Reproductive isolation prevents gene flow between populations, allowing them to evolve independently until they become separate species. In IB ESS, this mechanism is part of speciation; reproductive isolation is studied at HL only in subtopic 3.1 Biodiversity and evolution.
The Mechanism
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A population becomes divided by a geographical, ecological or behavioural barrier. Geographical isolation may occur when a mountain, river or island separates populations, while ecological or behavioural differences can isolate populations living in the same area.
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The barrier reduces or stops gene flow, meaning alleles are no longer exchanged through interbreeding. Each population therefore has an independent gene pool.
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Different environmental conditions create different selection pressures. Through natural selection, individuals with advantageous inherited characteristics survive and reproduce more successfully, changing allele frequencies in each population.
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Mutations and genetic drift can produce further genetic differences. Over many generations, the populations diverge genetically, physiologically or behaviourally.
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Eventually, they can no longer interbreed successfully to produce fertile offspring, even if the original barrier disappears. They are now reproductively isolated and are classified as separate species.
A named example is the Galápagos finches. Populations isolated on different islands experienced different food sources and selection pressures, producing differences in beak form and mating signals. These differences contributed to reproductive isolation and the evolution of multiple finch species.
A common misconception is that geographical separation automatically creates a new species. Separation begins divergence, but speciation is complete only when reproductive isolation prevents successful interbreeding.
Exam Technique
For an HL explain question, present the causal chain clearly: isolation reduces gene flow, different selection pressures cause divergence, and reproductive isolation produces separate species. Include a named example and use precise terms such as natural selection, gene pool and fertile offspring. SL students need the basic definition of speciation, but the detailed reproductive-isolation mechanism is HL-only content.
