8876 / 2027

Lesson 3 of 5 / Variation, selection and species

Read a change across generations

Does a higher allele frequency prove why it changed?

I can explain evolution through natural selection and distinguish a population change from evidence for its cause.

  • I can explain the importance of variation from mutation, meiosis and sexual reproduction in selection.Syllabus

    Syllabus 8876, Core 4(a). Explain the importance of variation from mutation, meiosis and sexual reproduction in selection.

  • I can explain environmental selection pressures using examples.Syllabus

    Syllabus 8876, Core 4(b). Explain environmental selection pressures using examples.

  • I can explain how natural selection contributes to evolution.Syllabus

    Syllabus 8876, Core 4(c). Explain how natural selection contributes to evolution.

About 5 min

Make a guess. You are not marked.

In a population, 80% show the dominant phenotype. Is the dominant allele frequency 0.8?

  1. Phenotype frequency equals allele frequency.
  2. Aa carriers hold recessive alleles, so it is lower.
  3. Dominant alleles are commoner, so it is higher.
Show the answer

Aa carriers hold recessive alleles, so it is lower.

Dominant phenotypes include AA and Aa. Here q squared is 0.2, so q is about 0.45 and p about 0.55, not 0.8.

The key idea

Natural selection can change allele frequencies over generations. A frequency change documents population change, while a causal explanation also needs evidence of inherited differences and unequal reproductive success.

Explore the idea

Count copies, not just visible individuals

100 diploid individualsAA20Aa60aa20A copies = 20 x 2 + 60100 / 200 = 50% A

AA contributes two A copies, Aa contributes one, and aa contributes none. The 100 individuals carry 200 allele copies at this locus.

Original teaching data. Green marks A copies and purple marks a copies; the bar does not show phenotype frequency.

Explanation

Evolution can be described as a change in inherited characteristics of populations across generations; changes in allele frequencies provide one way of measuring it. An individual grows, develops or responds to its environment, but does not evolve a population-wide allele frequency during its own lifetime.

Natural selection links inherited variation to differential reproductive success. When a variant helps its carriers leave more descendants in a particular environment, its alleles can increase in frequency. Repeated selection over generations can produce a population better suited to that environment.

An allele frequency counts copies of an allele, not just individuals showing a dominant phenotype. In a diploid population with 20 AA, 60 Aa and 20 aa individuals, there are 100 A copies among 200 allele copies, so the A frequency is 50%. The Aa individuals each contribute one A and one a.

A before-and-after difference is not, on its own, proof that natural selection caused it. Sampling, migration and chance can also affect measured frequencies. A strong selection account identifies an environmental pressure and evidence that an inherited difference changed reproductive success. No Hardy-Weinberg calculation is needed for this H1 explanation.

Step by step
  1. 1

    Count allele copies

    Two copies per diploid individual at this locus; heterozygotes contribute one of each.

  2. 2

    Compare generations

    Use frequencies rather than confusing allele counts with phenotype percentages.

  3. 3

    Evaluate the explanation

    Ask what evidence links the pressure to unequal reproduction.

Worked example

Calculate a model frequency change

An original population model changes from 20 AA, 60 Aa, 20 aa to 40 AA, 40 Aa, 20 aa. Calculate A frequency in each generation.

One way to explain it

Initially, A copies = 2(20) + 60 = 100 out of 200, or 50%. Later, A copies = 2(40) + 40 = 120 out of 200, or 60%. This is a 10 percentage-point increase. It does not alone identify the cause.

Why this answer works
  • Count both A copies in AA and one in Aa.
  • Divide by total allele copies in each generation.
  • Separate the measured change from the proposed mechanism.

Watch out for this

A student says: "If a dominant phenotype occurs in 80% of individuals, the dominant allele frequency must be 80%." What is wrong with this?

Show the answer

Dominant phenotypes may include both homozygotes and heterozygotes. Allele frequency requires allele counts or sufficient genotype information.

Check your understanding

What additional observation most directly supports selection as the cause of a frequency rise?

  1. The frequency rose after immigration, but reproductive success was not measured.
  2. Carriers of the inherited variant consistently leave more offspring under the measured pressure.
  3. The final generation was measured on its own, without a comparison population or earlier sample.
Show the answer

Carriers of the inherited variant consistently leave more offspring under the measured pressure.

This supplies the differential-reproduction link.

Quick recall

Card 1 of 3. Answer in your head, then check.

Does a higher allele frequency prove why it changed?

You can return to this lesson any time.