Lesson 3 of 5 / Variation, selection and species
Read a change across generations
Does a higher allele frequency prove why it changed?
In this lesson: Explain evolution through natural selection and distinguish a population change from evidence for its cause.
About 5 min
The key ideaNatural 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
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
Count allele copies
Two copies per diploid individual at this locus; heterozygotes contribute one of each.
- 2
Compare generations
Use frequencies rather than confusing allele counts with phenotype percentages.
- 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.
Is this true? "If a dominant phenotype occurs in 80% of individuals, the dominant allele frequency must be 80%."
Dominant phenotypes may include both homozygotes and heterozygotes. Allele frequency requires allele counts or sufficient genotype information.