Positive SelectionEdit
Positive selection is the evolutionary process by which advantageous genetic variants rise in frequency because they confer a reproductive or survival advantage. It is one of the central mechanisms by which organisms adapt to changing environments, pathogens, diets, and other ecological challenges. In the broader framework of evolutionary biology, positive selection stands in contrast to neutral drift, where changes occur by chance, and purifying selection, which removes deleterious variants. For organisms with genomes that recombine, the signature of positive selection can be subtle or dramatic, depending on the strength of selection, the size of the population, and the timing of the adaptive event. Natural selection
Across the genome, researchers identify footprints of positive selection through patterns such as reduced diversity around a beneficial variant due to a selective sweep, extended regions of shared haplotypes, higher genetic differentiation between populations in particular regions, and elevated rates of nonsynonymous changes in coding sequences relative to synonymous changes. These signals are interpreted in the light of demographic history, because population bottlenecks and migrations can mimic or obscure true selective events. Tools and concepts such as Selective sweep, extended haplotype analyses like Extended haplotype homozygosity, and statistics that compare populations (for example Fst) are used to disentangle selection from drift and structure. In practice, positive selection can act on single genetic variants with large effects or on multiple variants with small effects across the genome, a phenomenon that has driven the evolution of complex traits as well as species-specific adaptations. genetic hitchhiking
Positive selection operates through different molecular and population-genetic pathways. On the molecular side, changes in protein-coding sequences can alter function or efficiency, while regulatory variants can shift when, where, or how much a gene is expressed. Copy number variation, gene duplications, and structural rearrangements also introduce new material that selection can act upon. From a population-genetics perspective, the strength of selection is described by parameters such as the selection coefficient (s), and the rate at which advantageous alleles spread depends on population size, mating structure, migration, and recombination. Importantly, not all adaptive changes leave uniform or easy-to-detect signatures; some arise from shifts in networks of interacting genes or from changes in ecological contexts that alter selective pressures over time. dN/dS ratio genetic drift Hitchhiking
Notable examples underscore how positive selection has shaped human and other species’ evolution. In humans, lactase persistence—the continued ability to digest lactose into adulthood—is a classic case where a regulatory variant near the lactase gene enables lactase production beyond infancy, providing a dietary advantage in populations with a history of dairy consumption. This trait has arisen independently in several populations and is a widely cited example of recent, strong positive selection. Lactase persistence Other well-studied instances include adaptation to high altitude in Tibetans, where signals in genes such as EPAS1 are linked to improved oxygen utilization; and genetic changes related to diet, such as copies of the amylase gene that can be more numerous in populations with high-starch diets (the AMY1 gene). EPAS1 AMY1
Pathogen-driven selection also figures prominently in discussions of positive selection. In regions where malaria is or was historically prevalent, alleles that modulate red blood cell traits—such as variants in the HbS region of the HBB gene—have increased in frequency because they reduce the severity of disease in heterozygotes, a pattern that can be described as balancing selection with a strong selective component in certain environments. Other examples include variants that affect susceptibility to infectious diseases or to certain pathogens, as well as population-specific resistance traits. The full picture is nuanced, because some well-known cases—like the sickle-cell trait—are sometimes best described as forms of balancing selection when the fitness trade-offs are taken into account. Sickle-cell trait HBB
In pathogens and in microbial populations, positive selection underpins the rapid emergence of antimicrobial resistance. Burgeoning use of antibiotics imposes new selective pressures that favor resistant alleles, leading to sweeps in bacterial genomes and complicating medical treatment. This is a practical example of how human activity can alter the selective landscape and accelerate adaptation in another lineage. Antibiotic resistance
The study of positive selection often intersects with debates about how quickly and extensively adaptation occurs in natural populations. In humans and other species, scientists discuss questions such as the relative importance of single, high-impact changes versus many small-effect variants in complex traits, the prevalence of hard sweeps versus soft sweeps (where multiple haplotypes carry beneficial variants), and the role of demographic history in generating signals that resemble selection. Some researchers emphasize clear, recent, region-specific adaptations; others caution that polygenic adaptation—small shifts across many genes—can be difficult to detect and attribute with confidence. The discussions reflect ongoing refinements in methods, data, and interpretation, rather than a simple dichotomy about whether selection happened at all. Polygenic adaptation Selective sweep
Contemporary data and methods continually refine the narrative of positive selection. Ancient DNA, improved genome sequencing, and more sophisticated models help separate genuine adaptive events from demographic artifacts. As with many questions in evolutionary biology, positive selection is best understood as a spectrum of processes operating under different historical and ecological conditions, rather than a single uniform mechanism. The study of these patterns informs our understanding of how species adapt to environments, pathogens, and dietary changes over time. Genetic drift