The Theory of Evolution by Natural Selection in One Sentence

Darwin’s beautiful idea

Charles Darwin (1809-1882)

Charles Darwin’s theory of evolution by natural selection, published in 1859, explains how all of life on Earth today evolved over the eons from less complex forms. And the essence of this grand theory can be captured in a single sentence:

Within a species there’s always slight random variation in the hereditary traits of its members, and those variations that happen to be the most conducive to survival and reproduction will naturally tend to become widespread in the species.

Evolution by natural selection is therefore the non-random ‘selection’, by nature, of such random variations, which become established in the species. And the steady accumulation of innumerable such small changes over hundreds of millions of years has led to the extraordinary complexity, and diversity, of today’s plants and animals.

The ever-present slight random variation in hereditary traits within a species  –  the fuel of natural selection  –  is ultimately due to the continuous occurrence of random gene mutations, which have several causes, including gene copying errors during reproduction.

This beautifully simple theory, so often misunderstood or misrepresented, explains how the appearance of design in plants and animals arose from a mindless process. That is, it explains not just how today’s organisms came to exist, but also why, in the sense that there’s no reason other than the laws of physics and chemistry. Evolution by natural selection is simply as natural, and inevitable, as water flowing downhill.

(Photo by Syed Irfan)

Share article on Twitter

Receive updates:

To receive updates about new content on this site, submit your email address via Substack – you don't need a Substack account.


I welcome feedback about both my writing and its content – my email is

Support my independent research and writing

The content of this article can change, and so referencing of it should state the date of reading. Also, you can save the current version in the Internet Archive, and then link to the archived copy.


This article was inspired by the book The Moral Animal, by Robert Wright, which presents the theory of evolution by natural selection in a single sentence. The idea that the inevitability of evolution by natural selection is analogous to the inevitability of water flowing downhill came from the book The Adapted Mind, by Leda Cosmides and John Tooby. My understanding of the theory of evolution by natural selection comes mainly from the books The Selfish Gene and The Blind Watchmaker, both by Richard Dawkins.