1. Evolution: How Populations Change

Evolution is the change in the inherited traits of a population over many generations. The key word is population — a single organism does not evolve during its lifetime. Instead, individuals with different inherited traits survive and reproduce at different rates, so the makeup of the whole population shifts over time.

Evolution depends on variation: individuals in a population are not identical. New variation ultimately arises from mutations (changes in DNA), and it gets reshuffled through reproduction. If every individual were genetically identical, there would be no differences for the environment to favor — and no evolution.

Evolution can be small or large. A population of insects becoming resistant to a pesticide over a few years is evolution. Over much longer spans, repeated change can produce entirely new species (Lesson 6).

⚠️ Common misconception: organisms don't evolve because they 'want' or 'need' to. No individual chooses to evolve. Populations change when inherited traits affect survival and reproduction across generations.

💡 Tip: individuals survive or die; populations evolve. No variation means no evolution.