What Is Positive Selection Pressure in Evolution?

Positive selection is an evolutionary process where new, advantageous genetic traits become more common within a population. These traits improve an organism’s survival or reproduction, allowing the responsible genes to be passed on more frequently to subsequent generations. This process is a driving force of adaptation, enabling species to change over time in response to their environment.

The concept is summarized by the phrase “survival of the fittest,” but this refers to the fitness of genes rather than the physical strength of individuals. A trait that allows an organism to produce more offspring who also survive to reproduce will be selected for. This continuous filtering process ensures that beneficial genetic variations spread throughout a population, shaping the evolutionary path of a species.

The Mechanics of Positive Selection

Positive selection begins with a random genetic mutation that produces an advantageous trait. This advantage means the individual is better equipped to survive challenges, live longer, and reproduce more successfully than its peers. Over generations, the beneficial gene variant, or allele, is passed to offspring at a higher rate, increasing its frequency throughout the population.

This mechanism can be contrasted with negative selection, which actively removes harmful mutations from a population. While negative selection purges genetics that decrease an organism’s fitness, positive selection promotes the spread of variants that enhance it. Both are integral to how species adapt, with one preserving functional traits and the other driving the evolution of new ones.

Identifying Positive Selection in Genomes

Scientists can uncover the history of positive selection by searching for its distinctive footprints within a species’ DNA. A prominent signal is a “selective sweep,” where a beneficial allele spreads rapidly through a population. As the gene’s frequency increases, nearby DNA sequences on the same chromosome “hitchhike” with it, reducing genetic diversity in that genomic region.

Another method involves comparing mutation rates in protein-coding genes. Scientists analyze the ratio of non-synonymous mutations, which alter the protein sequence, to synonymous mutations, which are silent. A high rate of non-synonymous changes suggests the gene was under positive selection, as these alterations likely improved the protein’s function.

These genomic analyses help researchers pinpoint genes involved in a species’ adaptation to new challenges. By examining these patterns, scientists can reconstruct evolutionary histories and understand the pressures that shaped a species.

Positive Selection Shaping Human Evolution

Positive selection has influenced the trajectory of human evolution, with clear examples in our DNA. A well-known instance is the evolution of lactase persistence, the ability to digest lactose (milk sugar) as an adult. This trait is common in populations with a history of dairy farming, where milk provided a survival advantage.

Another example is the adaptation of certain human populations to high-altitude environments, such as those in Tibet and the Andes. Genetic studies have identified genes that help these individuals thrive in low-oxygen conditions. For instance, Tibetans have a variant of the EPAS1 gene that helps regulate the body’s production of red blood cells, preventing overproduction at high altitudes.

Our genomes also carry evidence of selection driven by disease. Variants of the CCR5 gene that confer HIV resistance are more frequent in some European populations, possibly because the variant also protected against a historical pathogen like smallpox. The sickle cell trait, while causing disease in some, provides protection against malaria, a strong selective pressure in certain regions.

Examples of Positive Selection in Other Species

Positive selection is a universal process that drives adaptation across all forms of life. An example is the rapid evolution of antibiotic resistance in bacteria. When exposed to antibiotics, bacteria with a random resistance mutation survive and reproduce, which quickly leads to entire populations of resistant bacteria.

In the insect world, the peppered moth of England is a textbook case of industrial melanism. Before the Industrial Revolution, light-colored moths were camouflaged against lichen-covered trees. As pollution darkened the tree bark, a rare, dark-colored variant gained a camouflage advantage against predators, and its frequency in the population increased.

This evolutionary force also drives the development of complex traits. The patterns on a butterfly’s wings that mimic toxic species result from positive selection, as this helps them avoid predators. The evolution of potent venom in snakes is also driven by the advantage it provides in subduing prey and defense.