The Hardy-Weinberg Principle and Allele Frequencies
In a population that meets a short list of conditions, allele and genotype frequencies stay put from one generation to the next. A departure from that baseline is evidence that one or more evolutionary forces may be acting.
What a learner can do afterwards
- Calculates allele frequencies from genotype counts.
- Lists the conditions the model assumes and names a real population that breaks one of them.
- Explains why a change in allele frequencies across generations is evidence of evolution and may point to selection or another force.
1 · Read
In equilibrium, p and q stay the same from one generation to the next. This baseline model expects no mutation, random mating, no gene flow, a very large population, and no selection.
Equilibrium keeps allele frequencies unchanged.
The model assumes no mutation, random mating, no gene flow, a very large population, and no selection. A small inbred group breaks the size and random-mating conditions. The Afrikaner population of South Africa is a real case: it grew from a small group of founders, so it broke the very-large-population condition, and alleles carried by a few founders became unusually common.
To find an allele frequency from genotype counts, count copies. AA gives two A copies, Aa gives one, and aa gives none. Divide A copies by all copies at the gene.
For 30 AA, 40 Aa, and 30 aa, there are 60 plus 40, or 100 A copies. There are 200 total copies, so p is 100 divided by 200, or 0.5.
If the condition is fully recessive and the population follows the model, affected people are aa. Four affected people in 100 gives q² = 0.04, q = 0.2, p = 0.8, and carrier frequency 2pq = 0.32.
A lasting frequency change means the population has evolved at that gene. The cause might be selection, drift, mutation, migration, or nonrandom mating. More evidence is needed before naming one force.
Hardy-Weinberg gives a still baseline. Count allele copies, use q squared carefully, and treat a frequency change as evidence of evolution rather than proof of one cause.
Original lesson, assessment, and flashcard text authored for this topic in the Open Source Taxonomy project.
The rewritten readings were written for this dataset. Other readings are adapted from the Khan Academy articles Allele frequency & the gene pool and Learn: Mechanisms of evolution. The Hardy-Weinberg Tutorial (via Canvas LMS) is described from its OER Commons listing. Videos are Khan Academy embeds. Publisher terms are unknown.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.