Evolution is the gradual process of change in the genetic makeup of a population over time, driven by natural selection and other factors.

Here's why this is the best description:

* Gradual Process: Evolution happens over many generations, not overnight.

* Change in Genetic Makeup: It's the genes of a population that are altered, leading to changes in traits.

* Population: Evolution affects a group of organisms, not just an individual.

* Natural Selection: This is the key driving force, where individuals with traits better suited to their environment survive and reproduce more successfully, passing those advantageous traits to their offspring.

* Other Factors: Besides natural selection, factors like genetic drift, mutation, and gene flow can also contribute to evolutionary change.

Important to note: Evolution is a scientific theory, meaning it is a well-substantiated explanation of some aspect of the natural world, supported by a large body of evidence. It is not a belief or guess.