Evolution and ecology

This topic zooms out from single organisms to whole populations and ecosystems. It starts with natural selection, one of the most misunderstood ideas in school biology. Individuals do not change because they need to. Instead, variation already exists, some variants survive and reproduce more, and the population shifts over many generations. Writing answers that say an animal adapted on purpose is the slip examiners look for. The second page covers food chains and energy flow. Arrows point in the direction energy moves, from the food to the eater, and only about ten percent of the energy passes to each level, which is why food chains rarely have more than four or five links. Both pages reward careful wording, so pay attention to the exact phrasing in the worked examples. For extra practice, take any animal you know and describe, in three sentences, how a population of it might change if its environment slowly got colder over many generations.