Why are populations, rather than individuals, the units of evolution?

The units of evolution are populations and not individuals because a variation or a mutation in an individual is not sure to last or produce offsprings having the same variation or mutation. The proof that a species has truly evolved is when the variation has allowed the species to survive its environment and produce generations carrying the same variation.

I’m not exactly sure what your trying to ask, but if it’s how an organelle is similar to a body system or a part a nucleus is like a in function. but i don’t fully understand what you’re trying to ask