Speciation
Prezygotic barriers reduce mating or fertilization; postzygotic barriers reduce hybrid survival or fertility. Allopatric populations may diverge under mutation, selection, and drift when gene flow is reduced. Sympatric divergence can occur without a geographic barrier; polyploidy can create rapid reproductive isolation in plants. The biological species concept emphasizes reproduction and has limits for fossils and asexual organisms. Similar appearance is not enough to establish interbreeding.
Two frog populations breeding in different months may be isolated before fertilization. Viable but sterile hybrid offspring indicate a barrier after fertilization.
A river splitting a population is the start of a possible process, not automatic proof that two species appeared overnight.