Here's why:

* Evolution is about changes in gene frequencies: Evolution is the process of change in the genetic makeup of a population over generations. This means the proportions of different alleles (versions of a gene) within a population change.

* Individuals don't evolve: Individuals have a fixed set of genes. While an individual might experience mutations in its lifetime, these changes are not passed on to offspring.

* Populations have gene pools: A population's gene pool is the collection of all the genes within that group. It is the gene pool that changes over time, leading to evolution.

Think of it this way: If you have a group of people (a population), some might be taller than others. If the environment favors taller individuals (perhaps for reaching food), the taller individuals might have more offspring. Over generations, the average height of the population will increase, demonstrating evolution.