Why is it that only populations evolve not individuals?

Populations evolve as changes in genes are passed down from parent to offspring. When a genetic change is passed down, it is there with the offspring organism from the start of its life and can affect how it develops. The organism with the altered genes can then pass those changes down to its own offspring, and thus the change can affect a population over the course of generations. So evolution occurs not by individuals changing, but from each new generation being slightly different from the previous one.

An individual organism keeps the same set of genes it is born with through its entire life.