Natural selection is the fundamental process that drives evolutionary change, where individuals with traits better suited to their environment tend to survive and reproduce more successfully. This differential survival and reproduction acts upon the natural variation within a population, ensuring that beneficial characteristics are passed to the next generation. The result is that populations become better adapted to the specific conditions of their habitat. Evolutionary biologists theorize that the most frequently observed form of selection in nature is stabilizing selection.
Distinguishing the Three Modes of Selection
Selection pressure manifests in distinct ways based on how it alters the distribution of a heritable trait. Directional selection occurs when one extreme of a trait is favored, causing the average trait value to shift toward that extreme over generations. For example, if larger body size conferred a survival advantage, the population distribution would move toward greater size. This selection is often seen during periods of significant environmental change.
Disruptive selection acts against the intermediate phenotype and favors individuals at both ends of the trait spectrum. This process results in a bimodal distribution, creating two distinct sub-populations with different optimal trait values. A notable example is a habitat where food resources are available only in two sizes, selecting against individuals with intermediate foraging structures. Stabilizing selection operates differently, favoring the average phenotype and actively selecting against both extremes of the trait distribution.
The Mechanism of Stabilizing Selection
Stabilizing selection functions by applying pressure against individuals who deviate significantly from the population mean for a specific trait. Organisms possessing traits that are either too large or too small are less likely to survive and reproduce. The intermediate phenotype, which is already the most common, experiences the highest reproductive success, or fitness.
This selection process does not change the average value of the trait but reduces the overall spread of its distribution. Over successive generations, the repeated elimination of extremes causes the bell-shaped curve representing the trait distribution to become narrower and taller. The primary effect is a reduction in phenotypic variance, meaning the population becomes more uniform for that characteristic. Stabilizing selection acts as a conservative force, maintaining the status quo and preserving a trait that is optimally adapted to the prevailing conditions.
Why Stable Environments Favor Intermediate Traits
The belief that stabilizing selection is the most common form stems from the reality that most organisms live in relatively stable environments. In these long-established conditions, populations have already reached an optimal level of adaptation for many traits. The intermediate trait value often represents the most effective compromise of biological trade-offs associated with survival and reproduction. For instance, a medium-sized animal might be large enough to deter predators but small enough to conserve energy efficiently.
This optimal trait value corresponds to a “fitness peak” on the adaptive landscape, and stabilizing selection works to keep the population centered precisely on this peak. Any deviation from this optimum results in a lower fitness score. Directional selection, which causes evolutionary change, requires a sustained alteration in the environment, such as a shift in climate or a new predator. Since major environmental changes are less frequent than periods of constancy, stabilizing selection dominates by maintaining the existing, highly adapted state.
Classic Empirical Evidence
The most commonly cited example of stabilizing selection involves human birth weight, demonstrating the clear advantage of an intermediate trait value. Historically, babies born with very low weight faced high mortality rates due to poor heat retention and disease susceptibility. Infants with very high birth weight also had lower survival rates due to complications during the birthing process. Babies of average weight, typically between 6.5 and 8 pounds, had the highest probability of survival and fitness.
Another classic illustration is the clutch size in birds, which is the number of eggs laid in a single reproductive effort. Birds laying too few eggs do not maximize reproductive output, while those laying too many eggs often cannot gather enough food to successfully feed all the hatchlings. This resource limitation means that an intermediate clutch size, like four or five eggs for many small bird species, is favored. In both cases, the extreme phenotypes are selected against, reinforcing the average trait value in the population.