The population is characterized. Population types
In nature, each existing species is a complex complex or even a system of intraspecific groups, which include individuals with specific structural features, physiology and behavior. This intraspecific association of individuals is population.
The word “population” comes from the Latin “populus” - people, population. Hence, population- a collection of individuals of the same species living in a certain territory, i.e. those that only interbreed with each other. The term “population” is currently used in the narrow sense of the word, when talking about a specific intraspecific group inhabiting a certain biogeocenosis, and in a broad, general sense - to designate isolated groups of a species, regardless of what territory it occupies and what genetic information it carries.
Members of the same population have no less impact on each other than physical environmental factors or other species of organisms living together. In populations, all forms of connections characteristic of interspecific relationships are manifested to one degree or another, but most clearly expressed mutualistic(mutually beneficial) and competitive. Populations can be monolithic or consist of subpopulation-level groups - families, clans, herds, packs and so on. The combination of organisms of the same species into a population creates qualitatively new properties. Compared to the lifespan of an individual organism, a population can exist for a very long time.
At the same time, a population is similar to an organism as a biosystem, since it has a certain structure, integrity, a genetic program for self-reproduction, and the ability to reproduce and adapt. The interaction of people with species of organisms found in the environment, in the natural environment or under human economic control, is usually mediated through populations. It is important that many patterns of population ecology also apply to human populations.
Population is the genetic unit of a species, changes in which are carried out by the evolution of the species. As a group of cohabiting individuals of the same species, a population acts as the first supraorganismal biological macrosystem. A population's adaptive capabilities are significantly higher than those of its constituent individuals. A population as a biological unit has certain structure and functions.
Population structure characterized by its constituent individuals and their distribution in space.
Population functions similar to the functions of other biological systems. They are characterized by growth, development, and the ability to maintain existence in constantly changing conditions, i.e. populations have specific genetic and environmental characteristics.
Populations have laws that allow limited environmental resources to be used in this way to ensure the preservation of offspring. Populations of many species have properties that allow them to regulate their numbers. Maintaining optimal numbers under given conditions is called population homeostasis.
Thus, populations, as group associations, have a number of specific properties that are not inherent in each individual individual. Main characteristics of populations: number, density, birth rate, death rate, growth rate.
A population is characterized by a certain organization. The distribution of individuals across the territory, the ratio of groups by sex, age, morphological, physiological, behavioral and genetic characteristics reflect population structure. It is formed, on the one hand, on the basis of the general biological properties of the species, and on the other, under the influence of abiotic environmental factors and populations of other species. The structure of populations therefore has an adaptive character.
The adaptive capabilities of a species as a whole as a system of populations are much broader than the adaptive characteristics of each individual individual.
Population structure of the species
The space or habitat occupied by a population may vary between species and within the same species. The size of a population's range is determined to a large extent by the mobility of individuals or the radius of individual activity. If the radius of individual activity is small, the size of the population range is usually also small. Depending on the size of the occupied territory, we can distinguish three types of populations: elementary, environmental and geographical (Fig. 1).
Rice. 1. Spatial division of populations: 1 - species range; 2-4 - geographical, ecological and elementary populations, respectively
There are sex, age, genetic, spatial and ecological structures of populations.
Sex structure of the population represents the ratio of individuals of different sexes in it.
Age structure of the population- the ratio in the population of individuals of different ages, representing one or different offspring of one or several generations.
Genetic structure of the population is determined by the variability and diversity of genotypes, the frequencies of variations of individual genes - alleles, as well as the division of the population into groups of genetically similar individuals, between which, when crossed, there is a constant exchange of alleles.
Spatial structure of the population - the nature of the placement and distribution of individual members of the population and their groups in the area. The spatial structure of populations differs markedly between sedentary and nomadic or migrating animals.
Ecological population structure represents the division of any population into groups of individuals that interact differently with environmental factors.
Each species, occupying a specific territory ( range), represented on it by a system of populations. The more complex the territory occupied by a species is, the greater the opportunities for the isolation of individual populations. However, to a lesser extent, the population structure of a species is determined by its biological characteristics, such as the mobility of its constituent individuals, the degree of their attachment to the territory, and the ability to overcome natural barriers.
Isolation of populations
If the members of a species are constantly intermingled and intermingled over large areas, the species is characterized by a small number of large populations. With poorly developed ability to move, many small populations are formed within the species, reflecting the mosaic nature of the landscape. In plants and sedentary animals, the number of populations is directly dependent on the degree of heterogeneity of the environment.
The degree of isolation of neighboring populations of the species varies. In some cases, they are sharply separated by territory unsuitable for habitation and are clearly localized in space, for example, populations of perch and tench in lakes isolated from each other.
The opposite option is the complete settlement of vast territories by the species. Within the same species there can be populations with both clearly distinguishable and blurred boundaries, and within the species, populations can be represented by groups of different sizes.
Connections between populations support the species as a whole. Too long and complete isolation of populations can lead to the formation of new species.
Differences between individual populations are expressed to varying degrees. They can affect not only their group characteristics, but also the qualitative features of the physiology, morphology and behavior of individual individuals. These differences are created mainly under the influence of natural selection, which adapts each population to the specific conditions of its existence.
Classification and structure of populations
A mandatory feature of a population is its ability to exist independently in a given territory for an indefinitely long time due to reproduction, and not the influx of individuals from the outside. Temporary settlements of different scales do not belong to the category of populations, but are considered intra-population units. From these positions, the species is represented not by hierarchical subordination, but by a spatial system of neighboring populations of different scales and with varying degrees of connections and isolation between them.
Populations can be classified according to their spatial and age structure, density, kinetics, constancy or change of habitats and other environmental criteria.
The territorial boundaries of populations of different species do not coincide. The diversity of natural populations is also expressed in the variety of types of their internal structure.
The main indicators of population structure are the number, distribution of organisms in space and the ratio of individuals of different qualities.
The individual traits of each organism depend on the characteristics of its hereditary program (genotype) and how this program is implemented during ontogenesis. Each individual has a certain size, sex, distinctive morphological features, behavioral characteristics, its own limits of endurance and adaptability to environmental changes. The distribution of these characteristics in a population also characterizes its structure.
The population structure is not stable. The growth and development of organisms, the birth of new ones, death from various causes, changes in environmental conditions, an increase or decrease in the number of enemies - all this leads to changes in various relationships within the population. The direction of its further changes largely depends on the structure of the population in a given period of time.
Sexual structure of populations
The genetic mechanism for sex determination ensures that the offspring are separated by sex in a 1:1 ratio, the so-called sex ratio. But it does not follow from this that the same ratio is characteristic of the population as a whole. Sex-linked traits often determine significant differences in the physiology, ecology and behavior of females and males. Due to the different viability of male and female organisms, this primary ratio often differs from the secondary and especially from the tertiary - characteristic of adult individuals. Thus, in humans, the secondary sex ratio is 100 girls to 106 boys; by the age of 16-18 this ratio levels out due to increased male mortality and by the age of 50 it is 85 men per 100 women, and by the age of 80 it is 50 men per 100 women.
The sex ratio in a population is established not only according to genetic laws, but also to a certain extent under the influence of the environment.
Age structure of populations
Fertility and mortality, population dynamics are directly related to the age structure of the population. The population consists of individuals of different ages and sexes. Each species, and sometimes each population within a species, has its own age group ratios. In relation to the population it is usually distinguished three ecological ages: pre-reproductive, reproductive and post-reproductive.
With age, an individual's requirements for the environment and resistance to its individual factors naturally and very significantly change. At different stages of ontogenesis, changes in habitats, changes in the type of food, the nature of movement, and the general activity of organisms can occur.
Age differences in a population significantly increase its ecological heterogeneity and, consequently, its resistance to the environment. The likelihood increases that in the event of strong deviations of conditions from the norm, at least some viable individuals will remain in the population, and it will be able to continue its existence.
The age structure of populations is adaptive in nature. It is formed on the basis of the biological properties of the species, but always also reflects the strength of the influence of environmental factors.
Age structure of plant populations
In plants, the age structure of the cenopopulation, i.e. population of a particular phytocenosis is determined by the ratio of age groups. The absolute, or calendar, age of a plant and its age state are not identical concepts. Plants of the same age can be in different age states. The age-related, or ontogenetic state of an individual is the stage of its ontogenesis, at which it is characterized by certain relationships with the environment.
The age structure of the coenopopulation is largely determined by the biological characteristics of the species: the frequency of fruiting, the number of produced seeds and vegetative rudiments, the ability of vegetative rudiments to rejuvenate, the rate of transition of individuals from one age state to another, the ability to form clones, etc. The manifestation of all these biological characteristics, in turn turn depends on environmental conditions. The course of ontogenesis also changes, which can occur in one species in many ways.
Different plant sizes reflect different vitality individuals within each age group. The vitality of an individual is manifested in the power of its vegetative and generative organs, which corresponds to the amount of accumulated energy, and in resistance to adverse influences, which is determined by the ability to regenerate. The vitality of each individual changes in ontogenesis along a single-peak curve, increasing on the ascending branch of ontogenesis and decreasing on the descending branch.
Many meadow, forest, steppe species, when grown in nurseries or crops, i.e. on the best agrotechnical background, they shorten their ontogeny.
The ability to change the path of ontogenesis ensures adaptation to changing environmental conditions and expands the ecological niche of the species.
Age structure of populations in animals
Depending on the characteristics of reproduction, members of a population may belong to the same generation or to different ones. In the first case, all individuals are close in age and approximately simultaneously go through the next stages of the life cycle. The timing of reproduction and the passage of individual age stages is usually confined to a certain season of the year. The size of such populations is, as a rule, unstable: strong deviations of conditions from the optimum at any stage of the life cycle immediately affect the entire population, causing significant mortality.
In species with single reproduction and short life cycles, several generations occur throughout the year.
When humans exploit natural animal populations, taking into account their age structure is of utmost importance. In species with large annual recruitment, larger portions of the population can be removed without the threat of depleting its numbers. For example, in pink salmon that mature in the second year of life, it is possible to catch up to 50-60% of spawning individuals without the threat of a further decline in population size. For chum salmon, which mature later and have a more complex age structure, removal rates from a mature stock should be lower.
Analysis of the age structure helps to predict the population size over the life of a number of next generations.
The space occupied by a population provides it with the means to live. Each territory can support only a certain number of individuals. Naturally, the complete use of available resources depends not only on the total population size, but also on the distribution of individuals in space. This is clearly manifested in plants, the feeding area of which cannot be less than a certain limiting value.
In nature, an almost uniform, ordered distribution of individuals within an occupied territory is rarely encountered. However, most often the members of a population are distributed unevenly in space.
In each specific case, the type of distribution in the occupied space turns out to be adaptive, i.e. allows optimal use of available resources. Plants in a cenopopulation are most often distributed extremely unevenly. Often the denser center of the aggregation is surrounded by individuals located less densely.
The spatial heterogeneity of the cenopopulation is associated with the nature of the development of clusters over time.
In animals, due to their mobility, the ways of regulating territorial relations are more diverse compared to plants.
In higher animals, intrapopulation distribution is regulated by a system of instincts. They are characterized by special territorial behavior - a reaction to the location of other members of the population. However, a sedentary lifestyle poses the risk of rapid depletion of resources if population densities become too high. The total area occupied by the population is divided into separate individual or group areas, thereby achieving the orderly use of food supplies, natural shelters, breeding sites, etc.
Despite the territorial isolation of members of the population, communication is maintained between them using a system of various signals and direct contacts at the borders of their possessions.
“Securing an area” is achieved in different ways: 1) protecting the boundaries of the occupied space and direct aggression towards a stranger; 2) special ritual behavior demonstrating a threat; 3) a system of special signals and marks indicating the occupancy of the territory.
The usual reaction to territorial marks—avoidance—is inherited in animals. The biological benefit of this type of behavior is obvious. If the mastery of a territory were decided only by the outcome of a physical struggle, the appearance of each stronger alien would threaten the owner with the loss of the site and exclusion from reproduction.
Partial overlapping of individual territories serves as a way to maintain contacts between members of the population. Neighboring individuals often maintain a stable, mutually beneficial system of connections: mutual warning of danger, joint protection from enemies. Normal behavior of animals includes an active search for contacts with members of their own species, which often intensifies during periods of population decline.
Some species form widely wandering groups that are not tied to a specific territory. This is the behavior of many fish species during feeding migrations.
There are no absolute distinctions between different ways of using the territory. The spatial structure of the population is very dynamic. It is subject to seasonal and other adaptive changes in accordance with place and time.
The patterns of animal behavior constitute the subject of a special science - ethology. The system of relationships between members of one population is therefore called the ethological, or behavioral structure of the population.
The behavior of animals in relation to other members of the population depends, first of all, on whether a solitary or group lifestyle is characteristic of the species.
A solitary lifestyle, in which individuals of a population are independent and isolated from each other, is characteristic of many species, but only at certain stages of the life cycle. Completely solitary existence of organisms does not occur in nature, since in this case it would be impossible to carry out their main vital function - reproduction.
With a family lifestyle, the bonds between parents and their offspring also strengthen. The simplest type of such connection is the care of one of the parents for laid eggs: protection of the clutch, incubation, additional aeration, etc. With a family lifestyle, the territorial behavior of animals is most pronounced: various signals, markings, ritual forms of threat and direct aggression ensure ownership of an area sufficient for feeding offspring.
Larger animal associations - flocks, herds And colonies. Their formation is based on the further complication of behavioral connections in populations.
Life in a group, through the nervous and hormonal systems, affects the course of many physiological processes in the animal’s body. In isolated individuals, the level of metabolism changes noticeably, reserve substances are consumed faster, a number of instincts do not manifest themselves, and overall vitality deteriorates.
Positive group effect manifests itself only up to a certain optimal level of population density. If there are too many animals, this threatens everyone with a lack of environmental resources. Then other mechanisms come into play, leading to a decrease in the number of individuals in the group through its division, dispersal, or a drop in the birth rate.
The biological term “population” was first used in 1903 by a biologist from Denmark. Wilhelm Ludwig Johansen (1857 - 1927) to denote the group growth of one plant species.
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General concept
What is a population? She (the ancient Latins said: populus from modern English population - population) is a collection of representatives a specific species of living organisms, a long period of time living or growing in one territorial space, separately from individuals of other groups similar in similar characteristics.
The term is used in various fields of natural sciences: ecology, medicine, demography.
If we take for example, then in the appropriate terminology, the concept is defined as a community of animals or plants of the same species, having a single gene pool(we will consider this term below) capable of sustainable self-reproduction. In biology, it refers to groups of organisms within a certain species.
The simplest example is the human population on Earth. If we take examples from the animal world: sika and red deer, brown and polar bears, cod and haddock in the seas of the Arctic Ocean basin. From the plant world: different types of pine and spruce, aspen and linden, oak and elm.
What parameters characterize each population? The generally accepted criteria are:
- general habitat (area);
- uniform origin of a community of organisms;
- the relative isolation of a given community from other similar groups (so-called interpopulation barriers);
- compliance with the principle of panmixia (free crossing) within the group, in other words, an equal probability of meeting all existing genotypes within the range.
Population types
There are a great many species of living organisms in the wild. First of all, we need to highlight two global populations- animals and plants. And they already define the subspecies of a particular group of organisms
In biology, geographically determined groups are structurally distinguished, for example, the settlement of squirrels in the forests of the Ulyanovsk region. Grouped animals of the same subspecies (in our case, squirrels), living in a geographically homogeneous space. Such an area is called a habitat.
In turn, geographic populations are divided into smaller ones - ecological (squirrels in coniferous and mixed forests of one area), and those - into even smaller ones - elementary or local (the same squirrels, but in different parts of the same forest).
According to the ability to reproduce, there is a division into:
- Permanent, which do not require an influx of individuals of their species from the outside to maintain numbers at the level necessary for full existence.
- Semi-dependent, in which a certain number of similar individuals come from outside, but even without them the population is able to exist for a long time.
- Temporary, in them the mortality rate of representatives is higher than the species birth rate, and existence is directly dependent on the influx of individuals from the outside. Temporary populations often form in places with an unfavorable climate and unstable food supply.
Attention! A population is very similar to a living organism, as a biosystem; it also has an organized structure that has its own integrity, a genetic program for self-reproduction and special characteristic mechanisms of self-regulation and adaptation.
Population structure
The structure of the predominant number of existing species settlements is determined by the representatives that form them, and the placement of the latter in the habitat (remembering squirrels - the total number and percentage of different-sex animals in the forest). To make it clearer, let's look at the points.
So, the population structure is
Spatial - the distribution of individuals across the occupied area - how many squirrels are running and where. It, in turn, is divided into:
- Random (if the forest is the same for all squirrels, and they jump in the same natural environment). In this case, there are few animals, they do not form “groups” and do not live crowded together in water.
- Uniform. It is found mainly in animals living in conditions of severe competition for food resources and habitats. Some species of predatory fish, birds and mammals (bears, for example) carefully guard their hunting grounds and do not favor strangers.
- Group. The most common in nature. Here we will look at the example of plants. Some trees have large, heavy fruits (nuts, acorns, plane nuts, etc.), which, falling next to the tree, immediately germinate, forming groups. And even lilies of the valley! But, they owe this to the vegetative method of reproduction (offshoots from rhizomes). These growth characteristics are caused the fact that the surrounding environmental conditions are heterogeneous, habitats are limited, the species has characteristic biological qualities and reproduction options.
Sexuality - the ratio of specimens of different sexes (how many male and female squirrels are in the forest).
Age is the most understandable. How many individuals of different ages. In any species, and sometimes in each population within a species, there are different ratios of age groups. As a rule, the following ecological ages are distinguished:
- pre-reproductive (organisms that have not reached sexual maturity);
- reproductive (sexually mature);
- post-reproductive (representatives who have lost the ability to reproduce).
For animals and plants, this structure has significant differences, but this is a separate topic for consideration.
Genetic structure of the population due to variability and diversity of genotypes(roughly speaking, the difference in squirrel color and size, and their variations during mating with subsequent offspring).
Ecological structure consists of dividing a species into groups of individual representatives that interact in their own way with environmental conditions. This is where local populations often show up. The whole point is that the difference between a type and a separate group of representatives existing in the special conditions of the general habitat is very conditional.
In principle, the system functions like almost any biological system. Accordingly, it is characterized by: growth, development, survival in changing environmental conditions. This determines the presence of certain parameters.
Squirrel population
Options
Most existing populations are characterized by: number, density, birth rate and death rate. All these characteristics are also closely interconnected and interdependent.
Population size- the total number of representatives of the species living in the territory. Density, respectively, is the number of individuals of a given species per unit area of the area.
In many large groups, the average size does not change much each year due to the fact that:
- approximately the same number of representatives die from natural causes;
- the intensity of reproduction of organisms increases with a low population density, and with an increase, it correspondingly decreases;
- Constantly changing natural conditions and climatic factors create obstacles to the high realization of reproductive potential.
But, even with a certain stability, the population size is characterized by fluctuations. The main reasons for these fluctuations associated with changes in living conditions. Namely:
These periodic fluctuations lead to changes in the total population, which consist of the following phenomena:
- fertility;
- mortality;
- immigration (movement - influx of individuals from outside);
- emigration (eviction of representatives of the species).
These factors are associated with so-called population waves.
Important! Population waves are sudden, significant numerical changes.
Example: a reduction in the number of foxes as a result of shooting (an abiotic factor) leads to a population increase in field mice (voles).
Populations are characterized by numbers, densities, birth rates, and deaths.
Gene pool
But of particular importance is the effective number - the number of sexually mature representatives of the species capable of producing offspring. It is they who form the gene pool. And now let's look at this concept specifically.
What is the gene pool of a population(gene pool). This is the totality of all the characteristics (genes) of a species and their variations that are inherited. It is thanks to genes that squirrels from Siberia differ from squirrels from Canada. Gene variations (alleles) determine the ability of organisms to adapt to constantly changing environmental features. The greater the diversity of genes, the more capable the organism is of adapting to life.
In biology there is such a thing as an ideal population. But, it is purely theoretical and is used to model processes. Ideal population can be defined as a hypothetical panmictic (i.e., individuals of which have the same chance of interbreeding), with an infinitely growing population that persists through generations and is independent of natural selection, external factors and mutations.
What is the main role of the concept for the existence of living organisms on the planet? In ecology it is defined as an elementary unit of the process microevolution(intraspecific small gene changes over several generations, leading to certain changes in the individual, both external and internal), responding to changing environmental factors by restructuring the gene pool.
Population functioning and population dynamics in nature
Population as a form of existence of species in nature
Conclusion
Based on the above , Let's summarize. A population is a collection of representatives of the same species living in a common territory, interbreeding freely, possessing a single gene pool, having its own structure, characteristics and parameters similar to existing biosystems, and is an elementary microevolutionary unit.
In biology, there are many concepts and many definitions of species. One of the simplest definitions of a species says: a species is a collection of organisms (individuals) similar to each other in a number of essential characteristics, inhabiting a certain area, capable of interbreeding with each other and producing fertile offspring similar to their parents.
A habitat is a section of the earth's surface (territory or water area) on which a given type of organism exists and reproduces. In most cases, the habitat area is so large that organisms of the same species must adapt to the effects of environmental factors in different conditions. Thus, the species has a certain ecological structure.
A population is a minimal self-reproducing group of individuals of the same species, inhabiting a certain area for a long series of generations, forming its own genetic system, forming its own ecological niche and more or less isolated from other similar groups of this species.
A population is a form of existence of a species and an elementary unit of evolution.
The main property of populations, like other biological systems, is that they are in continuous movement and constantly changing. This is reflected in all parameters: productivity, stability, structure, distribution in space. Populations are characterized by specific genetic and environmental characteristics that reflect the ability of systems to maintain existence in constantly changing conditions: growth, development, stability. The science that combines genetic, ecological, and evolutionary approaches to the study of populations is known as population biology.
Types of populations. Populations may occupy areas of different sizes, and living conditions within the habitat of one population may also not be the same. Based on this characteristic, three types of populations are distinguished: elementary, ecological, and geographical.
1. An elementary (local) population is a collection of individuals of the same species occupying a small area of homogeneous area. There is a constant exchange of genetic information between them.
EXAMPLES. One of several schools of fish of the same species in the lake; microgroups of Keiske lily of the valley in white birch forests, growing at the bases of trees and in open places; clumps of trees of the same species (Mongolian oak, larch, etc.), separated by meadows, clumps of other trees or shrubs, or swamps.
2. Ecological population - a set of elementary populations, intraspecific groups, confined to specific biocenoses. Plants of the same species in a cenosis are called a cenopopulation. The exchange of genetic information between them occurs quite often.
EXAMPLES. Fish of the same species in all schools of a common reservoir; squirrel populations in pine, spruce-fir, and broadleaf forests in one area.
3. Geographic population - a set of ecological populations that inhabit geographically similar areas. Geographic populations exist autonomously, their habitats are relatively isolated, gene exchange occurs rarely - in animals and birds - during migration, in plants - during the spread of pollen, seeds and fruits. At this level, the formation of geographical races and varieties occurs, and subspecies are distinguished.
EXAMPLES. The geographical races of Dahurian larch (Larix dahurica) are known: western (west of the Lena (L. dahurica ssp. dahurica) and eastern (east of the Lena, distinguished in L. dahurica ssp. cajanderi), northern and southern races of the Kuril larch. Similarly M.A. Shemberg (1986) identified two subspecies of stone birch: Erman birch (Betula ermanii) and woolly birch (B. lanata). 1000 km, to the north - 500 km. Zoologists distinguish tundra and steppe populations of the narrow-skulled vole (Microtis gregalis). The species "common squirrel" has about 20 geographical populations, or subspecies.
People) in ecology and genetics is a collection of individuals of the same species that occupies a certain space for a long time, and also reproduces itself over several generations. Individuals from the same population are much more likely to interbreed with each other than with individuals from other populations. This is due to the fact that a given individual is separated from other similar groups of individuals by varying degrees of pressure from various forms of isolation.
The main characteristic of a population, which determines its central position as an elementary unit of the evolutionary process, is its genetic unity: within the population, panmixia occurs to one degree or another. At the same time, the individuals that make up the population are characterized by genetic heterogeneity, which determines the population’s adaptability to various environmental conditions, creating a very important reserve of hereditary variability. Due to the heterogeneity of the environment, the population has a complex structure: individuals differ in belonging to different, usually overlapping generations, by sex and age.
The species includes many populations, and isolation between them is not absolute. Individuals of a population are capable of migration and settlement; their distribution depends on geographical barriers within the species, as well as on the nature of the habitat and the number of species. Mortality, birth rate, age composition and abundance are called demographic indicators. It is very important to know them in order to understand the laws governing the life of populations, in order to predict the constant changes occurring in them.
1. All organisms of a specific species (usually, but not necessarily, humans) within a specific geographic area at a specific point in time. 2. In statistics, the total number of cases about which a certain statement can be made. Note that in this sense, a population can be: (a) finite, actually existing and available for research (for example, all students enrolled in a particular school); (b) finite, actually existing, but inaccessible to research (for example, the population of Great Britain on the day of publication of this publication); or (c) infinite (for example, all possible coin tosses). To be distinguished from a sample, which is some observed or selected subset of a population. See sample population.
Philosophical Dictionary
- a collection of individuals of the same species with a common gene pool, over a large number of generations, inhabiting a certain space with relatively homogeneous living conditions.
Sampling, Population
Psychological Encyclopedia
Strictly speaking, a population from which a careful sample has been drawn is, by definition, a sample population. However, in practice it may not correspond to the actual theoretical population. As an example, consider a simple common case. Imagine that almost everything...
Population
Psychological Encyclopedia
A group of people constituting the object of psychological research. It is usually not convenient to study an entire population (for example, all university students, or all Liverpool 16-19 year olds) and a representative sample is used instead.
Population
A population is a group of individuals of the same species that interact with each other and jointly inhabit a common territory. The more complex the habitat of a species is dissected, the greater the opportunities for isolation of the population. The degree of isolation of neighboring populations of the species varies. In some cases, they are separated by territory unsuitable for habitation, and then they are clearly localized in space (for example, fish in isolated reservoirs).
In others, areas unsuitable for life are easily overcome during resettlement. It is possible to isolate the boundaries of populations only conditionally, based on population density. The main criterion for a population is the possibility of unhindered interbreeding. The main characteristics of populations include size, density, birth rate, mortality, population growth, and growth rate. The population is characterized by a certain structure: the distribution of groups across the territory, the ratio of groups by sex, age, morphological, behavioral and genetic characteristics, in general, reflecting the specifics of intraspecific relationships, both mutualistic (mutually beneficial) and competitive. Each population is programmed for a wide range of natural environmental changes, including catastrophic ones. Maintaining a certain number of individuals in a population is called population homeostasis and is an auto-regulated process. Any population under specific conditions is characterized by a certain average level of abundance, around which fluctuations occur. Deviations from the average level have different ranges, but after each deviation the population size begins to change with the opposite sign.
Modifying factors are factors that cause changes in population size but are not themselves affected by these changes. This includes all abiotic influences on the body and their consequences, expressed in changes in the quantity and quality of feed, the quantity and activity of competitors.
Regulating factors do not simply change the population size, but smooth out its fluctuations after the next deviation from the optimum. Interspecific and intraspecific relationships act as regulatory forces. Different types of relationships determine the speed of responses to changes in population size.
Natural population regulation has 2 features:
— regulatory mechanisms act in response to the change that has occurred and the effect is achieved with some delay;
— regulation has a one-sided effect aimed at limiting population growth. The rise in numbers after a strong decline occurs due to a decrease in the strength of the regulators.
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The word “population” came into our language from Latin and is literally translated as “population.” In the most general sense, a population is a group of creatures (organisms) belonging to the same species that live in the same territory for a long time. The term "population" is used in a variety of fields of science: biology, medicine, psychometrics, ecology, geography and demography.
To better understand what a population is, we will give several examples of the use of this word in different sciences.
- In ecology and biology, a population means individuals of the same species that have a stable ability for self-production and communicate primarily within their group.
- According to genetics, a population is determined by the degree of interbreeding within a species. Populations are usually groups within a species or subspecies of living organisms.
- The population is considered the basic unit of measurement of any evolutionary process.
It is ecology as a science that considers its task to be the study of education, development, and population dynamics.
- In medicine, a population is a collection of people from which individuals are selected for research. The results of the medical study will subsequently be applied to this population. The group of people for the study is determined by some general criterion, for example, by the presence of a disease.
You may also be interested in the article What ecology studies.
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TOPIC 2. TRAITS IN POPULATIONS
Every living organism, including the human one, has a whole set of inherent properties. Some of them are common to all representatives of this species ( species-specific features). For example, each representative of the species Homo sapiens differs from representatives of other species by the ability to walk upright, the absence of hair on most of the body, highly developed intelligence and the ability to communicate verbally. These are all species-specific features. Each individual, in addition to species-specific characteristics, has a whole set of individual qualities unique to him - this individual-specific peculiarities. All these features inherent in a given individual - both species-specific and individual - in genetics are usually called signs(Reader 2.1).
The species inhabiting the Earth form communities, that is, spatiotemporal associations. The reason for the formation of communities is the equal adaptability of individuals to certain environmental conditions. For example, deserts do not contain animals whose biology requires high climate humidity. One type of community is population. Since behavioral genetics works mainly at the population level, let's look at how this concept is defined in genetics.
In a broad sense, the concept of “population” can be applied to any collection of any objects. In biology and genetics populations
consider communities of individuals of the same species occupying a certain territory. Ideal population
in genetics, a group of interbreeding individuals living in the same territory is considered. If the probabilities of crossings between individuals do not depend on any additional circumstances (age of individuals, sexual preferences, etc.), then we are dealing with randomly interbreeding population. In other words, in such a population, any individual (or individual) of one sex has an equal chance of interbreeding (marriage) with any individual (individual) of the other sex. A randomly interbreeding population is sometimes called panmixia (the term "panmixia" means free random interbreeding; ideal panmixia is possible only in very large populations that are not subject to selection pressure, mutations and other factors). Many theoretical models in psychogenetics (and in population genetics in general) are based on the assumption of the presence of free random crossing in a population.
The principle of free and random crossing in populations may be violated if the formation of pairs based on any trait does not occur by chance. For example, in human populations there is a tendency for married couples to be non-randomly matched based on height. In genetics, this non-random selection of pairs is called "assortativity". For example, with regard to height, it is said that there is assortativity in height in a population. The presence or absence of assortativity can be checked by assessing the similarity of the spouses: if the correlation for this characteristic is close to zero, then they say that there is no assortativity; if the correlation is different from zero, this indicates the presence of assortativity (for more information on correlations, see topic 5 and statistics manuals) (Figure 2.1).
Populations are not static entities. Migration processes constantly occur in them, there are fluctuations in fertility and mortality and other changes. These processes lead to fluctuations in population size and changes in the frequencies of occurrence of various traits.
Since humans are one of the types of living organisms, they are also characterized by the formation of populations. In addition to biological reasons, various social factors also influence the formation of human populations. Human populations can vary in size. The term “population” can be applied to the population of megacities such as Moscow or Tokyo (in which migration processes occur daily, leading to renewal and change in numbers due to the influx or outflow of population), and to small communities or tribes inhabiting closed territories, for example, in mountainous areas. Closed populations are called isolates. Isolates in human populations can occur not only in geographically isolated areas, but also within large populations if certain groups of people, say, for religious or ethnic reasons, do not mix with the rest of the population.
Among representatives of any population there is a certain variability, that is, the diversity of characteristics among its members. The variability of the human population is easy to notice when you look at a crowd of people. Along with species-specific characteristics, each individual has a unique set of characteristics that are unique to him.
As a result, it is almost impossible to detect two identical individuals in a crowd of people. Individuals differ not only in their external structure - hair and eye color, height and physique, but also in their behavior - manner of speaking, gait, gestures, etc.
Depending on the type of variability in populations, two types of traits can be distinguished. Some of them are characterized discrete, that is, an intermittent series of variability. These characteristics occur in a limited number of variants, the differences between individuals are clearly expressed, and there are no intermediate forms. Signs of this kind are classified as quality. They are also called quality. External conditions have relatively little or almost no influence on their manifestation. Such characteristics include, for example, human blood groups. Carriers of different variants of qualitative traits in a population can occur with different frequencies: we can recall that people with Rh-negative blood factors are much less common than those with Rh-positive ones. In addition, the frequency of occurrence of variants of the same trait in different populations usually varies. Figure 2.2 shows an example of the frequency distribution of the most well-known blood groups of the AB0 system in the populations of Russians and indigenous people of Australia. It can be noted that the frequencies of occurrence of all four blood groups in these populations are different. Among human characteristics that are in one way or another related to behavior (in the broad sense), there are almost no qualitative signs that have a discrete nature of variability. Of those not associated with gross pathology, the best known are two related to sensory sensitivity. These are phenylthiourea taste sensitivity (some people can taste the substance, while others cannot) and color blindness (some people are unable to distinguish between individual colors of the spectrum). Most of the simple qualitative characteristics of a person known today are associated with biochemical blood factors or hereditary anomalies that do not have significant manifestations in behavior.
Often, qualitative polymorphisms, which are accompanied by certain behavioral characteristics, are associated with various hereditary anomalies, leading to degenerative changes in the central nervous system, accompanied by behavioral disorders. Examples of such polymorphisms affecting behavior are phenylketonuria, a metabolic disorder that leads to severe mental retardation, and Huntington's chorea, a degenerative disease of nerve cells that leads to involuntary movements, personality changes, and progressive dementia.
The bulk of human behavioral characteristics belong to the second category of characteristics that have continuous, or quantitative, variability - continuity manifestations. We observe similar variability in such morphological characteristics as height, weight, hair and skin color, and in such behavioral characteristics as intelligence, temperament traits, etc. All values of each of these characteristics can be placed on some continuous scale. Very high and very low values of a trait are, as a rule, less common in populations than average values. The frequency of occurrence of certain values of a characteristic can be calculated and presented in the form of a distribution, which is a set of values of a quantitative characteristic and their corresponding frequencies. The frequency distribution for a trait that exhibits continuous variability approximately corresponds to a normal distribution curve (Figures 2.3 and 2.4). Most people fall into the middle part of the distribution, and only a small part of the population ends up at its edges, representing extreme degrees of expression of the trait.
Often, when assessing quantitative characteristics, we use qualitative categories, such as “high-low”, “strong-weak”, “dark-light”, etc. Let us recall a well-known example from psychophysiology. People are often divided into having a strong and a weak type of nervous system, but this division is arbitrary. Only people from the extremes of the distribution fall into these groups, while the strength-weakness parameter itself is measured on a continuous scale, and everything from extreme nervous system weakness to extreme values of strength occurs in the population. In the same way, the division of people into extroverts and introverts is very arbitrary. Any typological division does not describe the complete picture of variability along a continuum (Chrestomat. 2.2 and 2.3).
There is another category of traits that occupies an intermediate position between qualitative polymorphisms and quantitative, continual traits. This is about signs with a threshold effect. Outwardly, these characteristics resemble qualitative polymorphisms, that is, they differ in the discrete nature of their manifestation. An organism either carries this trait or it does not. Most often, signs with a threshold effect are various diseases, for example, diabetes mellitus, bronchial asthma or schizophrenia, as well as various congenital developmental anomalies - anencephaly, spina bifida (anomalies of the central nervous system), cleft lip, cleft palate, etc. For these diseases and developmental defects, clear alternative distributions are observed, similar to distributions for qualitative polymorphisms: an individual either suffers from a given disease or does not. However, the type of inheritance in these diseases turns out to be closer to the type of inheritance of quantitative traits. In this case, we can say that the threshold is a conditional boundary in the normal distribution of the trait, dividing, on the one hand, affected individuals, and on the other, unaffected, but predisposed to the disease individuals. In some diseases, the border between normality and pathology can be traced quite clearly, especially with congenital developmental anomalies, while in others it is blurred and very conditional (for example, the border between mental retardation and normal intelligence). Thus, traits with a threshold effect are more likely to be classified as quantitative human traits. Accordingly, the same patterns of inheritance will be valid for them as for ordinary continual traits (Fig. 2.5).
conclusions
- Human individuals form communities, populations.
- There is interindividual variability within populations.
- Variation in populations can be discrete (discontinuous) or continuous (continuous). Most psychological signs are characterized by continuous variability.
- Depending on the type of variability, all human properties (traits) can be divided into two classes: discrete variability corresponds to qualitative traits, continuous variability corresponds to quantitative ones. A special place is occupied by characteristics with a threshold effect. By the nature of variability they correspond to qualitative traits, but by the nature of inheritance they are closer to quantitative ones.
- Variants, or different values of traits, occur with different frequencies in populations.
- Populations are not stable entities. Due to migration processes, changes in fertility and mortality and some other processes, the frequency of occurrence of various traits in a population may change
Glossary of terms
- Assortativity
- Population
- Sign
- Discrete variability
- Continuum variability
- Discrete variability
- Isolates
- Species-specific features
Self-test questions
- What types of variability do you know?
- Give examples of discrete and continuum variability.
- What type of variability is characteristic of most psychological traits?
- How is the frequency of occurrence in populations for qualitative and quantitative traits graphically depicted?
- What are threshold effect traits?
- Give examples of qualitative, quantitative, and threshold effect traits.
- What would the frequency distribution look like for a trait with a threshold effect?
- What is the specificity of the concept of population in genetics?
- Name the main criteria for classifying communities of organisms as populations.
- What are the reasons for the formation of populations?
- Why do certain traits occur in populations with different frequencies?
- What changes might occur in populations?
- What are the specific features of human populations?
- What types of human populations do you know?
- What can you say about the population of a metropolis (using the example of Moscow)?
- What is assortativity? Give examples.
Bibliography
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