Here's why:

* Evolution is about change in allele frequencies: Evolution is defined as a change in the genetic makeup of a population over time. Alleles are alternative forms of a gene, and their frequencies within a population are what evolve.

* Individuals don't evolve: While individuals can experience changes within their lifetimes (like developing a tan), these changes are not passed on genetically to their offspring. Evolutionary change happens across generations.

* Populations are the unit of evolution: A population is a group of interbreeding individuals of the same species. It's within this group that allele frequencies change due to factors like mutation, genetic drift, gene flow, and natural selection.

So, while individual organisms can experience changes, evolution itself is a process that occurs at the population level.