Natural selection and evolution

Explanation

Individuals in a population vary. Some differences are heritable, arising from genetic variation. More offspring may be produced than can all survive, and organisms can compete for limited resources such as food, light, water or space.

An environmental factor can make one heritable variant more successful under the current conditions. Predators may detect some prey more easily; drought may favour plants with traits that conserve water; an antibiotic can select resistant bacteria. The advantage depends on the environment.

Individuals with an advantageous heritable variant are more likely to survive and reproduce. Their offspring can inherit the relevant alleles. Over generations, those alleles and the associated characteristics can become more common in the population. Differential reproduction matters: survival alone does not transmit the variant.

Evolution is change in the inherited characteristics of a population over generations. Natural selection is one mechanism that can produce this change. An individual becoming stronger through exercise is not itself evolution, because this does not demonstrate an inherited population change.

Selection acts on existing variation; it does not cause organisms to choose mutations that match a need. A variant advantageous in one environment may be less advantageous after conditions change. Best fitted means greater reproductive success in the relevant environment, not simply the largest or strongest organism.