POPULATION (late lat. populatio, from armor. populus the people, the population) — community of individuals of one look capable to free crossing and possessing the general gene pool. In cytology, histology and microbiology the concept «population» is used also for association of homogeneous groups of the cells having homogeny from one cell or group of cells in culture of fabrics. Ambiguity of the use of the concept «population» of ecology, parasitology and population genetics led to attempt of allocation of elementary, geographical, ecological and other types P. The term «population» in his modern understanding for the first time was used by the Danish geneticist Iogansen (W. L. Johannsen, 1903) in work «About inheritance in populations and pure lines». In population genetics the term «Mendelian population» designating freely crossed group of individuals with the general gene pool is used. The concept «Mendelian population» emphasizes P.'s ability to free crossing — a panmixia. In foreign literature the term of «subjects» meaning elementary panmiktichesky (i.e. having ability to free crossing) unit was widely adopted. For P.'s description in space use the terms «topodeme» (group of individuals of one biotope) and «ecodeme» (group of the individuals living in one ekol, conditions, e.g. the ectoparasites living on owners of one look, but capable to live in the same biotope on other type of owners). For phenotypical excellent dem use the term «phenodeme», at distinctions in genotypes — «genody». At the description of degree of a panmiktichnost of dem for perekrestnorazmnozhayushchikh-sya types speak about gamodema with N percent degree of a panmiktichnost for p generations. At full sir-miktichnosti P. use the term «endody». To organisms with an autogamy (autogamy, napr, at infusorians) apply the concept «avtody», to types with preobladani-ekh\1 sexless (microorganisms) or vegetative (many plants) reproduction — «klonody».
Really the panmixia within one P. is provided with a community of habitals, synchronism of cycles of reproduction, coincidence of a consort behavior, compatibility of gametes etc., and partial genetic isolation of the next P. is caused by existence a nek-swarm of extent of reproductive isolation between the next Items. Intra population polymorphism on a genotype and a phenotype provides P.'s adaptability as whole to various conditions of the environment (see. Polymorphism in genetics ).
In case of an overshoot of generations of P. can have very complex age structure. At the same time crossings not only in one generation, but also between different generations are possible. In case of lack of an overshoot of generations of P. consists, as a rule, of individuals of two generation — parent and affiliated. Prrg it in the same territory various temporary populations can coexist with almost full isolation between different generations.
The item can have complex ethological structure, break up to herds, colonies, families. The P. is more numerous than animals, the more difficult it the hierarchical structure determined substantially by an originality of ethological relationship of individuals in the Item. The provision of an individual in ethological structure of P. can be not constant, it can change depending on age, a physical and mental condition of an individual, and also from that environment, in Krom it is. Disappearance of a dominant individual (leader) leads to occupation of her place one of subdominant or to full reorganization of hierarchical relationship of all individuals in P., at Krom the place the dominant can be occupied with the individual who was earlier on quite low levels of behavioural hierarchy. Reorganization of hierarchical structure of P. can be followed by the phenomena stress (see) at many members
of P. P. it is characterized by dynamics of number of individuals, in days of peaks of number P.'s area can extend, borders between the next P. can be erased, and in days of a depression of number P.'s area is narrowed to stations of experience; at the same time there is depletion of a gene pool of the Item.
Identification of geographical boundaries of separate P. in the absence of accurately expressed space barriers in many cases is complicated and demands use of various methods of population biology, first of all population genetics (see below the Geneticist of populations). Populations of different types are members various biocenoses (see), including. parazitotsenoz (see), also form together with so-called inert (lifeless) elements biogeocenosis (see).
Gives ambiguous delimitation of P. to the fact that in limits look (see) quite often it is necessary to allocate hierarchical system of populations. Population systems classify on the basis of the analysis of their phenotypical variability, geographical distribution and the nature of isolation and divide into three big groups: beyond all bounds crossed to a gradual intergradation of phenotypical signs (in this group allocate allopatric P. — i.e. the contacting, but not blocked geographical forms, or races, and sympatric P. — spatially undivided ecological forms, or races); restrictedly the disturbances of an intergradation of phenotypical signs which are crossed to bigger or smaller degree (allopatric P. with fiziol, or morfol, distinctions in average sizes — separate geographic races; allopatric P. with fiziol, or morfol. distinctions not only in average sizes of some sign, but also on limits of its variability — an allopatric semispecies; sympatric P. without reproductive isolation — ecological races; sympatric P. with partial reproductive isolation — a sympatric semispecies); the population systems divided by the nature of their morphological or physiological variability and, as a rule, not crossed with each other (allopatric population systems with disturbance of a gene-spread between them due to spatial isolation, but not developed fiziol, reproductive isolation — an allopatric semispecies; allopatric population systems without gene-spread between them for the account fiziol, reproductive isolation — allopatric types; the sympatric population systems living in the same territory, and reproduktivno isolated by P. — sympatric types). Thus, the concept of population systems stretches from the level of groups of elementary P. to the level of a look.
Genetics of populations
the Genetics of populations studies structure and dynamics of genotypic list of P. Nachalo to theoretical studying geneticists P. laid Hardy's works (G. N of Hardy) and W. Weinberg which in 1908 gave the mathematical description of a ratio of alleles in population.
As the sum of frequencies dominant (And) and recessive alleles is equal in P. 1, in the presence of a panmixia (i.e. at free crossing) there is a free recombination of these alleles (see) therefore distribution of genotypes in population can be described by a formula:
r 2 AA + 2pqAa + q 2 aa,
where r — the frequency of an allele And, q — the frequency of an allele of a (q = 1 — r).
In ideal P. of the frequency of genes (see. Gene ) and genotypes (see) are in balance and do not change among generations. Hardy's law — Veynberg allows to prove stability of genotypic structure of P. that is one of necessary conditions of its evolutionary importance. Though this law considerably simplifies natural situations, assuming existence of a panmixia, existence infinitely big P., lack of mutational process, an overshoot of generations in time, selection pressures — it with the known restriction is used for assessment of frequencies of a gene, heterozygotes and homozygotes, frequencies of crossings or marriages between individuals of the genotype defined *, etc.
Hardy's law — Veynberg further began to extend to cases of a multiple allelism, for the genes linked to X-chromosome for two genes with several alleles in everyone. The analysis of equilibrium states of genes of a blood group of the AB0 system (three alleles) and genes of a blood group of the MNS system is of special interest (four main alleles in two couples of closely linked genes).
The principles of studying of a genetic variety are proved by S. S. Chetverikov in 1926. He assumed that owing to mutational process and recession of the majority of again appearing mutations (see. Mutation ) in P. the considerable number of recessive mutations in a heterozygous state (so-called mutation pressure) in the presence of a panmixia and implementation of the law of Hardy — Veynberg shall collect. The experimental works which are carried out in the different countries and on different objects confirmed wide genetic heterogeneity natural P. Tak, the population statistical analysis of separate P. of the person on the genes defining blood groups, proteins of blood sera, enzymes of erythrocytes and other proteins showed that apprx. 30% of all studied genes are presented to P. by two and more alleles, at the same time the probability of heterozygosity on these genes for each individual reaches very noticeable size (R = 0,16). From natural P. all types of mutations are allocated: gene, chromosomal and genomic. Assume that wide genetic heterogeneity of natural P. provides a mobilization reserve and P.'s plasticity in the habitat which is constantly changing in time. Thus, existence in natural P. of genetic variability and existence of population mechanisms of maintenance and preservation of genetic polymorphism are necessary and sufficient conditions for manifestation of the factors providing genetic differentiation of P. as bases of microevolutionary process (see. Theory of evolution ).
Carry to factors of population dynamics: mutational process, interpopulation migrations, isolation and deviations from a panmixia in populations (various types of an assor-tativnost of marriages), the limited number of P., natural selection. All these factors can change frequencies of genes and genotypes and, therefore, to influence genotypic structure of P. and to cause emergence of genetic distinctions between P. relating to one look.
Natural mutational process is in character spontaneous and the nondirectional. It means that allocation of specific origins of new mutations is almost impossible and that for each gene there is a certain probability of its change in any possible structural form. As the most essential characteristic of spontaneous mutational process serves its speed (speed), i.e. frequency of emergence of mutations. The mutation rate of the genes responsible for display of an achondroplasia, a neurofibromatosis, Dyushenn's myopathy and other diseases of the person is defined, e.g. This frequency counting on one locus for one generation lies within 10 - 4 — 10 - 6 .
Significantly also definition of a mutation rate on a gamete. This size depends on number of loci in a genotype, a cut, according to different scientists, makes at the higher organisms, including the person, the size of an order 10b. Therefore, a considerable part of gametes shall bear new mutations. The absolute majority of again arisen mutations finds harmful manifestation. Mutational process provides emergence in P. wide genetic variability (see), the considerable part a cut presented by recessive mutations is hidden from action of natural selection and therefore can remain among generations.
Isolation (see. Isolates ) is a factor of population dynamics in that case when it is shown by disturbance of a panmixia. Isolation directly does not influence frequencies of genes, but leads to changes of frequencies of genotypes. Allocate several types of disturbance of a panmixia: assortativnost, inbreeding and the selection crossings. Assorta-tivny call such crossing when formation of marriage couples depends on a genotype of individuals. Distinguish a positive assortativnost (formation of marriage couples individuals of a certain genotype more frequent, than at a panmixia) and a negative assortativnost (formation of marriage couples individuals of a certain genotype more rare, than at a panmixia). Most on the effects in P. of the person such deviation from a panmixia as not casual is important inbreeding (see), i.e. more frequent in comparison with expected the conclusion of marriages between the individuals having the general ancestors. The average coefficient of inbreeding in P.'s most of the person makes 0,001. At the same time are found various size (The southern India, some Arab countries) isolated by P., coefficient of inbreeding in to-rykh significantly higher. More frequent gomozigotization of the rare recessive genes causing hereditary pathology is the main consequence of kinship marriages. If in the conditions of a panmixia the frequency of homozygotes on a recessive gene is r2, then in population with a widespread consanguineous relation the frequency of homozygotes will increase up to p2 + Fpq where F — average coefficient of inbreeding in the Item. From this it follows that the frequency of a gene is less in P. and value of coefficient of inbreeding is higher, the big share of homozygotes comes from kinship marriages.
Research of communication between coefficient of inbreeding of marriage couples and nek-ry demographic indicators (spontaneous abortions, a mert-vorozhdennost, early child mortality etc.) allows to estimate the size of a load of recessive mutations with harmful effect at P. of the person.
Leads the selection crossing to change of frequencies of genes in P., a cut means a high probability for individuals of a certain genotype to be crossed to individuals of any other genotypes.
One more factor of population dynamics operating in the direction opposite to isolation is migration, edges is followed by equalizing of frequencies of genes in P. exchanging migrants. For studying of effects of pressure of migrations a number of mathematical models is offered. From them the so-called island model of isolation is most common. According to this model P. it is subdivided into a great number of the isolated groups, between to-rymi there is an exchange of migrants. In distance the assumption of continuous distribution of individuals of P. in the big territory where the probability of crossing of individuals depends on extent of their dispersal is the cornerstone of model of isolation.
Drift of genes belongs to important factors of population dynamics. Accidental genetic drift, or genetiko-automatic process, is the fluctuation of frequencies of genes in population caused by its limited number. Drift of genes is implemented through so-called effect of a bottle neck, i.e. reduction of the sizes P. with the subsequent sharp increase; through effect of the ancestor, i.e. P.'s origin of rather small number of ancestors, one of to-rykh gives disproportionately big contribution of gametes to succeeding generations, through so-called population waves. The role of drift of genes is shown for cases of high frequency of the mutant genes causing hereditary pathology in many P. of the person. Thus, with a small number of P. the probability of random change in frequencies of alleles of separate genes and genotypes upon transition to each next generation is rather high. This change has the nondirectional character and leads eventually to elimination or fixing of certain alleles.
Unlike a deviation from a panmixia and action of other factors natural selection defines the directed change of frequencies of genes in P. and is considered as a major factor of speciation on the basis of hereditary variability, eurysynusic in P. of live organisms (see. Natural selection ).
Action of selection on P. is estimated in coefficients of adaptive value of a genotype and selection. Understand relative fitness, i.e. probability of achievement as adaptive value of a genotype an individual of this genotype of reproductive age and leaving of posterity. Simply, if individuals of a genotype aa give 60% of descendants of the number of descendants of individuals of a genotype of A A, a cut take for 100%, then adaptive value of a genotype aa will make 0,6, i.e. the value of coefficient of adaptive value of a genotype of an individual is less, the it is less at it probability to leave posterity. The selection coefficient also varies from 0 to 1. However in this case the value of coefficient is less, the probability is less that this genotype will be eliminirovan selection, and the it is more at an individual probability to reach reproductive age and to give posterity. Selection differently influences dominant and recessive alleles. If dominant alleles at once are exposed to selection, then recessive, hidden in a heterozygous state, escape it. In modern P. of the person, in to-rykh dispersion of the size of a family is sharply reduced, efficiency of selection pressure shall decrease. Intensity of the nek-ry directions of action of natural selection decreased also thanks to achievements of the modern medicine liquidating a row inf. diseases, offered effective methods of treatment of many infectious and noninfectious diseases which could act before as enough powerful selective factors. At the same time the expressed selection pressure on a mode of formation of gametes (gametny selection), zygotes and at early stages of ontogenesis easily is found in the person.
The mathematical theory of action of natural selection is developed. On its basis action of selection in system of one gene with two alleles (3 allelotypes), conditions of emergence limit (elimination of one of alleles) and stationary (preservation in population of all three allelotypes) states etc. are investigated. Action of selection in favor of heterozygotes i.e. when relative fitness of heterozygotes is higher than fitness of individuals, homozygous on dominant (AA) or a recessive allele is of the greatest interest. In this case there is a stable stationary state, at Krom equilibrium frequencies of both alleles are defined by values of adaptive value of homozygotes in comparison with heterozygotes. In a result in of P. the system of the balanced polymorphism forms. An example of such system at the person is the system of a gene of hemoglobin S, and also probably system of a gene of a beta talassemia. Frequencies of the called mutant genes in P. different in an ethnic origin and considerable on number, reached polymorphic level (i.e. 1% and above), and a number of researchers assume that heterozygotes on these genes have advantages even in comparison with normal homozygotes (homozygotes on a gene of hemoglobin S and a gene of a beta talassemia perish at children's age) in the conditions of action of such factor of natural selection as malaria.
In real natural P. all major factors of population stability and population dynamics constantly work and interact: mutational process, isolation, drift of genes, migration and natural selection. It follows from this that in different P. dynamics even of the same genetic system can be different depending on what factor of population structure is defining.
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