How is the direction selected?

What is direction selection? (With examples)

Directional Selection, also called diversification, is one of the three main ways in which natural selection acts on a certain quantitative character. In general, this type of selection occurs on a particular feature and increases or decreases in size.

Natural selection changes the parameters of a quantitative character in the population. This continuous character is usually represented in a normal distribution curve (also known as a bell graph, see picture).

Let's say we evaluate the size of the human population: on the sides of the curve we have the tallest and smallest people, and in the middle of the curve we have the average size people who are the most common.

Depending on how the character spread graph is changed, it is assigned some kind of selection. In case the smaller or larger people are the preferred, we have the case of directed selection.

What is natural selection?

Natural selection is an evolutionary mechanism proposed by the British naturalist Charles Darwin. Contrary to popular belief, it is not the survival of the fittest. In contrast, natural selection is directly related to the reproduction of individuals.

Natural selection is differentiated reproductive success. In other words, some individuals reproduce more than others

Individuals who carry certain beneficial and inheritable traits pass them on to their offspring, and the frequency of these individuals (especially this genotype) is increasing in the population. Therefore, biologists consider the change in allele frequencies to be evolution.

In the case of quantitative characteristics, the selection can be made in three different ways: directional, stabilizing and disruptive. Each is defined by the way they change the mean and variance of the character distribution curve.

Direction selection model

People at one end of the curve have a larger one fitness

The directional selection works as follows: When distributing the frequencies of phenotypic signs, the persons are selected who are on one of the sides of the curve, either left or right.

When the two ends of the distribution curve are selected, the selection is disruptive and undirected.

This phenomenon occurs because people at one end of the curve have a larger fitness or have biological effectiveness. This means that individuals with the trait in question are more likely to reproduce and that their offspring are more fertile than individuals lacking the trait under study.

Organisms live in environments that are constantly changing (both biotic and abiotic components). If a change persists over a long period of time, this can lead to a certain heritable trait being preferred.

For example, if it is convenient to be small in a given environment, the incidence of people who are smaller will increase.

How do the mean and the variance vary?

The mean is a central trend value and allows us to know the arithmetic mean of the sign. For example, the average height of women in the human population of a given country is 1.65 m (hypothetical value).

Variance, on the other hand, is a measure of the spread of values โ€‹โ€‹- that is, how much separate each of the values โ€‹โ€‹of the mean is from one another.

This type of selection is characterized by shifting the value of the mean (over the generations) and keeping the value of the variance relatively constant.

For example, if I measure the size of the tail in a population of squirrels and see that the average population moves to the left side of the curve over the generations, I can suggest that direction selection and the size of the tail is decreasing.


Directional selection is a common occurrence in nature and also with artificial selection by humans. However, the best-described examples correspond to the latter case.

Throughout history, humans have tried to modify their pets very carefully: chickens with larger eggs, larger cows, smaller dogs, etc. Artificial selection was very important to Darwin and served as an inspiration for the theory of natural selection

Something similar happens in nature, only that the different reproductive success between individuals is due to natural reasons.

Changes in the size of the insect's beak Jadera haematoloma

These insects are distinguished by the fact that the fruits of certain plants traverse their long beaks. They are native to Florida, where they source their diet from native fruits.

In mid-1925, a plant similar to the native one (but from Asia) and with smaller fruits was introduced to the United States.

J. haematoloma began to use the smallest fruits as a source of food. The new food source favored the increase in insect populations from shorter peaks.

This evolutionary fact was identified by researchers Scott Carroll and Christian Boyd after analyzing the highlights of the insects in collections before and after the introduction of Asian fruit trees. This fact confirms the great value of animal collections to biologists.

Size changes in pink salmon ( Onchorhynchus gorbuscha )

A decline in animal size has been observed in pink salmon over the past few decades. In 1945 fishermen began using nets for massive animal trapping.

With the continued use of fishing techniques, the salmon population became smaller and smaller.

Why? The fishing net acts as a selective force that pulls the population out of the larger fish (they die and leave no offspring), while the smaller ones are more likely to escape and reproduce.

After 20 years of intensive fishing with nets, the average salmon population has declined by more than a third.

Size of the brain of the genus homo

We humans are characterized by a large brain size if we compare it to our relatives, the great African monkeys (certainly our ancestors had a similar brain size and then increased in the course of evolution).

Larger brain size has been associated with a significant number of selective advantages in terms of information process, decision making, and more.