Variation is a prerequisite for natural selection to work
A note for Lesson no. 6, "Brains Make More Than One Kind of Mind," in Seven and a Half Lessons About the Brain by Lisa Feldman Barrett.
Some context from page 101 is:
One of Charles Darwin’s greatest insights was that variation is a prerequisite for natural selection to work.
Darwin’s idea about variation, known as population thinking, is one of his greatest innovations, according to the evolutionary biologist Ernst Mayr.
Additionally, degeneracy (lesson no. 1) provides a hidden reservoir of variation for natural selection to act on and contributes to the evolvability of our species.
More on population thinking
This lecture, which I presented at the American Association for the Advancement of Science, explains the basics of population thinking and its application to scientific research.