What must be true for natural selection to cause changes in a population over time?

What must be true for natural selection to cause changes in a population over time?

• A. Organisms must mutate in every generation
• B. There must be heritable variation
• C. The environment must be constant
• D. All individuals must reproduce equally

Answer: (B)

Natural selection can change a population only when there are heritable differences among individuals that affect survival or reproduction. Those differences must be passed from parents to offspring, so the advantageous traits become more common over generations. Variation that is not heritable won’t be inherited, so even if some individuals fare better in a given environment, the trait won’t accumulate in the population. Mutations in every generation aren’t required for the process to occur; existing heritable variation provides the material for selection, and new mutations can add more variation over time, but they aren’t a necessity in every generation. The environment does not have to stay the same for natural selection to act—changing environments can create shifting selective pressures. And if everyone reproduced equally, there would be no differential fitness to drive a change in trait frequencies. The key point is: there must be heritable variation for natural selection to cause changes over time.

Natural selection can change a population only when there are heritable differences among individuals that affect survival or reproduction. Those differences must be passed from parents to offspring, so the advantageous traits become more common over generations. Variation that is not heritable won’t be inherited, so even if some individuals fare better in a given environment, the trait won’t accumulate in the population.

Mutations in every generation aren’t required for the process to occur; existing heritable variation provides the material for selection, and new mutations can add more variation over time, but they aren’t a necessity in every generation. The environment does not have to stay the same for natural selection to act—changing environments can create shifting selective pressures. And if everyone reproduced equally, there would be no differential fitness to drive a change in trait frequencies.

The key point is: there must be heritable variation for natural selection to cause changes over time.