Lesson 4 of 5 / Population genetics
Know when the model applies
Does a different genotype ratio prove natural selection?
In this lesson: Explain equilibrium assumptions and interpret departures without overclaiming.
About 6 min
The key ideaThe model assumes random mating, a very large population, no selection, no mutation and no migration at the locus being studied.
Explore the idea
Same alleles, different genotype proportions
Gamete alleles combine independently at the locus. Non-random mating can alter genotype proportions even when allele frequencies are unchanged.
The second distribution is a counterexample, not a simulation of one specified cause. Its allele counts are unchanged, but its genotype proportions do not fit the equilibrium model.
Explanation
Random mating with respect to the locus allows gamete probabilities to combine independently. A very large population makes random changes due to genetic drift negligible. The model also excludes selection between genotypes, mutation changing alleles, and migration introducing or removing alleles.
Connect each broken assumption to a mechanism. Selection changes reproductive contributions when genotypes differ in survival or reproduction. Migration changes the allele pool if incoming or outgoing individuals have different allele frequencies. In a small population, chance sampling can change frequencies even without an advantage.
Non-random mating can change genotype proportions without immediately changing allele frequencies. For example, equal numbers of AA and aa individuals give p = q = 0.5 but contain no heterozygotes. Those allele frequencies alone do not guarantee that the present individuals have the equilibrium genotype distribution.
Observed counts rarely equal theoretical expectations exactly. A difference may reflect sampling variation, measurement problems, a mixture of populations or one or more broken assumptions. A suitable statistical comparison can assess a discrepancy, but the discrepancy alone does not identify natural selection as its cause.
Use the model as a stated baseline. Calculate what is expected, compare with the evidence and explain which additional information would distinguish possibilities. A good conclusion separates the pattern you observed from the biological mechanism you propose.
Step by step
- 1
State the expectation
Use the allele frequencies to calculate expected genotype proportions.
- 2
Inspect the context
Look for evidence about mating, population size, migration, mutation or selection.
- 3
Limit the inference
Distinguish an observed discrepancy from a proven cause.
Worked example
Work through the evidence
A population has 50 AA and 50 aa adults, with no Aa adults. Show that its allele frequencies can be 0.5 and 0.5 despite the absence of heterozygotes.
One way to explain it
There are 100 A copies and 100 a copies among 200 allele copies, so p = q = 0.5. The observed genotype frequencies are 0.5, 0 and 0.5, unlike the equilibrium expectation 0.25, 0.5 and 0.25. Allele frequencies and genotype frequencies describe different quantities.
Why this answer works
- Count alleles directly from the known genotypes.
- Calculate the equilibrium comparison separately.
- Do not claim that the cause of the departure has been established.
Is this true? "Any mismatch from Hardy-Weinberg expectations proves that a beneficial allele is being selected."
Sampling variation and several population processes can create a mismatch. Evidence about survival, reproduction and competing explanations is needed before attributing it to selection.