A typical example of stabilizing selection is natural selection associated with individuals with a deviation of the main characteristics, in comparison with the average norm.
Features of selection
Each generation gets rid of individuals that differ from the optimal average parameter for certain signs. An example of stabilizing selection in wildlife is associated with maintaining the state of the population. For a full-fledged existence, its representatives try to select the maximum conditions for adaptability to certain conditions.
Options in nature
An example of stabilizing natural selection in nature is the maximum contribution to the gene pool of new generations from the most prolific individuals. But scientists were able to prove by conducting numerous observations of natural populations of mammals and birds that in reality the situation is somewhat different. If there are a large number of chicks in one nest, it is rather difficult to feed them, therefore they are much smaller and weaker than those that grow on average. Thus, the researchers were able to reliably establish examples of the action of stabilizing selection, confirm the adaptability to survival in birds with an average degree of fecundity.
Opting for averages
When comparing birds with different numbers of offspring, it turned out that there are several signs that characterize examples of a stabilizing form of selection. Newborn mammals with low weight, as well as too much body weight, mostly died in the 1-2 weeks of life. As for cubs with average parameters, they easily tolerated the first weeks of their existence, developed, and died in minimal quantities.
Consider another example of stabilizing selection associated with birds. When during the experiment it was decided to analyze the size of the wings of birds that died after a severe storm, it turned out that most of them had either too short or, on the contrary, very long wings. This example of stabilizing selection also indicates better survival of individuals with average traits.
Causes of low fitness
Considering this example of the action of the stabilizing form of natural selection, we will try to identify the main reasons for the low fitness of individual individuals to constant conditions of existence. Why is it impossible to clear a certain population of evading undesirable forms using natural selection? The reason lies not only in the fact that, as new offspring are born, various mutations occur, but also because heterozygous genotypes will often be adapted individuals. In the process of crossing, they give splitting in the offspring, and new homozygous generations arise, in which the adaptability to survival conditions is significantly reduced. This phenomenon is called balanced polymorphism.
Examples of polymorphism
The main examples of the stabilizing form of natural selection (polymorphism) are sickle cell anemia. This severe blood disease is observed in people homozygous for hemoglobin with mutant alleles (HbS), leading to death at a young age. Most of the human populations have a low frequency of this allele, it is associated with certain mutations. But scientists were able to establish a relationship between the presence of this gene in the human body and the presence of malaria in the area. Research results showed that HbS-type heterozygotes are more resistant to a disease such as malaria than homozygotes with a normal allele.
Mechanism of variability
Examples of stabilizing and moving selection have a certain mechanism of accumulation of signs of variability in natural populations. For the first time, such a distinguishing feature of stabilizing selection was noted by the outstanding scientist I. I. Shmalgauzen. He managed to prove that even under stable conditions of existence, natural selection does not stop for a minute, evolution continues. Even with the same phenotype, the population continues to evolve. The example of the action of the stabilizing form of selection considered by him confirmed a constant change in the genetic composition. Thanks to stabilizing selection, such genetic patterns are created that help to create optimal phenotypes from various genotypes.
Purpose of the stabilizing form of natural selection
It is able to protect the formed genotype from the negative effects of the mutation process. An example of the action of the stabilizing form of selection is the existence of such ancient species as ginkgo, hatteria. It is stabilizing selection that has preserved to our time the "living minerals" that live in stable environmental conditions:
- Hatteria, which has the features of reptiles that existed during the Mesozoic era.
- Coelacanth, a descendant of cysteran fishes, known from the Paleozoic era.
- A North American possum, a marsupial animal that has existed since the Cretaceous.
- Ginkgo gymnospermous plant, similar to woody forms that became extinct during the Jurassic period of the Mesozoic era.
Such a stabilizing form of natural selection is valid up to the moment when there are those conditions under which a certain characteristic or property has been formed.
Environmental Impact on Volatility
Constant conditions are not necessarily constant over a long period of time. Due to constant changes in environmental conditions, adaptation occurs with the help of stabilizing selection of certain individuals to them. There is a change in breeding cycles, so that the young growth has developed at that time when there is enough food resources to support life. If descendants are born sooner or later than expected, they are eliminated by stabilizing selection. How do plants and animals “learn” about the onset of winter? Short-term temperature drops are very misleading. In addition, an annual shift in the boundaries of summer and winter is observed. Animals that hastily respond to signals may be left without offspring. Therefore, many birds and mammals are guided by the duration of daylight hours. It is this signal for many species of animals that stimulates the launch of important functions: molting, migration, reproduction. I. I. Schmalhausen was able to prove the connection of universal adaptation with stabilizing selection.
Deviation Options
Stabilizing selection completely eliminates all deviations from the established norm, contributes to the formation of genetic mechanisms that ensure the full development and formation of ideal phenotypes based on various genotypes. The result will be the full functioning of organisms even with fluctuations in the environment.
The doctrine of A. Wallace and C. Darwin
The theory of natural selection was created as the main creative force that guides the process of evolution and determines its forms. The process, due to which only those individuals who have hereditary traits useful for specific living conditions, survive and have offspring, began to be considered natural selection. When assessing natural selection from the point of view of genetics, we can conclude its significance for the selection of positive mutations and genetic combinations. They can occur due to sexual reproduction, and as the population exists, it can be improved by rejecting negative combinations and mutations.
Those organisms that have low-quality genes are not able to survive in certain conditions, die. Natural selection is able to "work" on the basis of the reproduction of living organisms, if the weakened individuals are not ready for full-fledged offspring or do not leave offspring at all. In this case, not only the selection and rejection of certain negative qualities of a living organism takes place, but genotypes that carry similar signs are completely destroyed.
About forms of natural selection
At the moment, it is customary to single out the following forms of such selection, it is precisely these that are discussed in biology textbooks in schools.
- Stabilizing natural selection.
- Driving selection.
- Tearing selection.
Driving selection is typical for changing environmental conditions under which a factor appears that has become mutational. For example, industrial melaninogenesis, characteristic of birch moth butterflies , is associated with darkening due to industrial soot trunks of birch trees. Since insects became visible against the backdrop of “new” trees, they were quickly destroyed by birds. The dark mutants of the butterflies survived, gave birth, and therefore gradually the dark mutant butterflies became the dominant form for this population.
Due to the shift of the average value towards the existing factor, the occurrence of cold-loving and heat-loving animals and plants is explained. Driving selection has led to the adaptation of bacteria, fungi, other pathogens of human diseases, animals to various pesticides and drugs. Driving selection can explain the occurrence of eye reduction in cave dwellers and moles, as well as the loss of wings of some birds. With this selection option, there is no branching of characters, as a result of which the bearing genotypes are gradually replaced by others, without forming evasive and transitional forms.
Tearing selection allows you to get extreme types of devices, while all intermediate forms die out. Due to disruptive selection, two or more forms of variability are formed, which lead to polymorphism. It is the struggle for existence that is the important factor that is the main mechanism of any natural selection. Competition, predation, amensalism are considered the three main types of struggle for existence.