The directional selection, Also called diversifying, it is one of the three main ways by which natural selection acts on a certain quantitative character. Generally, this type of selection occurs on a particular trait and increases or decreases its size.
Natural selection modifies the parameters of a quantitative character in the population. This continuous character is usually plotted on a normal distribution curve (also called a bell plot, see image).
Suppose we are evaluating the height of the human population: on the sides of the curve we will have the largest and smallest people and in the center of the curve we will have people with average height, which are the most frequent.
Depending on how the character's distribution chart is modified, a selection type is attributed to it. In the event that the smallest or largest individuals are favored, we will have the case of directional selection.
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Natural selection is an evolutionary mechanism proposed by the British naturalist Charles Darwin. Contrary to popular belief, it is not survival of the fittest. In contrast, natural selection is directly related to the reproduction of individuals.
Natural selection is differential reproductive success. In other words, some individuals reproduce more than others.
Individuals who carry certain advantageous and heritable characteristics transmit them to their descendants, and the frequency of these individuals (specifically of this genotype) increases in the population. Thus, the change in allele frequencies is what biologists consider evolution.
In quantitative traits, selection can act in three different ways: directional, stabilizing, and disruptive. Each one is defined by the way in which they modify the mean and variance of the character distribution curve..
Directional selection acts as follows: in the frequency distribution of phenotypic characters, individuals that are found on one of the sides of the curve, either the left or the right, are selected..
In case the two ends of the distribution curve are selected, the selection would be of the disruptive and non-directional type..
This phenomenon occurs because individuals at one end of the curve have greater fitness or biological efficacy. This means that individuals with the trait in question are more likely to reproduce and their offspring are fertile, compared to individuals who lack the trait studied..
Organisms live in environments that can constantly change (both biotic and abiotic components). If any change persists for a long period of time, it can lead to favor a certain heritable trait.
For example, if in a given environment it is favorable to be small, individuals of smaller sizes will increase in frequency..
The mean is a value of central tendency, and it allows us to know the arithmetic average of the character. For example, the average height of women in the human population of a certain country is 1.65 m (hypothetical value).
The variance, on the other hand, is a dispersion value of the values - that is, how much each of the values is separated from the mean.
This type of selection is characterized by shifting the value of the mean (as the generations pass), 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 in the course of generations the mean of the population shifts to the left side of the curve, I can propose that directional selection is occurring and the size of the queue is decreasing.
Directional selection is a common event in nature, and also in the events of artificial selection by humans. However, the best described examples correspond to the latter case..
In the course of history, humans have sought to modify their companion animals in a very precise way: chickens with larger eggs, larger cows, smaller dogs, etc. Artificial selection was of great value to Darwin, and indeed served as the inspiration for the theory of natural selection.
Something similar happens in nature, only that differential reproductive success between individuals comes from natural causes.
These insects are characterized by going through the fruits of certain plants with their long beaks. They are native species of Florida, where they obtained their food from native fruits.
In the middle of 1925, a plant similar to the native one (but from Asia) and with smaller fruits were introduced in the United States..
J. haematoloma began to use the smallest fruits as a source of food. The new food source favored the increase in the population of insects with shorter beaks.
This evolutionary fact was identified by researchers Scott Carroll and Christian Boyd, after analyzing the peak of insects in collections before and after the introduction of Asian fruit trees. This fact confirms the great value of animal collections for biologists.
In pink salmon, a decrease in the size of the animals has been identified in recent decades. In 1945, fishermen began to implement the use of nets for the mass capture of animals.
With the prolonged use of the fishing technique, the salmon population started to get smaller and smaller..
Why? The fishing net acts as a selective force that takes the larger fish from the population (they die and leave no offspring), while the smaller ones are more likely to escape and reproduce..
After 20 years of extensive net fishing, the average salmon population size decreased by more than a third.
We humans are characterized by having a large brain size, if we compare it with our relatives, the great African apes (surely our ancestor had a similar brain size, and then in the course of evolution it increased).
A larger brain size has been related to a significant number of selective advantages, in terms of information processing, decision-making, among others.
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