Final answer:
When members from separated populations do not reproduce with each other, they eventually become separate species through allopatric speciation, evolving genetic differences that prevent interbreeding with the original species.
Step-by-step explanation:
When members from separated populations do not reproduce with each other, the outcome is C) They become separate species. This process is known as allopatric speciation. Over time, due to differences in natural selection, mutation, and genetic drift, the populations acquire distinct genetic identities. If they remain isolated long enough, they may evolve such significant genetic differences that they can no longer interbreed successfully with members of the original species, confirming their status as separate species.