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Biology · Grades 8–12

Natural selection and population change

Avoid saying individuals evolve because they need a trait.

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Learn the idea

Variation

Individuals in a population vary, and some variation is heritable.

Selection

Differences in survival and reproduction can change inherited trait frequencies across generations.

Population

Evolution concerns population-level inherited change, not an individual choosing a new gene.

Context

A trait’s advantage depends on its environment; evolution has no required ladder toward perfection.

Worked examples

What is required for natural selection to change inherited trait frequencies?

Variation, inheritance and differential reproduction are central.

Heritable variation linked to differences in reproductive success.

Which evolves in the population-genetic sense over generations?

Evolution is a change across generations in populations.

A population's inherited composition.

Check your understanding

  1. A trait occurs in 20 of 100 organisms, then 40 of 100 in a later sample. What is the increase in percentage points?

  2. Why is “animals grew the trait because they needed it” incomplete or misleading?

  3. Is the same trait necessarily advantageous in every environment?

    1. Only for every dominant allele.
    2. Yes.
    3. No.

Answers and marking notes

Answers and marking notes
  1. 20

    40% − 20% = 20 percentage points, not a 20% relative increase.

  2. It omits pre-existing or arising variation, inheritance and differential reproduction.

    Replace need-based intent with a population mechanism.

  3. No.

    Benefits and costs depend on conditions.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.