What Are the Four Patterns of Natural Selection?

Natural selection is a fundamental process driving evolution. Within any population of organisms, there is a natural variation of traits. Individuals with characteristics better suited to their environment have a higher likelihood of surviving and reproducing. Consequently, these advantageous traits are more frequently passed on to the next generation, becoming more common over time. This process, described by Charles Darwin, explains how species adapt.

Directional Selection

Directional selection occurs when environmental pressures favor individuals at one extreme of the trait spectrum, causing the population’s average trait to shift in a single direction. As the environment changes, a phenotype that was once rare can become the most advantageous. This pattern is a direct response to challenges that make a specific, non-average trait newly beneficial for survival and reproduction.

A well-documented instance is the peppered moth in 19th-century England. Before the Industrial Revolution, light-colored moths were abundant because they blended with lichen-covered trees. As industrial pollution darkened the trees with soot, the once-rare, darker-colored moths gained a survival advantage. Their dark coloration provided better camouflage against the darkened tree trunks, making them less likely to be eaten.

This environmental shift caused the population’s coloration to shift from predominantly light to dark. Graphically, this change is represented by a shift in the peak of the population’s bell curve for the trait, moving toward the favored extreme. A similar process is observed in bacteria, where the application of antibiotics creates a strong selective pressure favoring individuals with mutations that confer resistance, leading to populations dominated by resistant strains.

Stabilizing Selection

Stabilizing selection is a pattern where the intermediate, or average, phenotype is favored over more extreme variations. This process reduces genetic diversity by selecting against individuals at either end of the spectrum. Over time, this leads to a population that is more uniform, with the majority of individuals displaying the average trait.

This form of selection is common in stable environments where it ensures populations remain well-suited to their surroundings. The result is a decrease in the variation of a particular trait without changing the population’s average value for that trait.

A classic example in humans is birth weight. Babies born with a weight much lower or higher than average have historically faced higher mortality rates. Small babies are often less developed and more susceptible to disease, while large babies can cause complications during childbirth. As a result, infants of an average weight have the highest survival rates, and this selective pressure results in a graph where the bell curve becomes narrower and taller.

Disruptive Selection

Disruptive selection, or diversifying selection, favors individuals at both extremes of the phenotypic range over those with intermediate traits. This occurs when an environment has distinct niches that can be exploited by different variations of a species. Individuals with average traits are at a disadvantage because they are not well-suited to capitalize on either extreme.

This process can lead to a population splitting into two distinct groups, which can be a precursor to the formation of new species (speciation). By selecting against the middle ground, disruptive selection promotes the existence of two or more different forms within a population. Over many generations, these groups may become so distinct they no longer interbreed.

An example involves African finches with two primary food sources: very soft and very hard seeds. Finches with small beaks handle soft seeds, while those with large beaks crack hard seeds. Birds with intermediate-sized beaks are not good at eating either seed type and are less successful, creating pressure that favors the two extreme beak sizes.

Sexual Selection

Sexual selection is a mode of natural selection where pressure comes from the competition for mates, not the external environment. In this process, certain traits become more common because they increase an individual’s reproductive success, even if they do not contribute to survival. This selection is driven by mate choice or by direct competition between individuals of the same sex.

These traits, known as secondary sexual characteristics, can sometimes appear disadvantageous for survival. A feature that makes an individual more attractive to mates might also make it more conspicuous to predators or hinder its movement. The persistence of such traits demonstrates that the reproductive benefit they provide outweighs their potential survival cost.

There are two main mechanisms for sexual selection. Intersexual selection, or mate choice, is evident in the vibrant plumage of the male peacock; the tail is costly and makes the bird vulnerable, yet peahens prefer males with impressive tails. Intrasexual selection, or mate competition, involves contests between individuals, such as male deer using large antlers in combat to secure access to females.