The definition

Biological evolution, simply put, is descent with inherited modification. This definition encompasses everything from small-scale evolution (for example, changes in the frequency of different gene versions in a population from one generation to the next) to large-scale evolution (for example, the descent of different species from a shared ancestor over many generations). Evolution helps us to understand the living world around us, as well as its history.

The explanation

Biological evolution is not simply a matter of change over time. Many things change over time: caterpillars turn into moths, trees lose and regrow their leaves, mountain ranges rise and erode, but they aren’t examples of biological evolution because they don’t involve descent with inherited modifications.

All life on Earth shares a common ancestor, just as you and your cousins share a common grandmother. Through the process of descent with modification, this common ancestor gave rise to the diverse species that we see documented in the fossil record and around us today. Evolution means that we’re all distant cousins: humans and oak trees, hummingbirds and whales.