Natural selection is the primary mechanism driving evolution, describing the process where organisms better adapted to their environment tend to survive and produce more offspring. This differential survival and reproduction is based on inherited traits that provide an advantage in a specific ecological context. The process ensures that advantageous characteristics become more common in a population across successive generations. Environmental selection pressures lead to distinct patterns of change in a population’s traits. These patterns are categorized into three main modes that describe how average or extreme trait values are favored or disfavored over time.
Prerequisites for Natural Selection
For natural selection to occur, several foundational requirements must be met within a population.
The first requirement is variation in traits among individuals. This variation provides the raw material upon which selection acts, meaning individuals must differ in characteristics like size, color, or behavior. This phenotypic variation often results from genetic differences, such as mutations or gene flow.
The second requirement is that these varying traits must be heritable, meaning they are reliably passed down from parent to offspring through genetic material. If an advantageous trait is not heritable, evolutionary change cannot occur. The final condition is differential reproduction, where some individuals survive and reproduce more successfully than others based on their traits. Successful reproduction is the ultimate driver of natural selection.
Directional Selection
Directional selection occurs when environmental pressure favors one extreme phenotype over the average or the opposite extreme. This selection causes the average trait value of the population to shift toward the favored extreme over generations. This consistent shift results in the population becoming better suited to an environment that has changed or features a sustained selective pressure.
A clear example is the development of antibiotic resistance in bacteria. Most individuals are susceptible to the antibiotic, but random mutations may produce a few resistant individuals. When an antibiotic is introduced, it acts as a strong selective pressure by eliminating susceptible bacteria. The few resistant bacteria survive and proliferate, passing the resistance trait to their offspring.
Over time, the frequency of the resistance gene increases rapidly in the population, shifting the average level of resistance higher. This process is rapid because bacteria have short generation times and high reproductive rates. Directional selection is responsible for many observable changes in species, such as the increasing body size of certain mammals or the darkening of peppered moths during the Industrial Revolution.
Stabilizing Selection
In contrast to directional selection, stabilizing selection favors the intermediate or average phenotype and acts against both extreme variants of a trait. This mechanism reduces phenotypic variation in the population, making the distribution curve taller and narrower while keeping the average trait value centered. Stabilizing selection is often the most common form of natural selection in stable environments because extreme deviations are typically less successful.
A classic example is human birth weight. Historically, infants born significantly below the average weight of approximately seven pounds had lower survival rates due to complications associated with prematurity. Similarly, babies born significantly heavier than average often faced lower survival odds due to complications during delivery.
Studies in the mid-20th century showed that infants weighing around seven to eight pounds had the highest rates of survival. This pattern demonstrates that environmental selective pressures, particularly the constraints of the birth canal and infant viability, favored the intermediate weight. Although modern medical interventions have reduced this selection pressure in developed countries, this pattern operated on human populations for a long time.
Disruptive Selection
Disruptive selection is the opposite of stabilizing selection, as it favors individuals at both extreme ends of a trait spectrum while selecting against the intermediate phenotypes. This process leads to a population with a bimodal trait distribution, meaning the population separates into two distinct groups. Disruptive selection increases the genetic variance within a population and is often seen as a step toward the formation of new species.
A remarkable example occurs in the African finch known as the black-bellied seedcracker (Pyrenestes ostrinus), which exhibits a dramatic polymorphism in beak size. These finches feed on sedge seeds, which present a bimodal food source—consisting primarily of either very large, hard seeds or very small, soft seeds. Finches with large beaks are highly efficient at cracking the large seeds, while those with small beaks are better suited for handling and processing the small seeds.
The intermediate-beaked finches, however, are poorly adapted for either food source. They struggle to efficiently crack the large seeds and are less adept at processing the small seeds than their specialized counterparts. This inefficiency results in lower survival and reproductive success compared to the birds at both extremes. The resulting strong selection against the average trait drives the population toward two distinct beak-size morphs, illustrating how environmental resource gaps maintain extreme variations within a single species.