Why is the unit of evolution considered not an individual, but a population?

First, let’s remember that a population is a collection of living organisms of one species that live in a certain territory. This is the only way the evolution of these living organisms can occur. They interact with each other. They multiply, compete with each other, and so on. This is how new features of certain representatives of the population appear. After a while, they completely change the entire population. One individual cannot be the driving force behind the evolutionary process, although changes occur first in one individual.