What is the smallest biological unit that can evolve in microevolution?

Populations
Populations. A population is a group of organisms of the same species that are found in the same area and can interbreed. A population is the smallest unit that can evolve—in other words, an individual can’t evolve.

What is the smallest biological unit that can?

The cell is the smallest structural and functional unit of living organisms, which can exist on its own.