Natural selection changes populations — Science, 14–17 years
Individuals vary, and some inherited differences affect survival or reproduction in a particular environment. Across generations, those differences can become more or less common in the population.
Populations change over generations
Natural selection happens when inherited differences make some individuals more likely to survive and leave offspring in a particular environment. Their helpful variants can then become more common. The individual does not change because it needs to; the population changes as generations replace one another.
The problem of explaining adaptation
Living things often fit their surroundings: a cactus stores water, and a moth may blend into tree bark. The problem was explaining this fit without assuming that species never change or that every feature was deliberately designed. Natural selection explains it through inherited variation, competition and many generations.
Peppered moths
Imagine a moth population with 60 light moths and 40 dark moths on pale tree bark. Birds spot dark moths more easily, so after many breeding seasons the dark form may become less common. If pollution darkens the bark, the advantage can reverse. The environment changes which inherited variant is more successful.
Individuals do not evolve on purpose
A reasonable mistake is to say that an animal develops a useful feature because it wants or needs it. Organisms do respond to conditions during life, but those changes are not automatically inherited. Evolution occurs when inherited differences affect reproduction across generations, without a plan or goal.
Resistance in bacteria
A bacterial population may contain a few cells with inherited resistance to an antibiotic. The medicine kills many non-resistant cells, while resistant ones survive and reproduce, making resistance more common. This is why antibiotics should be used only when appropriate and exactly as prescribed; they do not create a need-based plan.
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