Final answer:
Gene flow maintains genetic homogeneity across populations by allowing alleles to mix. When gene flow stops, separate populations can diverge, often leading to different evolutionary paths and potentially speciation due to environmental differences and genetic drift.
Step-by-step explanation:
When gene flow stops, allele frequencies in separate populations can diverge. Gene flow involves the movement of alleles within and between populations, mainly through migration. In a geographically continuous population, gene flow tends to maintain a homogeneous gene pool across the species' range, but when populations become geographically discontinuous, gene flow is prevented, essentially halting the mixing of genetic material.
Divergence in allele frequencies can lead to allopatric speciation, where two populations evolve distinct characteristics and potentially become distinct species due to differences in environmental conditions, natural section, and genetic drift. This differentiation is caused by new alleles arising by mutation in each population, and the genetic variability increases as these populations face different selective pressures and random genetic drift. Therefore, gene flow is vital for connecting populations and preventing quick divergence of genetic traits.