Modes of Natural Selection: Directional, Stabilizing, and Disruptive

Natural selection can act on a trait in three main ways: directional selection favors one extreme and shifts the population mean, stabilizing selection favors intermediate values and reduces variation, and disruptive selection favors both extremes and can split the population into two groups. In brief: in directional selection, the bell-shaped curve of a trait moves toward one side, as with the increase in the beak depth of finches in a drought. In stabilizing selection, the curve becomes narrower, as with human birth weight. In disruptive selection, the curve develops two peaks. Sexual selection is a form of selection for success in mating.

The three modes describe how selection changes the distribution of a quantitative trait, and how it changes allele frequencies, which links natural selection with population genetics. This guide explains each mode with examples and graphs described in words, covers sexual selection, compares the modes in a table, works through a numerical example with the breeder’s equation and corrects common mistakes.

How does selection change allele frequencies?

Selection acts on phenotypes, but its effect is on the allele frequencies in the gene pool. Individuals with phenotypes that give them higher fitness (more surviving offspring) pass on more copies of the alleles that contribute to the phenotype. Over generations, alleles that increase fitness become more frequent, as in the calculation in the article on the Hardy-Weinberg principle.

For quantitative traits, such as height, beak size or running speed, which show continuous variation and are controlled by many genes together with the environment, the distribution in the population is often a bell-shaped (normal) curve. The three modes are described by what happens to this curve.

What is directional selection?

Directional selection favors individuals at one extreme of the range of a trait, so the mean value moves in that direction in later generations. It occurs when the environment changes in a consistent way, or when a population moves into a new environment.

• Galápagos finches: Peter and Rosemary Grant found that during the drought of 1977 on Daphne Major, medium ground finches with larger, deeper beaks survived better, because they could crack the large, hard seeds that remained. The average beak depth in the next generation increased by about 0.5 millimeter, about 4 percent.
• Peppered moths: the dark form increased in sooty areas, and declined after pollution was reduced.
• Antibiotic and pesticide resistance: the mean resistance of the population rises under treatment.
• Artificial selection: breeding for greater milk yield.

If the same selection continues, the trait may keep changing until the cost balances the benefit, or until there is no more variation. Examples from the fossil record include the increase in the size of the brains of hominins and the increase in the size of horses during the Cenozoic.

What is stabilizing selection?

Stabilizing selection favors intermediate phenotypes, and acts against both extremes. The mean stays the same, but the variation is reduced, so the curve becomes narrower. It occurs in stable environments, and is probably the most common form of selection.

• Human birth weight: very small babies and very large babies have a higher risk of death than babies of average weight, around 3 to 3.5 kilograms. Medical care in modern times has reduced the pressure, particularly on the small side.
• Clutch size in birds: in the great tit, pairs with a medium number of eggs raise the most surviving chicks, as a very large clutch cannot be fed properly, and a very small clutch produces few young. David Lack studied this.
• Gall size in goldenrod: Arthur Weis and Warren Abrahamson showed that gall flies’ larvae in small galls are more often killed by parasitoid wasps, and in large galls by birds, so the intermediate size has the highest survival.
• Living fossils: species such as horseshoe crabs and coelacanths have changed little for millions of years in stable habitats.

What is disruptive selection?

Disruptive (diversifying) selection favors both extremes and acts against the average. The curve develops two peaks. It can occur when an environment has two distinct resources or habitats, and the intermediate forms are poorly adapted to either. If mating between the two forms is reduced, disruptive selection can lead to speciation.

• African seedcracker finches (Pyrenestes ostrinus): the birds have either large or small beaks, and few intermediate ones. Large-beaked birds are better at cracking hard sedge seeds, and small-beaked birds at soft seeds. Birds with medium beaks are less efficient with either type. Thomas Bates Smith documented this in the 1980s and 1990s.
• Male coloration: in some fish and lizards, males are either large and aggressive or small and sneaky, and the intermediate type has the lowest mating success.
• Color forms in snails or butterflies: when predators attack the intermediate morph more than the forms that match the different backgrounds.

What is sexual selection?

Sexual selection is a form of natural selection that acts on traits that influence success in obtaining mates. Darwin recognized it in 1871. It occurs in two ways:

• Intrasexual selection: competition between members of one sex (usually males) for access to the other. It favors weapons and size, such as antlers in deer and horns in beetles.
• Intersexual selection (mate choice): individuals of one sex (usually females) choose mates based on traits such as bright plumage, song or displays, as in the peacock’s tail and the bowerbird’s constructions. The traits may signal good genes or health. They may be costly for survival (conspicuous to predators), so a balance is reached between the advantage in mating and the cost in survival.

Sexual selection often leads to sexual dimorphism, a difference between males and females in size or appearance. It is a driver of the evolution of exaggerated traits, which would be eliminated by ordinary natural selection.

The three modes compared

Worked example: predicting the response to selection

In a population of ground finches, the mean beak depth is 9.0 mm. A drought kills the birds with small beaks, and the survivors that breed have a mean beak depth of 10.0 mm. The heritability of beak depth is h² = 0.6 (60 percent of the variation is due to genes).

1. Selection differential: S = mean of the breeding survivors − mean of the original population = 10.0 − 9.0 = 1.0 mm.
2. Response to selection (breeder’s equation): R = h² × S = 0.6 × 1.0 = 0.6 mm.
3. Mean of the next generation: 9.0 + 0.6 = 9.6 mm.

The population has evolved by 0.6 mm in one generation. If a second drought gave the same differential (S = 1.0 mm above the new mean), the next generation would average 9.6 + 0.6 = 10.2 mm. This continues as long as the selection is the same and variation remains. If h² were low, say 0.1, the response would be only 0.1 mm, although the selection was as strong. The size of the response depends on both the strength of selection and the heritability.

For comparison, the effect of stabilizing selection can be shown by a trait in which the survivors have the same mean as the original population (S = 0), so R = 0: there is no change in the mean, but the standard deviation of the trait is reduced, because the extremes are removed. In the case of disruptive selection, the survivors’ mean may equal the original mean, again giving no change in the mean, but the variance increases. This shows why the mean alone does not describe the effect of selection.

How were these modes recognized?

Darwin described natural and sexual selection in On the Origin of Species (1859) and The Descent of Man (1871). Hermon Bumpus studied house sparrows after a severe storm in Rhode Island in 1898, and reported that the birds that died tended to be extreme in size, which was an early demonstration of stabilizing selection, though his analysis has been criticized. Ronald Fisher, J. B. S. Haldane and Sewall Wright developed the mathematics of selection in the 1920s and 1930s. The names directional, stabilizing and disruptive selection were introduced by Kenneth Mather in 1953. Long-term studies by the Grants on the Galápagos since 1973 have documented directional selection in action.

Which mistakes are common?

Mistake 1: Saying that stabilizing selection means no selection

It is active selection against the extremes. The mean stays the same, but variation is reduced.

Mistake 2: Thinking that individuals change their traits

Selection changes the proportion of individuals with different traits. Individuals do not change.

Mistake 3: Mixing up directional and disruptive

Directional favors one extreme. Disruptive favors both extremes and acts against the average.

Mistake 4: Ignoring heritability

If the trait is not inherited, selection will cause no change in the next generation.