* Evolution is about changes in gene frequencies over time. An individual's genes don't change during their lifetime. They are born with a specific set of genes, and those genes are passed on to their offspring.

* Individuals are not the unit of evolution. Individuals can change during their lifetime, but these changes are not passed on genetically to their offspring. This is known as Lamarckism, and it has been disproven.

* Natural selection acts on variations within a population. Individuals within a population have different traits due to genetic variation. Some of these traits may make an individual more likely to survive and reproduce in a particular environment. Over time, these advantageous traits become more common in the population, leading to evolutionary change.

Here's a simple analogy:

Imagine a group of people trying to cross a river. Some are good swimmers, some are not. The good swimmers are more likely to survive and reach the other side. If those swimmers are more likely to have children who also inherit their swimming skills, the next generation will have more good swimmers. Over time, the population will become better adapted to crossing rivers.

In summary:

* Evolution is about the change in the genetic makeup of a population over time.

* Individuals don't evolve; they are either born with advantageous traits or not.

* Natural selection favors individuals with traits that make them more likely to survive and reproduce, which leads to changes in the overall genetic makeup of the population.

Therefore, evolution acts on populations because it is the change in the gene pool of a population over time, driven by natural selection acting on individuals within that population.