The genetic information that dictates all life processes acts as a fundamental blueprint for every organism. This blueprint, carried within the DNA, is not static but constantly subject to change across generations within a population. Allele frequency is a precise metric used by scientists to track and quantify this genetic change over time. By measuring the proportion of specific gene variants in a population, researchers gain insight into how populations are adapting, evolving, and responding to their environment. This statistical measure serves as a direct window into microevolution, defined as the change in the frequency of these gene variants over time.
Understanding Alleles and the Gene Pool
A gene is a segment of DNA that holds the instructions for building a specific functional product, typically a protein. Genes are located at fixed positions, or loci, on a chromosome. An allele is simply a variant form of that gene, like different versions of a software program that perform the same basic function but with slight differences in the code.
For example, a gene for flower color might have one allele that codes for purple flowers and another that codes for white flowers. Since most complex organisms are diploid, they carry two alleles for every gene, inheriting one from each parent. These alleles determine an organism’s traits, and the collection of all these different alleles within a reproductive population is known as the gene pool. The gene pool represents the total genetic diversity available to the population.
Defining and Measuring Allele Frequency
Allele frequency is a measure of how common a specific allele is compared to all other alleles for that gene within the gene pool. It is the relative proportion of a particular allele at a given genetic location. This frequency is typically expressed as a decimal between 0 and 1, or as a percentage. The sum of the frequencies for all possible alleles of a single gene must always equal 1 (or 100%).
To calculate this, scientists count the number of times a specific allele appears and divide that count by the total number of alleles for that gene in the population. For a diploid population, the total number of alleles is twice the number of individuals, since each individual carries two copies of the gene. For instance, if the dominant ‘A’ allele appears 110 times in a group of 100 individuals (200 total copies), its frequency is 110/200, or 0.55.
Forces That Shift Allele Frequencies
Allele frequencies are not static; their shifts are the definition of evolution occurring at the population level. Four primary forces constantly act on the gene pool to cause these changes:
- Natural selection: Certain alleles offer a survival or reproductive advantage in a specific environment. Individuals carrying these beneficial alleles are more likely to pass them to the next generation, causing the allele’s frequency to increase over time.
- Genetic drift: This describes random fluctuations in allele frequencies that occur by chance, often having a significant impact in small populations. Two specific forms are the bottleneck effect, where a population is drastically reduced by a random event, and the founder effect, where a small group breaks off to establish a new population. Both events can lead to a non-representative sampling of the original allele frequencies.
- Gene flow: This involves the transfer of alleles between different populations through the migration of individuals. When individuals move into a population and successfully reproduce, they introduce new alleles or alter the existing frequencies. This exchange tends to make the gene pools of the connected populations more genetically similar over time.
- Mutation: This creates new genetic variation by generating novel alleles or changing one allele into another through alterations in the DNA sequence. Although the immediate effect of mutation on frequency is usually minor, it is the original source of all new genetic material upon which the other forces can act.
Allele Frequency and Population Change
Monitoring allele frequencies over multiple generations is the direct method scientists use to observe and measure evolution in action. If a population’s allele frequencies change from one generation to the next, it is undergoing microevolution. This change signifies that one or more of the evolutionary forces are actively influencing the population’s genetic structure.
The Hardy-Weinberg Principle provides a baseline for this measurement by describing the theoretical state in which allele frequencies do not change. This principle represents a hypothetical, non-evolving population where conditions like no mutation, no genetic drift, and no selection are met. Since real populations almost always violate at least one of these conditions, they are rarely in this state of equilibrium. Scientists use the Hardy-Weinberg prediction as a null hypothesis, and any observed deviation from the expected constant frequencies proves that evolution is occurring.