Population genetics explores how genetic variation behaves within groups of organisms, examining the distribution of genes and their variations across generations. A central question is whether genotype frequencies, the combinations of genes an individual carries, remain stable or change over time. This article clarifies the conditions under which these frequencies do, or do not, change.
What Hardy-Weinberg Equilibrium Means
Hardy-Weinberg Equilibrium (HWE) represents a foundational concept in population genetics. It describes a theoretical state where allele and genotype frequencies in a population remain consistent from one generation to the next. If a population is in HWE, the frequencies of particular gene combinations will stay the same over generations.
Keeping Frequencies Constant: The Conditions
For a population to achieve and maintain Hardy-Weinberg Equilibrium, five specific conditions must be met simultaneously:
- No mutation: No new alleles are created, and existing ones do not change.
- No gene flow: No migration of individuals into or out of the population.
- Random mating: Individuals choose mates without regard to any specific genotype.
- Infinitely large population: Prevents random fluctuations in allele frequencies (genetic drift).
- No natural selection: All genotypes have equal survival and reproductive rates.
The stability of genotype frequencies relies entirely on all these conditions being perfectly met.
When Frequencies Do Change
While Hardy-Weinberg Equilibrium describes a theoretical state of genetic stability, real-world populations rarely fulfill all five conditions perfectly. Violations of these conditions cause allele and genotype frequencies to change across generations, representing the forces that drive evolutionary change.
Mutation, which involves changes in DNA sequences, introduces new genetic variation and can alter allele frequencies. Gene flow, resulting from the migration of individuals between populations, can introduce or remove alleles, thereby shifting frequencies. Genetic drift causes random changes in allele frequencies, particularly impactful in smaller populations where chance events have a greater effect. Non-random mating, such as individuals preferring mates with specific traits, can alter genotype frequencies, even if allele frequencies remain unchanged initially. Finally, natural selection occurs when certain genotypes have differential survival and reproductive success, leading to an increase in the frequency of advantageous alleles over time.
The Importance of This Genetic Baseline
Despite its theoretical nature, the Hardy-Weinberg Principle holds importance in population genetics. It serves as a baseline for comparing the genetic makeup of real populations. When observed genotype frequencies deviate from HWE predictions, it indicates that evolutionary forces are at work. This comparison allows scientists to identify and understand the mechanisms driving genetic change. HWE provides a benchmark for genetic stability, helping researchers understand how evolution occurs.