase differences between their gene pools because it reduces gene flow between the populations. Here's why:

* Gene Flow: Gene flow is the transfer of genetic material from one population to another. It happens when individuals migrate between populations and breed.

* Isolation Prevents Mixing: Geographic isolation physically separates populations, preventing individuals from easily moving between them. This means there's less opportunity for gene flow.

* Independent Evolution: With reduced gene flow, the isolated populations begin to evolve independently. They may experience different mutations, selection pressures (environmental factors), and genetic drift (random changes in allele frequencies), leading to divergence in their gene pools.

In short: Isolation acts as a barrier to gene flow, allowing the isolated populations to evolve along different paths, leading to greater differences in their genetic makeup over time.