Natural selection changes populations — Biology, 14–17 years
Individuals vary, some differences are inherited, and resources are limited. When a heritable difference affects survival or reproduction, it can become more common over generations.
Populations, not individuals, evolve
Natural selection is a change in the inherited features common in a population over generations. Individuals already differ from one another. If a difference helps some survive or leave more offspring in a particular environment, the associated gene versions may become more frequent. The environment filters variation; it does not design organisms with a plan.
What problem does it explain?
Species often fit their surroundings surprisingly well, yet environments change and no individual can predict the future. Natural selection explains how useful features can spread without a designer choosing each one. The idea grew from observations of variation, fossils and living species, especially Darwin’s comparison of organisms and his study of selective breeding.
Example: moth colour
Suppose a population has 80 pale moths and 20 dark moths. On pale tree bark, birds spot 15 dark moths but only 8 pale ones, so more pale moths survive to reproduce. In the next generation, pale-colour gene versions may be more common. The birds did not choose a goal, and no moth changed colour during its lifetime.
Trap: organisms adapt because they need to
This sounds reasonable because people can train their bodies when they need to. But an individual animal does not produce a useful inherited change simply by wanting it. Random mutation and recombination create differences first; selection then changes their frequency. Practice can alter an individual, but that is not the same as inherited evolution.
Where this appears
Natural selection helps explain antibiotic resistance. In a bacterial population, a few cells may already carry a mutation that makes a drug less effective. The antibiotic kills many others, leaving those cells to reproduce; resistance can then become common. This is why completing prescribed treatments and developing new medicines matter, although no treatment removes every risk.
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