Groups of individuals which can interbreed and produce fertile offspring. Trilobite eyes were the most advanced ever produced For reasons unrelated to the eye, trilobites went extinct hundreds of millions of years ago.
Natural selection can only work with what's available.
Members of a species will live across a geographic range. The largest group will live in the main part of the range, where conditions are most favorable. Small groups will live along the margins of the range, under stressed conditions. If a marginal group becomes reproductively isolated from the main group by some change in the environment, then local selective pressures can gradually shift the predominant genotype. Change occurs gradually in the stressed marginal populations, while the main population doesn't change.