Natural selection, the fundamental mechanism driving evolutionary change, operates in multiple ways to shape the traits of living organisms. One of its common modes is stabilizing selection, a process that actively maintains the status quo within a population. This form of selection focuses on preserving a particular, non-extreme trait value that has already proven successful in a given environment. Instead of pushing a trait toward a new extreme, stabilizing selection favors the average individual, ensuring that the most common form of a trait continues to dominate in future generations. This force is responsible for the consistency of many characteristics we observe in the natural world.
How Stabilizing Selection Works
The underlying mechanism of stabilizing selection involves a direct relationship between a physical characteristic, or phenotype, and the fitness of an individual. Fitness, in evolutionary terms, refers to an organism’s ability to survive and successfully reproduce, passing its genes to the next generation. Individuals who possess the mean, or average, version of a trait have the highest fitness.
This process targets and removes individuals at both extremes of the trait spectrum, such as those that are either unusually large or unusually small. Selection pressures act against these extreme variations, which are often less suited to the environment and therefore have a lower chance of survival and reproduction. Graphically, this is often represented by a bell-shaped curve where the peak, representing the average phenotype, becomes taller and narrower over time.
The consequence of this consistent pruning of outliers is a reduction in the population’s genetic variance for that specific trait. By continually favoring the intermediate variants, stabilizing selection minimizes the range of phenotypes present. This mechanism explains why so many traits across species do not undergo continuous, drastic change over long periods.
The Classic Case of Human Birth Weight
One of the most widely cited biological examples of stabilizing selection is the determination of birth weight in human infants. Studies have shown a strong correlation between an infant’s weight at birth and its chances of survival. This relationship provides a textbook illustration of a trait stabilized around an optimum mean.
The pioneering work of biologists Karn and Penrose in 1951 demonstrated that infants with birth weights significantly deviating from the average of approximately 7.5 pounds had a much higher mortality rate. Infants born with very low birth weights often face severe viability issues outside the womb, including difficulty regulating body temperature and a higher susceptibility to infectious diseases, which reduces their fitness.
Conversely, infants with very high birth weights also experience decreased survival, historically due to complications during the birthing process. A larger fetus increases the risk of injury to both the mother and the baby during delivery, constrained by the size of the human female pelvis. The selection pressure comes from two opposing directions: the physiological constraints of the baby outside the womb and the mechanical constraints of birth. This dual pressure stabilizes the trait, favoring the intermediate weight that maximizes the probability of survival.
Avian Clutch Size and Resource Limits
Stabilizing selection is clearly evident in the reproductive strategies of the avian kingdom, particularly in the determination of clutch size, which is the number of eggs a female bird lays in a single nesting attempt. For species like the European Starling, the optimal number of eggs is a balance between reproductive potential and resource availability.
Laying too few eggs represents a missed reproductive opportunity, resulting in a lower number of offspring contributing genes to the next generation. This outcome reduces the parent’s evolutionary fitness relative to others in the population. The selection pressure here favors greater reproductive output.
However, laying too many eggs introduces a resource-based constraint. A very large brood may exceed the parents’ capacity to effectively forage and provide sufficient food. This poor resource allocation leads to offspring that are malnourished, weaker, or smaller, increasing the likelihood of nestling mortality. The average clutch size, often around four to six eggs for many passerine birds, is the stabilized compromise that maximizes the number of healthy, surviving fledglings.
Size and Survival in Plant Populations
The plant kingdom provides examples where stabilizing selection maintains an optimal physical form in response to environmental forces. A common trait subject to this selection is plant height, where the selective pressures are derived from a combination of competition and physical stress. The environment selects for a medium height that balances competing needs.
Plants that are significantly shorter than the average may struggle to compete for sunlight, a fundamental requirement for their survival and reproduction. Being shaded by taller competitors significantly reduces their photosynthetic output and limits their ability to produce seeds. This places a strong selective disadvantage on the shortest individuals.
In contrast, extremely tall plants face challenges related to structural and energetic demands. They require more energy to build and maintain their extensive support structures, and they are more susceptible to damage from wind and storms. Therefore, an intermediate height strikes the optimal balance, ensuring sufficient access to light without incurring the high energy costs and physical risks associated with being overly tall. This mechanism preserves the medium-sized plant as the most successful variant in many ecosystems.