Explain Hardy-Weinberg equilibrium and list factors that disrupt it.

• A. A null model of allele frequencies in a non-evolving population; factors include mutation, migration, natural selection, genetic drift, nonrandom mating.
• B. A principle describing how alleles disappear; factors include aging and environmental changes only.
• C. A theory about gene flow in bacteria; factors include horizontal transfer only.
• D. A model of mutation rate in a population; factors include time of day.

Answer: (A)

Hardy-Weinberg equilibrium is a baseline model for allele frequencies in a population that is not evolving. In this ideal scenario, allele and genotype frequencies stay constant across generations as long as certain conditions hold: a very large population, random mating, no mutation, no migration (gene flow), and no natural selection. When any of these conditions are violated, allele frequencies can change, signaling evolution.

The factors that disrupt this equilibrium are precisely the evolutionary forces that violate those conditions: mutation introduces new alleles; migration brings in or removes alleles from outside populations; natural selection changes frequencies based on differential fitness; genetic drift causes random fluctuations, especially in small populations; and nonrandom mating alters genotype frequencies by skewing who mates with whom. So the statement that describes Hardy-Weinberg as a null model for allele frequencies in a non-evolving population and lists these disruptive forces best captures the concept.