A mountain range rises and splits one population into two that can no longer meet. This is:

1. Speciation begins when gene flow between groups is cut off. 2. Geographical isolation is a physical barrier — a mountain, a river, an ocean — separating the groups. 3. Reproductive isolation instead means the groups can meet but cannot successfully breed. 4. Genetic isolation arises from chromosomal or genetic changes that block interbreeding. 5. Here the barrier is a landform, so the isolation is geographical, and the separated populations then diverge under their own selection pressures. _Source: NECTA ACSEE 2023 Biology 133/2, Question 5: Organic Evolution_