WHAT IS DISRUPTIVE SELECTION? (WITH EXAMPLES) - BIOLOGY - 2025

The disruptive selection is one of the three ways by which natural selection acts on quantitative traits in organisms. Disruptive selection is responsible for selecting more than two values ​​of a character from the population, and the average forms decrease.

For example, consider some kind of bird that eats seeds. If we graph the frequency of the size of the peaks, we will obtain a normal distribution: a bell-shaped curve, where the maximum point represents the individuals with the most frequent peaks.

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Suppose that the climatic conditions of the animals' habitat only allow the production of very small and very large seeds. Finches with very small and very large beaks will be able to feed, while individuals with intermediate size beaks will be adversely affected.

What is natural selection?

Selection can occur in nature under different modalities, depending on the relationship between phenotype and fitness.

One of the many faces of selection is disruptive selection. However, before defining this type of selection, it is necessary to understand a basic concept in biology: natural selection.

The year 1859 represented a stage of radical change for the biological sciences with the arrival of the theory of natural selection. This was formulated by the famous British naturalist Charles Darwin in his book The Origin of Species, where he proposed this mechanism.

Natural selection occurs as long as three conditions are met in a population: there is variability, organisms have certain characteristics that increase their fitness, and this characteristic is heritable.

In evolutionary biology, the term fitness or biological efficacy refers to the ability of an individual to reproduce and have fertile offspring. It is a parameter that goes from 0 to 1.

It is worth noting that natural selection is not the only evolutionary force, gene drift also has a relevant role in evolutionary change, particularly at the molecular level.

Disruptive natural selection model

Individuals at both ends of the curve have greater

Directional selection occurs when individuals located at both ends of the frequency distribution have greater fitness than central individuals. Over the generations, favored individuals increase their frequency in the population.

In disruptive selection models, more than two genotypes may be favored.

From a genetic perspective, disruptive selection occurs when the heterozygote has a lower fitness than the homozygous.

Let's take the hypothetical example of body size. Suppose that in a population of organisms, the smallest and largest have the advantage (escape from predators, obtaining food, among other reasons). In contrast, organisms of average height will not have as high a reproductive success as their counterparts.

How do the mean and variance vary?

A common and fairly widespread methodology among biologists is the measurement of the effects of natural selection on phenotypic variation by means of changes in the mean and in the variation of traits over time.

Depending on how they change, the selection is classified in three main ways: stabilizing, directional, and disruptive.

In the frequency distribution graphs of the quantitative traits evaluated we can quantify several of the mentioned parameters.

The first is the mean or arithmetic average of the trait under study. For example, measure body size in a population of rodents and calculate the mean. This is a measure of central tendency.

The variance is the dispersion of the data with respect to the population mean. If the variance is high, then there is considerable variability of the character studied. If it is low, all the values ​​obtained are close to the mean.

If we study a character in the population and observe that the variance increases over the generations, we can infer that disruptive selection is occurring. Visually, the bell of the graph is expanding with each generation.

Theoretical and evolutionary implications

Disruptive selection has been of great interest to biologists for two main reasons. First, it promotes variation within a species in a population, as we will see later with the beak of finches.

Second, it is proposed that disruptive selection acting for long periods of time could promote speciation events (generation of new species).

Examples

Although disruptive selection events may seem unlikely, they are common in nature - at least in theory. The most prominent examples of disruptive selection are in different species of birds.

The African finch

Generalities of the finch and its diet

Finches of the species P. ostrinus live in central Africa. The diet of this animal is made up of seeds. Most of the populations have small and large forms, both in males and females.

In the environment where finches live, there are multiple species of plants that produce seeds and that these birds include in their diet. The seeds vary in terms of their hardness and size.

Smith's studies on variation in beak size

Smith in 2000 studied the morphometric variation in the beak of finches and found very interesting results.

The researcher quantified the time it takes for a finch to open the seed to consume it. At the same time, he measured the biological fitness of the individuals and related it to the size of the beak. The time period for this experiment was about seven years.

Smith concluded that there are two predominant beak sizes because there are two primary species of seeds that are consumed by finches.

One of the plant species produces very hard seeds, and the larger finches with more robust beaks specialize in consuming this species of seeds.

The other abundant species produces small, soft seeds. In this case, the finch variants that specialize in their consumption are small individuals with small beaks.

In an environment with a bimodal distribution of resources, natural selection shapes a bimodal distribution of the species.

References

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