Reproductive isolation stops gene flow, allowing different selection pressures and genetic changes to drive populations along separate evolutionary paths toward speciation.
Coverage 2012–2024 · Updated 15 Jul 2026
Speciation is the splitting of one pre-existing species into two or more species; it increases the total number of species, whereas extinction decreases it.
Reduced gene flow lets separated gene pools diverge. Mutation supplies variation, differential selection changes allele frequencies under different conditions, and drift can add chance divergence until reproductive isolation prevents fertile gene flow.
Geographic isolation can begin the process. The Congo River separated populations ancestral to bonobos and common chimpanzees; different conditions on either side supported differential selection and divergence.
If two isolated populations accumulate different mating traits and eventually cannot produce fertile offspring when reunited, one ancestral species has split into two.
Geographic separation alone is not speciation, and gradual evolutionary change within one continuing lineage does not increase species number. The defining outcome is reproductive isolation after splitting.
This objective is assessed through structured response, commonly using Identify / Analyse / State.
Identify / Analyse / State / Describe / Outline / Distinguish / Explain
Build the answer around this relationship: Reproductive isolation reduces or prevents gene flow between populations.
Listing a barrier but not connecting it to reduced gene flow.
Representative question
Explain how isolation leads to speciation.
The core A4.1 answer moves from evidence to mechanism. Define evolution as heritable population change across generations. Use molecular universals and sequence differences, selective breeding, homology, and convergence as evidence for common ancestry and divergence. Then explain speciation as reduced gene flow followed by divergence of isolated gene pools through selection, mutation, and drift.
