An allele is a specific variation of a gene, representing one of the possible forms at a particular genetic location. All alleles for every gene within a breeding population collectively form the gene pool. Allele frequency describes how common a specific version of a gene is within that gene pool, expressed as a proportion of all copies. Evolution is the change in these allele frequencies within a population across successive generations.
How New Alleles Emerge
All genetic variation originates from mutation, which is an alteration in the nucleic acid sequence of an organism’s genome. These changes can range from the substitution of a single nucleotide base to large-scale chromosomal rearrangements. Mutation provides the raw material upon which all other evolutionary forces act to change allele frequencies. Mutation is considered random; it does not occur in response to environmental pressures. Errors during DNA replication or repair mechanisms introduce these novel alleles into the gene pool. While a small fraction of new alleles may be beneficial, the vast majority are either neutral or harmful.
Selection Pressure and Fitness
Once a new allele is present, its frequency is subject to the non-random process of natural selection, which acts based on differential reproductive success. Biological fitness is a measure of an organism’s ability to survive and produce viable offspring relative to others in the population. Individuals possessing traits better suited to their environment are more likely to pass their alleles to the next generation, causing advantageous alleles to become more common.
Environmental factors, such as predators, climate shifts, or new diseases, exert selective pressure that favors certain phenotypes. The rapid increase in antibiotic resistance in bacterial populations demonstrates this process. Bacteria with resistance alleles survive drug treatment to reproduce, quickly dominating the population. Natural selection alters allele frequencies in three distinct ways, depending on which traits are favored.
Directional Selection
Directional selection favors one extreme of a trait, causing the entire distribution of the trait to shift over time. A classic example is the change in the peppered moth population in England during the Industrial Revolution. Soot darkened trees favored the previously rare dark-colored moths over the common light-colored ones.
Stabilizing Selection
Stabilizing selection favors the intermediate or average phenotype, acting against both extremes. This mode reduces variation around the mean. This is illustrated by human birth weight, where babies of average weight have a higher survival rate than those who are very small or very large.
Disruptive Selection
Disruptive selection favors both extreme phenotypes at the expense of the intermediate forms. This process can create a bimodal distribution of traits, effectively splitting the population into two distinct groups. The African seedcracker finch shows this pattern. Individuals with very large or very small beaks are favored, while those with medium-sized beaks are less efficient at feeding and are selected against.
Random Chance Genetic Drift
Genetic drift is a change in allele frequency that occurs purely by chance, independent of an allele’s survival or reproductive advantage. This mechanism is a sampling error affecting which alleles make it into the next generation due to random events. Drift is present in all populations but is significantly more pronounced and rapid in small populations.
Bottleneck Effect
The Bottleneck Effect happens when a large population is severely and suddenly reduced in size by a random, non-selective event like a natural disaster. The few survivors carry only a random subset of the original gene pool. The resulting population often has dramatically reduced genetic diversity and altered allele frequencies. Some alleles may be entirely lost, while others may become fixed at 100% frequency by accident.
Founder Effect
The Founder Effect occurs when a small group separates from a larger population to establish a new colony. The genetic makeup of this small founding group may not accurately represent the diversity of the original population, leading to a new gene pool with different allele frequencies. This effect is observed in human populations where a small group migrates and, through isolation, exhibits a higher frequency of certain rare genetic traits.
The Impact of Gene Flow
Gene flow, or migration, is the transfer of alleles between populations, typically occurring when individuals move and interbreed. This movement of genetic material changes the allele frequencies in the recipient population. For example, plant populations experience gene flow when pollen is carried by wind or insects over long distances. The primary effect of sustained gene flow is the homogenization of populations, making them more genetically similar over time. Gene exchange can counteract the effects of genetic drift and selection, preventing divergence. Conversely, gene flow can introduce novel alleles, increasing genetic variation within a specific group.