3.1.11 (HL)—Reproductive isolation

Speciation evidence is strongest when a barrier or ecological choice reduces gene flow and divergence follows.

Geographic separation can split populations; the separation of bonobos and common chimpanzees is a geographical example. Ecological or behavioural choices can reduce mating even in one region, as host choice does in apple-maggot flies. Islands add founder effects, low immigration and distinct niches.

Apple-maggot flies mating on different host plants show reduced gene flow, but that is an early-stage case, not automatically complete speciation.

Persistent reproductive isolation and inability to produce fertile offspring, not separation alone.

An island endemic is evidence of isolation and divergence, not proof that every island lineage is a separate species.