The Hardy-Weinberg principle, named after the scholars who introduced the idea, states that if a population is not evolving, the allele and genotype frequencies will not change between generations. An allele is a version of a gene, and a genotype is the genetic makeup of an individual. The mechanisms of evolution all have one thing in common: They all change the frequency of at least one gene within a population (interbreeding group). As long as the frequency of one gene has changed between generations, evolution has occurred. In the case that no evolution is occurring, which is rare in natural populations, a population is said to have reached Hardy-Weinberg equilibrium. Formally defined, Hardy-Weinberg equilibrium is the state of a population in which allele and genotype frequencies do not change between generations because evolution is not occurring.
In addition, if a population is in Hardy-Weinberg equilibrium, the genotype frequencies are predictable based on the allele frequencies. Because the frequency of genes does not change between generations, knowing the frequencies in one generation allows for accurate calculations of the frequencies in each successive generation as long as the population stays in equilibrium. This is significant because the Hardy-Weinberg principle can be applied and the values compared with genotype frequencies of a real population to answer the question, "is this population evolving?"
Conditions of Hardy-Weinberg Equilibrium
Hardy-Weinberg equilibrium requires the complete absence of evolution (genetic change between generations). This would indicate that none of the forces of microevolution, such as natural selection, gene flow, genetic drift, and mutation (direct change in DNA sequence), are operational. A population is considered to be in Hardy-Weinberg equilibrium if, for a gene or locus (location on a chromosome) of interest, there is no change in genotype or allele frequency. An allele is a version of a gene, and a genotype is the genetic makeup of an individual. The frequencies are the proportion of occurrences of a particular allele or genotype within a population. From the perspective of a whole genome, a population would never be in Hardy-Weinberg equilibrium. Hardy-Weinberg equilibrium requires the following:
- No mutation
- No natural selection
- Random mating
- No genetic drift (very large populations are less affected by random events)
- No gene flow
These requirements are very difficult to meet in natural populations. However, the Hardy-Weinberg principle is a useful tool for determining whether a population is evolving at a particular site.
Allele Frequencies and Using Hardy-Weinberg
To know whether evolution is occurring at a particular on a chromosome, one must know whether alleles or genotype frequencies are changing. An allele is a version of a gene, and a genotype is the genetic makeup of an individual. Genotype frequency is the proportion of individuals in the population that have a particular genotype. The first step is to calculate the current genotype and allele frequencies. Then, that information can be used to predict what the genotype frequencies should be in Hardy-Weinberg equilibrium (the state in which allele and genotype frequencies do not change). If the predicted genotype frequencies match the actual genotype frequencies of a population, then evolution is not occurring at that locus.For example, a butterfly population might have some butterflies that are blue (BB), some that are purple (Bb), and some that are pink (bb). To calculate the genotype frequencies, count the number of individuals with each genotype and divide by the total number of individuals. In this example population, the genotype frequencies are:
Next, to predict what the genotype frequencies should be if the population was in Hardy-Weinberg equilibrium, consider the entire gene pool (set of genes of the population).Assuming random mating, there is a 25% chance that a sperm from this population will be B and a 75% chance that a sperm will be b. The same probabilities apply to eggs from this population. Use a diagram similar to a Punnett square to calculate the expected genotype frequencies of the next generation. A Punnett square is used to calculate the expected genotype frequencies of offspring from two parents. These same principles can apply to an entire gene pool. The expected genotype frequencies are as follows:
Observed vs. Expected Hardy-Weinberg Frequencies
|Observed Genotype Frequencies||Expected Under H-W Equilibrium|