Describe Charles Darwin's theory of evolution by natural selection.

Charles Darwin (1809-1892) first proposed the idea of evolution, which says that species of living things change over time. Darwin focused on the effect that environmental conditions have on the makeup of a population and the characteristics of individuals, which we call natural selection. Natural selection is not the only mechanism of evolution; biologists today recognize that another effect, genetic drift, also results in changes in a population or species over time.
An important thing to remember about natural selection is that individuals do not adapt; a population or species does. Individuals can be more or less successful under the existing environmental conditions. "Success" in natural selection is all about reproduction. The most successful individual is the one that has the most progeny. Some individuals will not only survive, but attract mates, be fit enough to reproduce, and produce maximum numbers of progeny. Others may produce only a few progeny, may barely survive but without the surplus of energy needed to reproduce, or may die before reaching reproductive age.
Offspring of the most successful individuals will be over-represented in the next generation. Assuming that the successful parents passed on some of all of the traits that made them successful, these offspring will continue to be more successful than their contemporaries, and will in turn be over-represented in the next generation. Over succeeding generations, the population as a whole includes more and more individuals who have the "successful" traits, and, as a whole, the population comes to resemble those individuals.
Today we know a great deal about genetics, DNA, and the mechanisms by which traits are inherited, which Darwin did not. But he did know that offspring tend to resemble their parents, which was enough for him to elucidate his theory.
Two things that are necessary for natural selection are (1) diversity or variation within a population and (2) some environmental condition that favors one trait or set of traits over another.
https://www.biography.com/scientist/charles-darwin