Natural selection drives evolutionary change through the differential survival and reproduction of individuals. When selection acts on traits controlled by multiple genes, known as polygenic traits, it results in three distinct patterns of change within the population. These traits, such as height or skin color, exist across a broad spectrum of possibilities rather than being simply “on” or “off.” The three primary modes by which natural selection acts upon this continuous variation are stabilizing, directional, and disruptive selection.
The Continuous Nature of Polygenic Traits
Polygenic traits are influenced by the cumulative effect of many genes and environmental factors. This complex architecture prevents the trait from falling into discrete categories, resulting instead in continuous variation where individuals express a range of values.
When measured across a population, these values typically form a bell-shaped curve, known as a normal distribution. This curve shows that most individuals possess the intermediate phenotype, while extremes are less common. Natural selection acts by favoring or disfavoring individuals at specific segments of this existing distribution.
Stabilizing Selection
Stabilizing selection favors the intermediate phenotype while actively selecting against both extremes of the trait. This mode is common in stable environments where the current average trait value is the optimal adaptation for survival. The primary effect is a reduction in overall variation, causing the bell curve to become narrower and taller over time. The average value of the trait remains largely unchanged across generations.
A classic example is human birth weight. Historically, infants with very low birth weights struggled with higher mortality rates due to issues like disease. Conversely, very high birth weights presented a greater risk of complications and mortality for both the baby and the mother during delivery. Babies of intermediate weight, typically around seven pounds, have historically demonstrated the highest survival rates. This environmental pressure consistently selects for the middle ground, conserving the best-adapted average trait and reducing the frequency of genes contributing to the extreme phenotypes.
Directional Selection
Directional selection occurs when one extreme of the phenotypic range is favored over the intermediate phenotype and the opposite extreme. This mode is typically observed when the environment changes consistently or when a species migrates to a new habitat. The consequence is a consistent shift in the population’s average trait value toward the favored extreme over successive generations. The entire bell curve shifts horizontally across the phenotypic spectrum.
A well-known instance is the evolution of antibiotic resistance in bacterial populations. When an antibiotic is introduced, it kills most non-resistant bacteria, acting as a strong selective pressure. Bacteria with genetic variations conferring resistance are more likely to survive and reproduce. Over time, the frequency of resistance alleles increases, causing the average level of resistance in the population to shift toward the higher end of the spectrum. This mechanism was also seen in the peppered moth during the Industrial Revolution, where darker-winged moths became advantageous due to soot-darkened trees.
Disruptive Selection
Disruptive selection, also known as diversifying selection, involves selection against the intermediate phenotype, favoring both extremes of the trait. This mechanism occurs when individuals with average traits are at a disadvantage in an environment offering two distinct types of resources or niches. The result is that the population begins to diverge, with the bell curve splitting into a bimodal distribution. This process maintains or increases the phenotypic and genetic variation within the population.
A clear example is the African seedcracker finch, where the available seeds are either very large and hard or very small and soft. Finches with large beaks successfully crack the tough, large seeds, while those with small beaks handle the tiny seeds better. Finches with medium-sized beaks are inefficient at cracking either type of seed, placing them at a nutritional disadvantage. Consequently, the intermediate phenotype is selected against, leading to two distinct subpopulations specializing in different food sources. When this divergence becomes pronounced, disruptive selection can contribute to the formation of new species.