Final answer:
Among the options listed, geography is the answer that cannot function to isolate populations. Gametes, external fertilization, and timed reproduction can all function to isolate populations.
Step-by-step explanation:
Among the options listed, geography is the answer that cannot function to isolate populations. Geography refers to physical barriers or distances that separate populations, preventing gene flow and promoting speciation. On the other hand, gametes (reproductive cells), external fertilization (where gametes combine in the external environment), and timed reproduction (where individuals have different mating seasons or reproductive periods) can all function to isolate populations by limiting gene flow and promoting reproductive isolation.