Positive selection is an evolutionary process where new genetic variants that provide an advantage to an organism become more frequent in a population over generations. This process is a driver of adaptation, enabling species to adjust to changing environmental conditions. The advantageous trait gives the organism a distinct edge in survival or reproduction. Individuals with this beneficial trait are more likely to thrive and pass it on, leading to its widespread presence.
The Engine of Adaptation
Positive selection begins with a random genetic mutation, a change in an organism’s DNA. Most mutations are neutral or harmful, but occasionally, a mutation arises that confers a survival or reproductive advantage in a specific environment. An individual carrying this beneficial mutation may live longer or produce more offspring than its peers. This concept is often summarized as ‘survival of the fittest,’ but more accurately means ‘survival of the best adapted’ to current environmental pressures.
These advantageous traits are heritable, meaning they are passed from parents to their offspring. As generations pass, offspring inheriting the beneficial gene also experience greater reproductive success. Consequently, the frequency of this advantageous allele—the specific version of the gene—steadily increases throughout the population, allowing a species to become better suited to its habitat.
This process is responsible for the diversity of life and the ways organisms are tailored to their surroundings. From the evolution of bipedalism in early humans to the development of complex immune systems, positive selection has shaped the traits of species. It allows populations to innovate and thrive when facing new challenges.
Positive Selection in Action
A clear example of positive selection is the development of antibiotic resistance in bacteria. When a bacterial population is exposed to an antibiotic, most are killed. However, due to random mutations, a few individuals may possess a gene that allows them to survive the drug’s effects. These survivors then reproduce, passing the resistance gene to their offspring, leading to a population dominated by antibiotic-resistant bacteria.
Another example is the evolution of lactose tolerance in humans. In most mammals, the gene that produces lactase, the enzyme needed to digest milk sugar, switches off after infancy. In human populations with a history of dairy farming, a mutation allowing for lactase production into adulthood became widespread. This trait, known as lactase persistence, provided a nutritional advantage, especially where other food sources were scarce.
The peppered moth illustrates how environmental changes can drive positive selection. Before England’s Industrial Revolution, light-colored moths were well-camouflaged against lichen-covered trees, while a rare, dark variant was easily spotted by birds. As industrial pollution blackened the trees, the dark moths gained a camouflage advantage and their numbers soared. This shift demonstrates how a trait can be selected for when the environment changes.
Comparison with Other Evolutionary Forces
One evolutionary force is negative, or purifying, selection, which works in opposition to positive selection. While positive selection promotes beneficial traits, negative selection removes harmful new mutations from a population’s gene pool. It is a conservative process that maintains the stability of genes by weeding out variants that decrease an organism’s fitness.
Another evolutionary mechanism is genetic drift, which describes changes in gene frequency due to random chance. Genetic drift’s effects are most pronounced in small populations, where random events can impact which individuals reproduce. For instance, if only a few individuals in a small population survive a natural disaster, their specific genetic traits will become more common, regardless of whether those traits were advantageous. This process highlights that not all evolutionary change is adaptive.
Positive selection, therefore, is the force that actively drives adaptation by favoring beneficial traits, setting it apart from the random fluctuations of genetic drift and the conservative action of negative selection. While genetic drift can lead to non-adaptive changes and negative selection maintains the status quo, positive selection allows species to innovate and adapt to new challenges.
Identifying Positive Selection in the Genome
Scientists can identify the historical footprint of positive selection by analyzing an organism’s DNA. When a beneficial gene spreads rapidly, it often drags neighboring DNA segments with it in a process called a ‘selective sweep.’ This event leaves a distinct signature in the genome: a region with unusually low genetic diversity. The rapid rise of the advantageous allele doesn’t allow time for recombination to shuffle the surrounding genetic material, making that chromosomal section look very similar across the population.
Another method involves comparing the DNA sequences of related species or individuals. Researchers look for genes with a higher rate of nonsynonymous mutations—those that change the protein’s amino acid sequence—compared to synonymous mutations that do not. A surplus of these protein-altering changes suggests they were beneficial and therefore preserved and spread by positive selection.