Darwin's explanation for evolution was dependent on which scientific concept?

Darwin's explanation for evolution fundamentally relies on the existence of variations within a population. He observed that individuals within a species have differences, or variations, in their traits. These variations can affect their survival and reproduction. In the natural world, some traits make individuals better suited to their environment, allowing them to survive longer and produce more offspring. This concept of variation is crucial because it provides the raw material for natural selection. Without variation, there would be no differences for natural selection to act upon, and thus no evolution could occur. This understanding of variation paved the way for the theory of natural selection, which is a cornerstone of Darwinian evolution.