A population is a set of living organisms of one species that are partially or completely isolated from their counterparts. In this article we will consider its main "wealth", the reason for the feature. Namely, we will answer the question about what is the gene pool of a population. Does it change and why? Does the driving forces of evolution affect him? Why are the gene pools of populations of the same species heterogeneous? Let's start by defining the concept.
What is the gene pool of a population?
The gene pool, the gene pool (also known as the gene pool) is one of the basic concepts of population genetics. It denotes the entire set of alleles (variations of the same gene) of a certain isolated group of living organisms.
Having learned what the population’s gene pool is, the reader will ask: “Why does the population need this variety of alleles?” This is necessary for more optimal adaptation to the environment. Today, the survival rate is higher for brownish sparrows, and tomorrow the conditions will change, and grayish birds will be more likely. Single individuals will die, but the population will live thanks to the saving gene.
Note that gene exchange is characteristic not only for representatives of one population, but also for the whole species, since isolation between them is not absolute. Therefore, we can talk about the gene pool of the species.
Origin of the concept
For the first time he told the world what the gene pool of a population is, in 1928, the Soviet geneticist A. S. Serebrovsky. It was he who formed the concept of this phenomenon. And the name "gene pool" was chosen by him to emphasize that the wealth of gene variations is the most valuable for the biological world.
In Western science, the concept appeared thanks to F. G. Dolzhansky. In English, it sounds like a gene pool.
Types and features of gene pools
All populations, based on the subject of our conversation, can be divided into two types:
- Monomorphic - there is only one allele of a gene.
- Polymorphic - a completely different situation is observed. The gene has several different variations.
If we consider a species in which the set of chromosomes is more than one, then the total number of alleles will exceed the number of organisms in the group. In most cases, there are more members of the population than gene variations. With strong inbreeding (crossing closely related organisms within the same population), monomorphic groups of organisms with only one allele can be observed.
The gene pool volume indicator is the effective population size (N e ). The higher it is, the larger the gene pool. But how useful is the population? It all depends on environmental conditions. If they are constantly changing, then a population with a rich gene pool is more likely. It will adapt to a new life much faster than the desired allele appears in another group as a result of mutations.
But if the living conditions are quite stable, then the population with the smaller gene pool is “in plus” - the probability is lower that individuals with an “unsuccessful” genotype will be born whose chance of survival and reproduction of offspring is very small.
Population and evolution
A change in the gene pool of a population is a change in the gene pool of a species. After all, it is she, and not a separate individual, that is the unit of evolution. A population is a stable holistic entity that has the following characteristics:
- Being in relative isolation, which does not allow free exchange of hereditary information with other groups of the species.
- Crossing freely among themselves, individuals form the gene pool of the population (in the lesson of class 11, the "Gene Pool" this issue is examined in detail). There is a constant exchange of genes: only those that pass the test of natural selection remain. They are the wealth of the gene pool of the population.
- It exists in a certain area for several years. During this time, a considerable number of generations are changing, which makes it possible for evolutionary processes.
Population and driving evolutionary forces
Consider the influence of the driving forces of evolution on the gene pool of a population.
Mutations . Under normal conditions, they are quite rare. They provoke radioactive, ultraviolet radiation, chemical components, a number of other factors. Mutations harmful in some conditions can become extremely useful in others; therefore, this is a very valuable phenomenon for the population gene pool.
In overwhelming amounts, they are recessive, therefore, they appear in the phenotype only in a homozygous state. If the mutation is dominant, then its signs can be seen already in the first generation.
"Waves of life" (population waves) . Significant fluctuations in the population. Sometimes the reasons for this are natural (for example, an increase in the number of insects in spring and a decrease in autumn), sometimes caused by natural disasters.
"Waves of life" also affect the change in the gene pools of populations (in the 11th grade this is considered in biology classes), because affect the direction and intensity of natural selection.
The drift of genes . A random change in a small population of allele frequencies. The consequence may be the disappearance of recessive genes, an increase in the number of homozygous individuals, and the appearance of genetic variations rare for the species. It is determined by various factors - isolation, population waves, mutations.
Isolation . It is divided into geographical (the presence of any spatial barriers) and biological (limiting or eliminating the possibility of interbreeding between individuals of the same species - different mating seasons, different behavior during the mating season). Preventing the free crossing of individuals of different populations, isolation exacerbates the genetic differences between them.
It is important to note that isolation does not contribute to the enrichment of the gene pool with new material, the emergence of other genotypes.
All factors listed by us act spontaneously. If, as a result of their impact, a change in the gene pool of populations occurred (in the lesson in grade 11, the teacher explains this phenomenon in detail), then this is a purely random result. The directed action is only for one driving force of evolution - natural selection. It is he who contributes to the survival of the fittest, preserving their genotypes in the general pool of the population.
From here we can divide the causes of changes in the gene pool of populations into two branches - random and directed. Let's consider them in more detail.
Random reasons for changing the gene pool
So what we can highlight here:
- Migration of some individuals to another place of residence. Their gene pool will no longer exactly repeat the "parent" due to the fact that the genotypes of these representatives do not include all the variety of alleles. Rare genes can spread, qualitatively new mutations can pass.
- The population was separated by a natural or artificial barrier. Here, gene pools will also become qualitatively different after a while, not similar either to the former or to each other.
- The fact of natural disaster. The general fund will consist of randomly selected genotypes of a small group of survivors. However, some mutations may disappear altogether, while others will spread widely.
- Adverse conditions causing the mass death of individuals. Some genes are lost, including rare ones, which may have become the reason for less vitality.
- Periodic fluctuations in numbers. For example, the pool of the insect population in the spring consists of the genotypes of individuals who managed to survive the winter.
From here we see that the effect of random factors on the gene pool is negative - they impoverish it. But in the end, a very viable, qualitatively different population arises with its own peculiar gene pool.
Directed cause of gene pool change
Here we take only one factor - natural selection. It helps to increase the frequencies of the genes most useful for certain conditions and to reduce unnecessary, harmful to a given location. Therefore, it enriches the gene pool, being a kind of "orderly".
Natural selection changes the phenotypes of individuals, their behavior, appearance, lifestyle. The purpose of these changes is the greatest adaptability to certain living conditions.
Causes of Population Differences
From the foregoing, it is easy to isolate the causes of differences in the gene pools of isolated populations of the same species:
- Different living conditions. For some factors, individuals with one genotype are better adapted, for some - with another.
- Different directions of mutagenesis. Somewhere, one kind of mutation can happen, somewhere else. In one case, it is dominant, in the other recessive.
- The phenomenon of crossing over is the exchange of chromosome sites.
First of all, the difference in gene pools causes isolation. Some changes accumulate, are inherited, increasingly alienating populations of the same species category in similarities.
Is the stability of the gene pool of a population possible?
Can a population’s gene pool not be variable, mobile? Theoretically, yes. This is confirmed by the well-known Hardy-Weinberg law: after a certain period, the frequencies of the genes reach equilibrium, and then the total fund remains unchanged.
However, for such stability, a number of conditions must be met:
- Lack of migration of individuals with certain genotypes.
- Infinitely large population.
- Only random crossing.
- The constancy of living conditions.
However, in the real world, compliance with all these conditions is almost impossible, which is why changes in the population gene pool cannot be avoided. Stability can only be created artificially.
We figured out the reason for the difference in the gene pools of isolated populations. We also learned what the gene pool is in general, and what causes (including the driving forces of evolution) affect its change.