In modern biology, living matter is considered to be a hierarchical structure. Each level is a system of interconnected elements. Moreover, a separate structural unit is at the same time a collection of “details” of a lower order. The population-species level of organization of life is one of such steps in the hierarchical ladder of organisms. It is on it that all evolutionary changes begin to fully manifest themselves.
Hierarchical model
Living systems are usually combined in four groups:
Molecular genetic level. It contains such components of living organisms as lipids, carbohydrates, proteins and nucleic acids. This level cannot yet be called living, but macromolecules, its components, create the basis for the next stage of development.
Ontogenetic level. It houses cells, organs, tissues and multicellular organisms, from hydra to humans. It is at this level that life first appears.
Population and species level. This article is devoted to the presentation of its features.
Biogeocenotic level. Includes community of organisms, biocenoses and biosphere. This is the level at which the organization of living matter reaches its greatest difficulty.
Some features
Structures located at each of the levels are systemic. They tend to consist of a certain number of elements, constantly interact with the environment, and manage internal processes through self-regulation. They have a boundary that determines where the system ends and the outside world begins. The population-species level is a structure with similar properties. The border separating it from the environment is not a certain physical structure, but the complex relationships of individual individuals and genetic factors.
The population-specific level of organization of life is essential for understanding evolutionary processes. It is at this stage that all the main selection mechanisms are clearly visible. The main elements of the level are species and population.
Selection criteria
There are many kinds of living creatures on our planet. The differences between them are determined by a whole set of characteristics. All of them are different variants of similarity of individuals of the same species:
Morphological sign. In other words, this is the similarity of the external structure.
The uniformity of physiological and biochemical processes. In individuals of one species, metabolism proceeds in a similar way, and the molecular composition of tissues and organs coincides.
Geographical sign. All individuals of this species are located within the same range.
Ecological sign. Organisms of the same species respond similarly to changes in living conditions. For normal functioning, they need a certain level of temperature, humidity, lighting and other parameters.
- Genetic trait. For individuals of one species, the same nucleotide sequence of DNA is characteristic. They have the same number of chromosomes.
Lack of the main
Any of these characters, taken separately and found in a group of individuals, does not guarantee that we have before us the described element that makes up the population-species standard of living. Only all the parameters taken together allow us to state that the group of organisms in question is a single whole. According to their morphological characteristics, the so-called double species may be similar. An example of them are roundworms, generally identical in structure, but differing in habitat. It also happens that individuals of one species differ externally. A common example is a mismatch in the color and size of the females and males of some birds or insects.
A single habitat in isolation from other indicators can also lead to an erroneous assignment of individuals to one species. The range is often fragmented due to some landscape features. And vice versa, individuals of completely different species often live together on the same territory.
Definition
Similar examples can be found for any of the listed parameters. The elements that make up the population-species level of organization of life can be distinguished only using the entire set of characters. The most significant is the free crossbreeding of individuals and prolific offspring. Based on these features, a definition of the concept can be derived. A species is a collection of individuals that are similar in internal and external structure, as well as the course of life processes, occupying the same range and able to freely interbreed with each other, leaving posterity capable of reproduction.
Divisions
The population-species level, examples of which are found in any territory, is that stage of the hierarchy of life, where all the mechanisms of natural selection are fully developed. This is where the so-called unit of evolution is located. This is a population, which is also a structural element of the species. The latter is, rather, a systematic unit. In nature, it is impossible to detect a species that is not divided into populations.
This element, which is part of the population-species level, has several characteristics:
- all individuals belong to the same species;
- they inhabit a relatively isolated area in the territory of this species;
- individuals freely interbreed and leave prolific offspring.
Indicators
The division of the species into populations most often occurs as a result of the geographical or biological isolation of one group of individuals from others. In the first case, they are separated by mountains, lakes, rivers or other natural barriers. In the second - as a result of slightly different needs in the environment, differences in behavior or the presence of mutations, the possibility of crossing individuals of different groups disappears.
Populations have a set of indicators such as abundance, birth rate, mortality, and growth. The first is a collection of all individuals. The population is distinguished by the ability to self-regulate its numbers. The restraining factor is the environmental resistance: as a result of an increase in the number of individuals, the food supply in this territory decreases, other conditions worsen. The answer to this will be a decrease in numbers - its restoration to a certain average level.
Important indicators of this element, which is part of the population-species level of living organization, are fertility and mortality. They represent the number of individuals that appeared and died over a given period, respectively. The difference between them is called growth. It is negative and positive. In the first case, the population decreases, and in the second it increases.
Structure
Individuals of the element under consideration, which is part of the population-species level of organization of life, differ by sex and age. These indicators formed the basis for identifying the corresponding structures. The ratio of males and females is usually one to one, however, due to external factors, disharmony in this parameter may occur. The simultaneous presence in the population of individuals of different ages contributes to its greater adaptability. Moreover, an increase in the number of “young growth” allows us to predict an increase in the future population size.
A behavioral structure that is characteristic only of animals is also distinguished. Individuals in a population can be single or form flocks, families, and herds. The former will sooner or later seek societies of their own kind, since otherwise reproduction is impossible. The flock is characterized by the presence of a large number of imitative reactions, a clear internal order developed by the alarm system. During the breeding season, as a rule, it splits into pairs. In the family, the relations of offspring and parents become stronger. A good example of this type of behavioral structure is the Lion Pride, which consists of one male, several females and their cubs.
A herd is the most permanent association of animals. It is characterized by a strict hierarchy led by a leader.
Unit of evolution
As already noted, the population-species level of organization is that step in the hierarchy of living systems at which the evolutionary process can be fully traced. Changes begin with a population. Individuals, its components, have a gene pool, that is, a set of hereditary material of all organisms. It is characterized by the ability to directional change. A population is called a unit of evolution, since an individual organism cannot change during life due to the fixed set of its genes.
Evolutionary material
A change in the gene pool occurs as a result of the appearance and accumulation of mutations. They appear quite rarely and can affect any symptom. Dominant and recessive mutations are distinguished. The first, appearing, immediately appear. Individuals with a new trait are then subjected to natural selection. If the mutation is useful, then it is fixed. Gradually, the number of individuals with this trait increases in the population.
Recessive mutations that occur in nature much more often than dominant ones are initially inactive. They accumulate in the gene pool for an often quite long period. Upon reaching a certain level of concentration of such mutations, they can manifest themselves in the form of a new trait, and the process will proceed similarly to that described above.
Also, the appearance of various characteristics of individuals is possible on the basis of mixing (combination) of the material available in the gene pool as a result of free crossing. Moreover, the number of possible variations will be the greater, the more impressive the population size.
Directional change
In relatively calm, that is, constant, conditions in the population, individuals with a different set of characters coexist. In this case, a certain average composition of genes is supported. In the case when individuals are subject to the constant influence of aggressive environmental factors, only the most adapted organisms will survive. This is how natural selection works , vigilantly “controlling” the population-species level. Examples of its effects make up the entire evolution of the animal world. Such changes in the gene pool are a prerequisite for all larger transformations.
In the natural hierarchy, the most important and necessary structures cannot be distinguished. Each higher level of development is impossible without previous developments. However, a new step is always qualitatively different from a simple sum of features of the underlying ones. Thus, the population-species level of organization of living matter becomes the "field of activity" of natural selection, the main evolutionary process.