Coping with Consistency: Normalizing Selection in Uniform Environments

Normalising selection is a kind of natural selection in which the population mean stabilises on a certain non-extreme trait value. Since, most features do not appear to change much over time, this is regarded to be the most typical method of action for natural selection. Stabilising selection tends to eliminate the more severe phenotypes, resulting in the reproductive success of the more moderate or average phenotypes.

This signifies that the population's most frequent phenotype is chosen for and will continue to dominate in future generations.

Natural Selection in Uniform Environments: Normalizing Selection

In the absence of large-scale environmental change, populations frequently retain a stable genetic constitution for numerous features in more or less homogenous settings. Natural selection preserves genetic equilibrium in the absence of environmental change. This is referred to as normalizing selection. Several of an individual's phenotypic characteristics can be placed on a linear scale. The distribution curve of the attributes generally takes the shape of a bell, with more people at intermediate levels and fewer people at the extremes.

This inclination is passed down from generation to generation. If there is a significant selection pressure against the phenotypes at the extremities of the standard curve, the population may exhibit reduced variability even if the mean remains constant. Natural selection tends to normalize or stabilize populations with intermediate trait values, and individuals with intermediate trait values have a greater chance of surviving.

For example, newborns weighing significantly less or much more than average have a high death risk. Infants of moderate weight, on the other hand, have fewer survival issues. To understand the notion of normalizing selection, we will look at two examples, one from nature and one from Dobzhansky and Spassky's research.

The Observation of Bumpus

An American scientist, H.C. Bumpus (1899), presented an intriguing insight that reasonably explains normalizing selection. After a heavy snow and sleet storm with high winds at Woods Hole, Bumpus gathered 136 wounded house sparrows. Sixty-four of these birds perished, resulting in two groups of sparrows: those killed by the storm and those that survived.

Bumpus measured various randomly selected features like wing length, wing span, tarsus length, and so on and discovered that those killed by the storm had measures that fell at the extremities of the bell-shaped curve. In other words, birds with mean or near-mean measures were the ones who survived.

Normalizing selection often removes people whose features deviate significantly from the mean values during a disaster or a stressful event. According to Bumpus' observations, the birds that were easily blown down by the winds either had wings that were too long for their body weight and thus presented a larger surface area, or they had wings that were too short for their body size and thus could not fly against strong winds.

Before the disaster, the range of individual measures was more comprehensive than the range of measurements that survived the disaster. Every disaster can reduce population diversity, selecting inferior genes.

The Experiments of Dobzhansky and Spassky

Boris Spassky and Theodosius Dobzhansky established the operation of normalizing selection on a behavioral feature in two populations of Drosophila pseudobscura. Artificial selection was used on both populations. One group was chosen for positive phototactic conduct, while the other was for negative behavior. Flies were placed in a container and allowed to fly toward light or darkness.

Photopositive flies, or those that went towards the light, were caught and bred, as were photonegative flies (those that moved away from light). The breeding trials were repeated for numerous generations, with artificial selection maintained for each generation. After multiple generations of artificial selection, two populations emerged, one highly photopositive and the other becoming photonegative.

More crucially, after the artificial selection was removed, natural selection rewarded individuals with neutral light behavior, and both populations recovered to an intermediate phototactic score. The above two examples demonstrate how selection often favors phenotypes in the middle of the distribution, lowers variability around the mean, but does not affect the mean value.

Example of Normalising Selection

  • Weight of Human baby at birth is a famous illustration of this. newborns with low body weight lose heat more rapidly and are more susceptible to infectious infections, whereas newborns with high body weight are more difficult to deliver through the pelvis. Infants of a more moderate weight survive far more frequently. The newborn mortality rate is substantially greater for bigger or smaller babies. The human population bell curve peaks at a birth weight at which newly born has the lowest mortality rate.

  • The African butterfly Bicyclus anynana demonstrates stabilising selection with its wing eyespots. It has been argued that the circular eyespots on the wings are more useful than other forms and sizes.

  • The Eurosta solidaginis fly lays its eggs on the tips of plants, encasing the larvae in a protective gall. Predation determines the size of this gall, which is subject to stabilising selection. Parasitic wasps, which deposit a single egg in galls containing flies, pose a hazard to these larvae. To live, the solitary wasp offspring devour the fly larvae. As a result, a bigger gall is preferred to provide more locations for larvae to hide from the wasp. Larger galls, on the other hand, attract a distinct sort of predation from birds, who may pierce huge galls with their beak. As a result, the ideal gall is modest in size to escape predation from both birds and wasps.

  • In terms of leg muscles, the Siberian husky has stabilising selection. These dogs must be strong enough to pull sleds and move rapidly. They must, however, be light enough to stay on top of the snow. This indicates that the husky's leg muscles are best fit when they are reasonably big, allowing them to balance their strength and weight.

  • Plant height is another feature that might be influenced by stabilising selection. A plant that is too short may be unable to compete for sunlight with other plants. Extremely tall plants, on the other hand, may be more vulnerable to wind damage. These two selection forces work together to keep plants of medium height. The quantity of medium-sized plants will rise, while the number of short and tall plants will drop.


Selection operates on genotypes based on the contexts in which they exist. If the environment is uniform and homogenous, the type of selection that occurs is the normalizing or stabilizing type, which weeds out phenotypes with extreme measurements, providing the selection pressure on such characteristics is quite strong. Under these conditions, population variability is reduced, but the mean value of the characteristic stays constant.

However, if the environment changes, the mean value of the feature will vary since selection acts unequally on the two extremes of the normal distribution. The mean value is moved to a new location. Insect resistance to pesticides is one example of this selection, also known as directed or progressive selection.

Updated on: 04-May-2023


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