Here's why:

* Evolution of an individual: Individuals do not evolve in their lifetime. Their genetic makeup is largely fixed at birth. While environmental factors might cause changes in an individual's phenotype (observable traits), these changes are not passed on to offspring.

* Evolution of a species: Evolution occurs over generations within a species. Genetic mutations occur randomly and are passed on to offspring. Natural selection then acts upon these variations, favoring traits that increase survival and reproduction. Over time, these beneficial traits become more common in the population, leading to a change in the overall genetic makeup of the species.

Therefore, while individuals can change during their lifetime, it's the accumulation of changes over generations within a species that defines evolution.