Natural Selection

Look closely at any wild population and no two members of it are quite the same. One runs a little faster. One is a shade darker. One survives a cold night that kills its neighbours. Those differences look too small to matter until you notice what the surroundings are doing with them, quietly, generation after generation: keeping some, discarding others, and never once consulting the organisms about it. Given enough generations that sorting rebuilds a species. Given a strong enough pressure it can do it inside a few years, which is why a poison that worked last decade may not work now.

This lesson takes you through the mechanism doing the rebuilding, and it is a short mechanism: four steps that fit on one line and are worth four marks when they arrive in the right order. You will see why an individual organism never evolves although a population does, why a weightlifter's children are not born strong while a beetle's colour is inherited, how a population of rats can stop responding to a poison without a single rat ever changing, and how one species split in half by a stretch of sea can drift so far apart that the two halves can no longer breed together at all. Everything in this topic is Core Tier content, which means every word of it is examinable whichever tier you sit, and the specimen papers make that point more forcefully than the specification does.