Disruptive selection, also called diversifying selection, is a mode of natural selection where extreme traits in a population are favored over intermediate ones. Over time, this can cause a single population to split into two distinct groups, each with traits better suited for different environmental niches.
The Mechanics of Disruptive Selection
Disruptive selection occurs when environmental pressures make the intermediate phenotype less fit for survival and reproduction. These pressures can arise from factors such as resource availability or predation. When competition for scarce resources is high, individuals with extreme traits may have an advantage by being able to exploit alternative resources that intermediate individuals cannot.
This shift in the population’s trait distribution can be visualized. A population might show a bell-shaped curve for a particular trait, with most individuals clustered around the average. Under disruptive selection, this curve begins to change. A dip forms in the middle of the curve as the intermediate individuals are selected against, and two new peaks emerge at either end, representing the favored extreme phenotypes.
Consider a hypothetical bird population where food availability is limited to either very small seeds or very large, hard seeds. Birds with small beaks are efficient at handling the small seeds, and birds with large, robust beaks can crack the large seeds. Birds with intermediate-sized beaks, however, are not well-suited for either food source. They struggle to manipulate small seeds and lack the force to break large ones, putting them at a competitive disadvantage.
In this scenario, the birds with small and large beaks will have higher survival and reproductive rates. Over generations, the alleles for small and large beaks will increase in frequency, while the alleles for intermediate beaks will decrease.
Contrasting Selection Types
To understand disruptive selection, it is helpful to compare it with the other modes of natural selection: directional and stabilizing. Each type describes how environmental pressures shape the distribution of traits within a population, resulting in a distinct outcome.
Disruptive selection favors individuals at both ends of the phenotypic spectrum, selecting against intermediate individuals. This process increases genetic variance and can lead to a distribution curve with two peaks.
Directional selection favors one extreme phenotype over others, causing the average trait of the population to shift in a single direction over time. This occurs when the environment undergoes a change, making a particular trait more advantageous. For example, in an environment with progressively darker tree bark, darker-colored moths would be better camouflaged from predators, and the frequency of the dark-colored phenotype would increase. This results in the population’s trait distribution curve shifting to one side.
Stabilizing selection is the opposite of disruptive selection; it favors the intermediate phenotype and selects against extreme variations. This is the most common mode of natural selection in environments that are stable over long periods. It reduces genetic variance by weeding out individuals with traits that deviate from the population average. A classic example is human birth weight, where infants of average weight have a higher survival rate than those who are significantly smaller or larger.
Real-World Examples of Disruptive Selection
A clear example of disruptive selection can be seen in the African finch, Pyrenestes ostrinus. This bird population exhibits a distinct polymorphism in beak size, with individuals having either large or small beaks, but very few with intermediate sizes. This difference is not related to sex or age but is directly linked to their primary food sources. The environment provides two main types of seeds: a soft-shelled species and a hard-shelled species.
Finches with small beaks are highly efficient at cracking and eating the soft seeds, while those with large, powerful beaks are specialized for the tough, hard seeds. Birds with intermediate-sized beaks are at a disadvantage because their beaks are not specialized for either type of seed, making them less efficient at feeding. This lower feeding efficiency leads to lower survival rates, selecting against the intermediate phenotype and maintaining the two specialized beak sizes.
Another well-documented case involves the peppered moth, Biston betularia, in England. Before the Industrial Revolution, the light-colored form of the moth was predominant because it was well-camouflaged against the lichen-covered trees, while the dark-colored form was rare. However, as industrial pollution killed the lichens and covered the trees in black soot, the dark moths gained a camouflage advantage.
In heavily polluted industrial areas, the dark moths thrived, while in clean, rural areas, the light moths remained more common. In certain transitional environments, both light and dark moths could be effectively camouflaged, but intermediate, grey-colored moths would have stood out against both the soot-blackened bark and the lighter, lichen-covered trees.
The Role in Speciation
Disruptive selection can play a part in the formation of new species, a process known as speciation. When the selection against intermediate phenotypes is strong and sustained over many generations, the two favored groups can become increasingly different from one another. This divergence can affect not only physical traits but also behaviors, such as mating preferences.
If the two extreme groups begin to mate preferentially with individuals that share their traits—a phenomenon called assortative mating—it can reduce or eliminate gene flow between them. This reproductive isolation is an important step in the creation of new species. Even without a physical barrier separating the groups, they can diverge into distinct species within the same geographic area, a process called sympatric speciation. The continued pressure of disruptive selection reinforces these differences until the two groups are no longer able to interbreed, resulting in the emergence of two separate species from a single ancestral population.