Hardy–Weinberg Equilibrium Requires Specific Conditions to Prevent Evolution
- The Hardy–Weinberg principle predicts that allele and genotype frequencies will remain constant across generations but only if specific conditions are met.
- Any violation of these conditions leads to evolutionary change.
- Think of the Hardy–Weinberg equilibrium as a snapshot of a population’s genetics.
- If the population changes, it’s like seeing movement in what should be a still photo
Five Conditions Must Be Met for Genetic Stability
- A population remains in genetic equilibrium only if the following five conditions are maintained:
- Mutations Do Not Occur: No new alleles are introduced, and existing alleles remain unchanged.
- Mating Is Completely Random: Individuals do not choose mates based on phenotype.
- No Gene Flow Alters the Population: Immigration or emigration does not bring in or remove alleles.
- Population Size Is Large Enough to Prevent Drift: Random changes (genetic drift) in allele frequencies are minimized.
- Natural Selection Does Not Favor Any Phenotype: All individuals have an equal chance of surviving and reproducing.
These conditions are theoretical; in real-world populations, deviations are common, and evolution is inevitable.
Genotypic Frequencies Reveal If a Population Is in Equilibrium
- If observed genotypic frequencies match predictions from the Hardy–Weinberg equation ($p^2 + 2pq + q^2 = 1$), the population is in equilibrium.
- If frequencies deviate from expectations, evolutionary forces such as selection, mutation, or gene flow are influencing the population.
Use $p + q = 1$ to quickly solve for the missing allele frequency.
Violations of Conditions Indicate Evolution
- When any of the five conditions are not met, the population evolves:
- Mutations: Introduce new alleles, altering allele frequencies.
- Non-Random Mating: Increases the prevalence of certain genotypes.
- Gene Flow: Adds or removes alleles through migration.
- Small Populations: Genetic drift causes random fluctuations in allele frequencies.
- Natural Selection: Favored phenotypes contribute more alleles to the next generation, driving change.
Hardy–Weinberg Predictions Help Detect Evolutionary Forces
- The Hardy–Weinberg principle provides a baseline for comparing observed and expected frequencies.
- Deviations from equilibrium highlight which evolutionary forces such as selection or drift are acting on a population.
To determine if a population is evolving, calculate allele and genotype frequencies and compare them to Hardy–Weinberg expectations.
