Reproductive isolation occurs when barriers prevent two populations from interbreeding and producing fertile offspring. It reduces or stops gene flow, allowing the populations to evolve independently and potentially become separate species.
This concept is part of A4.1 Evolution and speciation and is assessed at both SL and HL, with additional mechanisms studied at HL.
When members of a population normally reproduce, their alleles are exchanged within a shared gene pool. If a barrier separates the population, alleles are no longer regularly exchanged between the separated groups.
Different environments may then impose different selection pressures. Natural selection favours different heritable traits in each population, while mutations introduce new alleles. Over many generations, the populations diverge genetically and phenotypically.
If the differences become large enough that individuals from the populations cannot produce fertile offspring together, reproductive isolation is complete. Under the biological species concept, they are then considered different species.
| Isolating mechanism | Effect |
|---|---|
| Geographical isolation | A physical barrier, such as a river, prevents interbreeding. For example, the Congo River separated populations that diverged into bonobos and common chimpanzees. |
| Courtship differences (HL) | Different courtship behaviours prevent mating between members of different populations. |
| Hybrid sterility (HL) | Mating may occur, but the hybrid offspring cannot reproduce. A mule is a sterile hybrid. |
When geographical isolation leads to divergence and reproductive isolation, the process is called allopatric speciation. Isolation alone does not automatically create a new species: genetic divergence must occur and reproductive isolation must persist.
A common misconception is that individuals deliberately adapt after becoming isolated. In fact, mutations are random, and natural selection changes the heritable characteristics of populations across generations.
Exam technique: For an “explain” question, link the stages clearly: barrier prevents gene flow → different selection pressures cause divergence → reproductive isolation develops → speciation occurs. Do not state that geographical separation by itself proves that two populations are different species.