AP Biology · Unit 7 · Topic 7.2
Selection acts on what an organism is, not on what it carries. That one restriction explains why a harmful recessive allele can never quite be driven out, and the model says how much survives.
Learning objective 7.2.A Describe the importance of phenotypic variation in a population, and 7.2.B explain how variation in molecules within cells connects to fitness.
Topic 7.1 ran a haploid population, where every individual wears its allele on the outside. Here the identical harmful allele faces the identical selection in a diploid population, where it can hide.
Selection can only act on an allele when it shows. Follow the last column down as the allele gets rarer.
A genetic disease caused by a recessive allele cannot be removed from a population by selection against affected individuals, however severe the condition. At a frequency of one in a thousand, 99.9% of the copies are in healthy carriers who are unaffected and unselected. This is not a limitation of medicine or of time — it is arithmetic, and it is why carrier screening exists and why eugenic programs aimed at eliminating recessive conditions were doomed on their own terms as well as being monstrous.
Give the heterozygote the highest fitness and something odd happens: the allele stops going anywhere. Start it high or start it low — it arrives at the same place.
When the allele is rare, almost every copy is in a heterozygote — the fittest genotype — so it rises. As it becomes common, homozygotes appear and start being removed, so it falls. There is exactly one frequency where those two pressures cancel, and the population goes there from wherever it starts. That is what a stable equilibrium is, and the tool reaches it from both directions.
This model explains why an allele that is severely harmful in homozygotes can nevertheless be common in a population — something a simple “harmful alleles get removed” account cannot do at all. It does not explain why the heterozygote is fitter; that comes from what the molecule does, which is Topic 7.2's second learning objective and the next tab.
Note also what the model does not require. There is no foresight, no benefit to the species, and nothing being preserved for a purpose. Every generation, selection simply favors the genotypes that leave more offspring, and the equilibrium falls out of the arithmetic.
The same three genotypes, the same starting frequency, two environments. Only the surroundings differ.
A population carries several versions of an enzyme. A new challenge arrives, and each version has only a small chance of coping with it. Pick how many versions the population happens to hold.
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