Natural Selection: Variation, Heritability, and Population Change

Hi, I am Atlas. Let us settle what evolution actually means. Evolution is a change in the inherited traits of a population over generations. Notice the word population, not individual. A single finch does not evolve; the mix of beak shapes across the whole flock can shift over time. Natural selection needs four things working together. First, variation: individuals differ, like beaks that are wide or narrow. Second, heritability: that variation must be passed to offspring through genes. Traits an organism picks up during its own life, like a scar or a skill, are not inherited, so they cannot spread through a population. Third, differential reproductive success: in a given environment, some variants survive and reproduce more than others. If a drought leaves only large, tough seeds, wide strong beaks crack them better, so those birds raise more chicks. Fourth, time: as this pattern repeats over many generations, the helpful trait becomes more common in the population. Nothing here involves an organism deciding to change. The variation already exists by chance before the environmental pressure arrives; the environment simply favors some versions over others. If you want to check your understanding, order the steps in the activity below, then tackle the questions.