Biological evolution is the change in heritable traits in populations over successive generations. The key word is populations. Individual organisms live, develop, and die, but evolution is tracked by asking how variation is distributed across a population and how that distribution changes over time.

That population-level view helps clear away several common misunderstandings. Evolution is not a story about organisms changing because they need to. It is a story about heritable variation already present or newly introduced, and about what happens to that variation when drift, selection, mutation, migration, and reproductive patterns shape later generations.

The current Notebook is built to help a reader learn that way of thinking in a manageable order. The first-ring path starts with allele-frequency change, then adds null-model reasoning, mechanism comparison, lineage splitting, and shared ancestry.

• If you are new: start with Allele Frequency Change, then continue into the current learning path.
• If you already have a question: go straight to the concept index.
• If you want to know how well grounded a page is: check Source Trails for current review status.