Do you need geographic separation for speciation in IB Biology?

No, and IB Biology often tests this directly. Geographic separation is central to allopatric speciation, but sympatric speciation happens in the same area. The heart of the process is not distance; it’s gene flow stopping. If individuals do not interbreed because of behavior, timing, habitat preference, or chromosomal incompatibility, the populations can still diverge. Over many generations, allele frequencies shift differently, and reproductive isolation becomes stronger. In exams, the safe move is to state clearly that physical barriers are one route to isolation, not the definition of speciation.

Why is gene flow so important in speciation questions?

Because gene flow is what keeps populations genetically similar. In IB Biology terms, it mixes alleles between populations and resists divergence. If there is frequent interbreeding, selection in one area can be “diluted” by alleles arriving from another population. That’s why reproductive isolation is a turning point: it allows mutation, drift, and selection to accumulate differences without being reset by mating. Many markschemes reward the explicit phrase “reduced gene flow leads to divergence in allele frequencies.” If you include that idea, your explanation usually lands.

How do I tell whether a barrier is prezygotic or postzygotic?

Use the timing test: ask whether fertilisation happens. If mating doesn’t occur, fertilisation is prevented, or gametes can’t fuse, it’s prezygotic. If a zygote forms but the hybrid fails to survive, is sterile, or produces weaker later generations, it’s postzygotic. IB Biology questions sometimes hide this in a scenario, so underline clues like “mating call,” “breeding season,” or “pollen tube fails” for prezygotic barriers. For postzygotic barriers, look for “hybrid offspring,” “reduced fertility,” or “breakdown in F2 generation.” Practising a few data-response items in the Questionbank helps you recognise these patterns quickly under time pressure.