Speciation and Reproductive Isolation

Under the biological species concept, two populations count as separate species once they can no longer interbreed and produce fertile, viable offspring. That separation comes from reproductive isolation, barriers that stop gene flow. Prezygotic barriers act before a zygote can even form: different mating seasons, incompatible courtship behaviors, mismatched genitalia, or habitats that never overlap. Postzygotic barriers act after fertilization happens: the hybrid embryo fails to develop, or it survives but is sterile or weak. Both kinds stop genes from one population mixing permanently into the other, but prezygotic barriers prevent the mating attempt from succeeding at all, while postzygotic barriers let mating happen and then waste the effort.

Speciation is gradual, not a single event. Two populations split by a barrier, geographic or otherwise, and stop exchanging migrants. Each population keeps mutating, drifting, and adapting to its own local pressures, so their allele frequencies diverge further every generation. Early on, gene flow between them is low but not zero; occasional migrants can still interbreed successfully. Given enough time and enough accumulated genetic difference, interbreeding stops working, or stops happening altogether, and gene flow reaches zero. At that point the split is permanent and irreversible, and the two populations are separate species.

This connects directly to everything else in natural selection: divergence only becomes speciation once isolation is complete. If two diverging populations meet again too soon, while barriers are still weak, they can interbreed and blend their gene pools back together, undoing some of the earlier divergence instead of finishing the split.