Textbook for grades 10-11

§ 49. Isolation - an evolutionary factor

Even Charles Darwin pointed out that isolation is a very important evolutionary factor, since it leads to divergence in the characteristics of individuals within the same species and prevents the crossing of individuals different types among themselves.

Geographical isolation. Let us consider the ways in which isolation is carried out in nature, leading to divergence of characteristics in the population. The most common is spatial, or geographic, isolation. Its essence lies in the rupture of the single habitat in which the species lived into parts that do not communicate with each other. As a result, individual populations become isolated, so free crossing of individuals from different parts of the range turns out to be either impossible or extremely difficult.

Mutations can occur randomly in any isolated population. Due to genetic drift and the action of natural selection, the genotypic composition of isolated populations becomes more and more different.

The reasons leading to the emergence of geographic isolation are numerous: the formation of mountains or rivers, isthmuses or straits, the extermination of populations in certain areas, etc.

Due to the impossibility of crossing individuals from different isolated populations, each of them develops its own direction evolutionary process. Over time, this leads to significant differences in their genotypic structure and a weakening and even complete cessation of gene exchange between populations.

Environmental insulation. Another way leading to population divergence is environmental isolation. It is based on differences in the preferences of animals or plants to settle in certain place and interbreed at strictly defined times of the year. Some salmon fish, for example, they spawn not annually, but every other year. Moreover, one population of fish comes to spawn in the same spawning area in an even year, and another in an odd year. For this reason, representatives of different populations cannot interbreed and populations become isolated.

Another type of ecological isolation is associated with the preference of living organisms for a particular habitat. Sevan trout is an example of such isolation. Different populations of trout spawn at the mouths of different streams and mountain rivers flowing into the lake, so free crossing between them is extremely difficult. Ecological isolation thus prevents the interbreeding of individuals from different populations and serves, like geographic isolation, as the initial stage of population divergence.

Biological mechanisms that prevent individuals of different species from interbreeding. There are complex mechanisms that prevent individuals of different species living in the same territory from interbreeding. Differences in animal behavior are especially important in implementing such isolation.

The dissimilarity in mating songs, courtship rituals, emitted odors, preferred habitats - all this reliably protects individuals of different species from mating. Many species, in addition, have differences in the structure of the genital organs, which is an additional obstacle to crossing. In plants, the inability of pollen of some species to germinate on the stigmas of other species is observed. In the event that fertilization does occur, the death of zygotes is observed. genetic reasons. In cases where all barriers are overcome and hybrid offspring are nevertheless born, they are often infertile due to meiotic disorders due to differences in the structure and number of chromosomes.

So, various types isolation, on the one hand, creates the prerequisites for the divergence of populations and subsequent speciation, and on the other hand, they contribute to the preservation of the genetic structure of the species.

1. Explain how new species form under conditions of geographic or environmental isolation.
2. What is the difference between geographic and environmental isolation?

Page 30. Remember

1. What levels of organization of living nature do you know?

Molecular (For example, fat molecules). Cellular (erythrocyte - red blood cell). Tissue is no longer one cell, but a group of cells similar in structure, origin and performing the same functions. (For example, in animals there is nervous, epithelial, fatty, blood and lymph, and in plants it is conductive, integumentary, mechanical and educational). Organ (An organ is several tissues connected to each other). For example: heart, liver, kidneys. Organismal (For example, human, dog, cat.). Species (species Homo Sapiens). Population (one species in one territory partially or completely isolated from other such populations). Biocenotic. Sometimes they say that there is also a biosphere level, but on it all living organisms of the planet and inert things (atmosphere, hydrosphere and lithosphere) are located.

2. What is a species?

A species is a collection of individuals that are similar in structure, capable of interbreeding and having a common habitat (area of ​​distribution).

3. What other systematic categories do you know?

Overkingdom

Kingdom

Sub-kingdom

Type/department

Squad/order

Family

Page 33. Questions for testing and assignments

1. Define the concept of “species”.

Species - a set of individuals similar in structural and functional organization (biochemical, cytological, histological, anatomical and physiological criteria), having the same origin (evolutionary criterion), the same karyotype (cytogenetic criterion), similar behavior (ethological criterion), freely interbreeding with each other (reproductive criterion) and producing fertile offspring, occupying a certain habitat (geographical criterion) and characterized by certain relationships with other organisms and factors environment (ecological criterion).

2. Explain what biological mechanisms prevent the exchange of genes between species.

One of the important characteristics of a species is its reproductive isolation, i.e. the presence of mechanisms that prevent crossing with individuals of other species and, as a result, preventing the flow of genes from the outside, the protection of the gene pool from the influx of genes from other, including closely related species, is achieved in different ways. The following mechanisms can be identified that prevent the exchange of genes between species:

1. differences in the timing of reproduction among representatives of different species;

2. differences in places preferred for reproduction;

3. discrepancy between standards of specific sexual behavior;

4. mismatch of acrosome enzymes (the front part of the sperm head) chemical structure egg membranes;

5. discrepancy in the structure of the genital organs of representatives various types;

6. non-viability or sterility interspecific hybrids.

Consequently, a species is a really existing, genetically indivisible unit of the organic world.

3. What is the reason for the infertility of interspecific hybrids? Explain this phenomenon using your knowledge of the mechanism of meiosis.

The timing of reproduction in closely related species may not coincide. If the timing is the same, then the preferred breeding sites do not coincide. For example, females of one species of frogs spawn along the banks of rivers, and of another species - in puddles. Accidental insemination of eggs by a male of another species is excluded. Many animal species have strict mating rituals. If one of the potential mating partners has a behavioral ritual that deviates from the specific one, mating does not occur. If mating does occur, sperm from a male of another species will not be able to penetrate the egg and the eggs will not be fertilized. But sometimes during interspecific crossing, fertilization occurs. In this case, the resulting hybrids either have reduced viability or are infertile and do not produce offspring. Famous example– A mule is a hybrid of a horse and a donkey. Although fully viable, the mule is infertile due to disturbances in meiosis: non-homologous chromosomes do not conjugate, and bivalents are not formed. Chromosomes do not separate different cells. As a result, sex cells are not formed, and the body cannot leave offspring. The listed mechanisms that prevent the exchange of genes between species have unequal effectiveness, but in combination they natural conditions they create virtually impenetrable genetic isolation between species.

4. What criteria do scientists use to characterize a species? What criteria do you consider most important when determining a species?

Characteristics and properties characteristic of a given species are called species criteria. There are morphological (similarity of structure), genetic (same set of chromosomes), physiological (similarity of physiological processes), biochemical (similarity of biochemical processes), geographical (area of ​​the species) and ecological (similarity of living conditions) criteria. None of the criteria can be considered absolute, that is, to characterize a species, the entire set of criteria must be taken into account. So, for example, a similar structure may not be a sign of one species, since in nature there are morphologically indistinguishable species - doubles (the common vole and the Eastern European vole; in the first case, the set of chromosomes is 46, and in the second - 54). Important characteristic species is that a species is a closed genetic system, that is, there is no exchange of genes between the gene pools of two species. This phenomenon is based on differences in the set of chromosomes (that is, a genetic criterion), discrepancies in the timing of reproduction (that is, an environmental criterion), differences in the structure of the genital organs (morphological criterion), differences in mating behavior (physiological criterion) and other factors. The genetic structure of a species changes under the influence of evolutionary factors, so the species is heterogeneous.

5. What is the species' range?

The range of a species is the area of ​​distribution of the species. The size of their habitats can vary greatly among different species. For example, Scots pine grows almost throughout the entire territory of Russia, and snowdrop is characteristic only of the North Caucasus.

Species that occupy large areas and are found everywhere are called cosmopolitans, and those that live only in small specific areas are called endemics. It is endemic species that make the greatest contribution to the diversity of life on our planet. And they also need the most careful protection - due to their small numbers, strict attachment to certain living conditions, certain food, etc.

6. Characterize the species Domestic cat according to the main criteria.

Morphological criterion: the mammal is small in size, has four legs and a tail, is covered with hair, has developed fangs and retractable claws.

Genetic - the karyotype of a cat is represented by 19 pairs of chromosomes, of which 18 pairs are somatic chromosomes and one pair is sex chromosomes.

Physiological: prefers an evening-night lifestyle, lies in wait (rather than chasing) prey, and, if necessary, makes characteristic meowing and purring sounds. Biochemical: chemical composition polymers are standard for warm-blooded mammals. Ecological: is a predator, hunts small rodents and birds.

Geographic: the species is cosmopolitan, lives almost everywhere and is attached to human habitation.

7. Define the concept “population”.

Due to unequal environmental conditions, individuals of the same species within the range break up into smaller units - populations. In reality, a species exists precisely in the form of populations. A population - the elementary structure of a species - is a collection of freely interbreeding individuals of a given species, living for a long time in a certain area of ​​the species' range. Within a population, the frequency of crossings between individuals is much higher than between populations. Species are the sum of populations; Natural selection occurs within a population, and under certain conditions, first subspecies and then new species can arise.

Think and do it.

1. Why can one species be distinguished from another only by a combination of various criteria? Which criteria do you think are most important?

Because there are twin species that are similar in morphological criterion and species similar in karyotype.

2. Do you know of examples when the formulation “a species as a genetically closed system” turned out to be incorrect?

Karpechenko's experiments. Radish and cabbage treated with colchicine with polyploid sets of chromosomes were able to give rise to a new species. Triticale. Animal hybrids (mule, hinny, zebroid, liger). Transduction (transfer of genes by bacteriophage from one type of bacteria to another). All the achievements of genetic engineering.

5. Provide evidence indicating that species objectively exist in nature.

To include a new species in the lists of the fauna of a particular region or the fauna of the World, there are so-called faunal commissions, consisting of qualified experts, who make this or that decision. Members of the Commission consider the irrefutable evidence of the presence of a particular species in a given territory. Typically, such evidence is complex materials from biological, environmental or genetic (molecular) studies. If a new species is described, its name and systematic status are later confirmed International Committee according to biological nomenclature.

>> Isolating mechanisms

1. What is the reason for the difference between the organisms discovered by Charles Darwin on the Galapagos Islands and closely related forms on the mainland?
2. What natural factors isolate some populations organisms from other populations of the same species?

Darwin discovered that differences between populations of the same species manifest themselves in the form of adaptation to different conditions life. In the light modern knowledge this means that individuals in populations develop certain genetically fixed properties that distinguish them from each other and ensure the best adaptation of organisms to the conditions of a particular area. Here's an example. Atlantic herring populations in different ocean areas multiply V different times year. A necessary condition The survival of juvenile herring is due to the coincidence in the timing of the hatching of larvae from eggs and the development of small phytoplankton - their main food. Depending on the latitude of the area, the peak development of phytoplankton occurs in spring, summer, autumn or winter.

Accordingly, spring-, summer-, autumn- and winter-spawning herring are distinguished, the populations of which live separately, have slight external differences, but belong to the same species and can interbreed, producing fertile offspring.

Can differences between populations lead to their reproductive isolation, to the loss of the ability of individuals from different populations to freely interbreed with each other?

Populations living on different islands are isolated from each other, and their individuals practically do not mix. It is clear that under conditions of such purely geographical isolation, differences in structure or behavior gradually accumulate, and this may ultimately lead to the formation of new species of animals or plants.

How do individuals of different populations lose the ability to interbreed and exchange genes with other individuals of the same species? Is this simply due to geographical separation or are there other mechanisms? The answers to these questions provide the key to understanding the mechanisms of speciation.

It has been established that physical factors environment habitat and biological properties organisms can lead to restrictions on gene exchange. This occurs as a result of the activation of various types of isolating mechanisms

Let's look at some examples.

On Hawaiian Islands There are two species of fruit flies that are very similar in appearance. Both species live in the same places, feeding on the sap of the same woody plant. In this case, one species feeds on sap flowing down the trunks and branches in the upper tiers of the tree, while the other feeds on puddles of sap on the forest floor. Crossing between these species never occurs due to their spatial separation. This example shows that genetic differences between populations can arise from different ecological specializations.

An interesting example of behavioral isolation is demonstrated by various species of fireflies. Each of the species living together is characterized by a certain light trajectory and types of emitted light signals. The trajectories can be zigzag, straight or in the shape of a loop, and the light pulsations can be short or long in the form of stable reflections (Fig. 76). When mating, individuals select each other, strictly focusing on the type of light signal. This example shows that isolation between populations can be reinforced by forming certain types behavior - the development of reflex reactions only to signals of one type or another.

The pollen of some plant species, such as orchids, is carried only by certain species of animals, whose instinctive behavior ensures that gene exchange will occur only among individuals of their own species.

In animals with external fertilization, isolating mechanisms operate at the molecular level. U starfish and some species of mollusks the role of isolating factors play differences in the structure of special protein molecules that connect sperm and eggs. Being on the surface of eggs, these molecules react only to sperm of “their” species, which excludes the possibility of fusion of reproductive products of different species. In animals with internal fertilization, this role is played by differences in the structure of the genital organs.

Finally, in many animals the breeding season begins with strictly defined combinations external factors(for example, temperature and light). These factors act on them as signals to begin mating. Different species react to the same factors differently, which is why their breeding times do not coincide. Figure 77 shows the differences in breeding times among different species of amphibians living in the same areas .

Isolating mechanisms prevent the development of an organism from a zygote formed as a result of the fusion of gametes of a male and female of different species. Hybrids that arise in this way usually die quickly or remain infertile. For example, a mule - a hybrid of a horse and a donkey - is sterile; it cannot produce offspring due to the fact that meiosis is impossible with its set of chromosomes. Hybrids of the white hare and brown hare, marten and sable are sterile.

Reproductive isolation. Isolating mechanisms.

1. What are isolating mechanisms? What is the importance of isolating mechanisms?
2. What types of isolating mechanisms do you know? Give examples.
3. Why are hybrids of different species of organisms sterile?

Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 9th grade
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Question 1. Define the species.

A species is a collection of individuals that have similar genetic, morphological, physiological characteristics, are capable of crossing with the formation of fertile offspring, inhabit a certain area, have common origin and similar behavior. A species is a basic systematic unit. It is reproductively isolated and has its own historical destiny. Species characteristics ensure the survival of both the individual and the species as a whole. At the same time, behavior that is beneficial for the species can even suppress the instinct of self-preservation (bees die defending the family).

Question 2. Tell us what biological mechanisms prevent the exchange of genes between species.

The exchange of genes between species is prevented by reproductive isolation, that is, the impossibility of crossing with individuals of another species. There are several reasons for reproductive isolation.

Geographical isolation. Species that live at great distances or are separated by an insurmountable barrier are not able to exchange genetic information.

Seasonal isolation. The difference in breeding seasons for different species is ensured. For example, in one species of Californian pine, pollen ripens in February, and in another, in April.

Behavioral isolation. Characteristic of higher animals. For example, in many closely related species of waterfowl mating behavior has its own characteristic features, which excludes the possibility of interspecific crossing.

Question 3. What is the reason for the infertility of interspecific hybrids?

Each individual species has its own karyotype, which differs in the number of chromosomes, their shape, size, and structure. Differences in karyotypes lead to impaired fertilization, the death of embryos or the birth of infertile offspring. The sterility of the offspring is due to the fact that in the absence of paired homologous chromosomes, conjugation is disrupted in prophase I of meiosis. As a result, bivalents are not formed, meiosis is disrupted, and the development of full-fledged gametes does not occur in the hybrid offspring.

Question 4: What criteria do scientists use to characterize a species?

There are several main criteria for the type.

Morphological - similarity between external and internal structure organisms. With its help, it is easy to identify individuals of relatively well-differentiated species.

Genetic - structural features of the karyotype (number of chromosomes, their shape, size) and DNA. Used to separate closely related species and sibling species.

Physiological - the similarity of life processes. For example, different times of sexual activity in closely related species of Drosophila. This criterion can also include features of sexual and child-parent behavior: courtship rituals, care for offspring, etc.

Biochemical - similarity or difference in the structure of proteins, chemical composition of cells and tissues. It is used to separate, for example, closely related species of fungi that synthesize different chemicals.

Ecological - certain forms of interaction with other species and factors inanimate nature. For example, there are similar species of oaks that live on different soils: one on limestone, another on sandy soil, and a third on volcanic soil.

Geographic - by area (area of ​​distribution). For example, Galapagos finches are isolated from South American ones, i.e., island and mainland species have completely different habitats.

Question 5: What is the range of a species?

The range of a species is the area of ​​distribution of the species. The size of their habitats can vary greatly among different species. For example, Scots pine grows almost throughout the entire territory of Russia, and snowdrop is characteristic only of the North Caucasus.

Species that occupy large areas and are found everywhere are called cosmopolitans, and those that live only in small specific areas are called endemics. It is endemic species that make the greatest contribution to the diversity of life on our planet. And they also need the most careful protection - due to their small numbers, strict attachment to certain living conditions, certain food, etc.

Question 6. Describe the type of domestic cat according to the main criteria.

Morphological criterion: the mammal is small in size, has four legs and a tail, is covered with hair, has developed fangs and retractable claws.

Genetic - the karyotype of a cat is represented by 19 pairs of chromosomes, of which 18 pairs are somatic chromosomes and one pair is sex chromosomes. Material from the site

Physiological: prefers an evening-night lifestyle, lies in wait (rather than chasing) prey, and, if necessary, makes characteristic meowing and purring sounds.

Biochemical: the chemical composition of polymers is standard for warm-blooded mammals.

Ecological: is a predator, hunts small rodents and birds.

Geographic: the species is cosmopolitan, lives almost everywhere and is attached to human habitation.

Question 7. Define the concept of “population”.

A population is a collection of individuals of the same species that inhabit for a long time certain territory, freely interbreeding with each other and partially or completely isolated from individuals of other similar populations.

The species' range is usually home to a fairly significant number of populations, each of which is an elementary unit of evolution.

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Question 1. What is a species?

A biological species is a set of individuals that have the ability to interbreed with the formation of fertile offspring; inhabiting a certain area; possessing a number of common morphological and physiological characteristics and similarities in relationships with the biotic and abiotic environment.

Question 2. What biological mechanisms prevent individuals of different species from interbreeding?

The isolation of a species is maintained by reproductive isolation, which prevents it from mixing with other species during reproduction. Isolation is ensured by differences in the structure of the genital organs, fragmentation of habitats, differences in timing or places of reproduction, differences in behavior, ecological isolation and other mechanisms.

Geographical isolation. Species that live at great distances or are separated by an insurmountable barrier are not able to exchange genetic information.

Seasonal isolation. The difference in breeding seasons for different species is ensured. For example, in one species of California pine, pollen ripens in February, and in another, in April.

Behavioral isolation. Characteristic of higher animals. For example, in many closely related species of waterfowl, mating behavior has its own characteristic features, which eliminates the possibility of interspecific crossing.

Question 3. What is the reason for the infertility of interspecific hybrids?

Each individual species has its own karyotype, which differs in the number of chromosomes, their shape, size, and structure. Differences in karyotypes lead to impaired fertilization, the death of embryos or the birth of infertile offspring. The sterility of the offspring is due to the fact that in the absence of paired homologous chromosomes, conjugation is disrupted in prophase I of meiosis. As a result, bivalents are not formed, meiosis is disrupted, and the development of full-fledged gametes does not occur in the hybrid offspring.

Question 4. Define the concept of “species range”.

The range of a species is the area of ​​distribution of the species. The size of their habitats can vary greatly among different species. For example, Scots pine grows almost throughout the entire territory of Russia, and snowdrop is characteristic only of the North Caucasus.

Question 5. What is the radius of individual activity of organisms? Estimate the range of individual activity of some animal species typical of your region.

The radius of individual activity is the distance over which an organism can move, based on its vital characteristics and physical capabilities.

Activity of organisms: 1) in plants - the rate of horizontal growth of rhizomes and plagiotropic shoots (per year), as well as the distances over which diaspores and pollen are dispersed in one way or another; 2) in animals - the form of their behavior: the duration of active activity, taking into account the distances (radii of activity) of movement in individual habitats, as well as migration.

Question 6. What is a population? Do you think the range of a population can coincide with the range of a species? Prove your opinion.

A population is a collection of individuals of the same species, inhabiting a certain territory for a long time, freely interbreeding with each other and partially or completely isolated from individuals of other similar populations.

The species' range is usually home to a fairly significant number of populations, each of which is an elementary unit of evolution.

Question 7. How are the boundaries of a species’ habitat determined?

There is a division of boundaries into climatic, landscape, and biocenotic ones. They can be called environmental boundaries, since they are determined by a combination of factors. On the other hand, there are many cases where the border remains stable and dispersal does not occur, despite the fact that environmental conditions inside and outside the range are homogeneous and quite favorable for the species.

Biocenotic boundaries are determined by the relationships of different species in a community.

Range boundaries are often determined by changes climatic conditions, especially hydrothermal regime, day length, snow cover thickness, etc. The essence of climatic boundaries varies; in some cases, the limit of distribution of a species is determined by a lack of heat or a reduction in the duration of the warm period; in others, the severity of winters or a decrease in the amount of precipitation during a certain part of the year may be decisive.

The boundaries of the range are determined various factors, among which the most important role is played by climatic, edaphic, biocenotic and anthropogenic ones.