The formation of new species in nature. Formation of new species. Links and notes

Competition and other interspecies interactions can stimulate rapid speciation when natural selection joins in to improve the mechanisms for successful mating within one species. This conclusion was made by Australian scientists based on the study of different natural populations of one species of tropical tree frogs.

Even Charles Darwin drew attention to the fact that in some species-rich systematic groups (genera, families), the most closely related species differ primarily in features associated with the choice of a marriage partner and mating. These can be both structural features of the genital organs, and specific signals to attract a partner - for example, the croaking of a male frog or odorous substances (attractants) secreted by females (and sometimes males) of some insects. If in some part of the population there is a selection to change such traits, then it may be reproductively isolated (sometimes called "sexually isolated") from the maternal population. This happens especially quickly when hybrids between individuals from the separated group and the maternal population are characterized by reduced viability. Such an increase in the degree of sexual isolation and, accordingly, the acceleration of the formation of a new species received a special name in the English-language literature - "reinforcement" (which literally means "strengthening").

Currently, specialists in the field of evolutionary biology are intensively studying "reinforcement" and often associate it with such a long-described phenomenon as "character displacement" - a change in a trait not in all populations of a species, but only in those that are in contact with ecologically closely related competing species. So, in two species of Galapagos finches living on different islands (under allopatry), beaks are approximately the same size, but when the same species live on the same island (under sympatriy), then the average size of their beaks differ significantly, which suggests and differences in the food they eat. If a trait that shifts in the presence of a competitor is responsible for ensuring a meeting with a sexual partner, then this can lead to very rapid speciation, since the exchange of genes between the budding and maternal populations stops.

In the journal " Ecology Letters A recent review article by two Australian researchers, Conrad Hoskin and Megan Higgie, examines in detail how a new species can separate from a population of one species by changing mating traits. The object of their research is the Green-eyed tree frog, Litoria genimaculata(Fig. 1), a species fairly widespread in the tropical rainforests of Australia and New Guinea. Where the work was directly carried out, in the area of ​​the Barron River (Barron, northeastern Queensland, Australia), there are two (this is proven by molecular genetic methods) allopatric - that is, occupying different territories - populations of green-eyed frogs, conventionally designated as " northern” and “southern” (Fig. 2). These populations were apparently formed sometime in the Pleistocene, during a period of colder and arid climate, when tropical forests remained only in the form of isolated islands in the open landscape.

Later, about 6,500 years ago, when the climate became warmer and wetter and the tropical forests again formed a continuous cover, the distribution areas of these frog populations closed. Hybrids between them in the border area are possible, although they are characterized by reduced viability. A peculiar situation developed, however, in the north of the surveyed territory, where a small “enclave” represented by individuals of the “southern” population was surrounded by the “northern” population (in Fig. 2 it is marked as iS). It was in this "enclave" that an unusually rapid shift in the signs responsible for the meeting of the sexes (characteristics of the calling cry of males and the response of the female) was discovered. As a result of this shift, the individuals of this group have lost the ability to normally interbreed with individuals of the mother (the main "southern" population). How this could happen is shown schematically in Figures 3 and 4 below.

Rice. 3 illustrates three possible variants of evolutionary changes in individual genetic lines of the population: in the "southern" ( S) and "northern" ( N). Each circle corresponds to a certain line at a certain point in time. The upper half of the circle is post-zygotic (coming after fertilization) isolation. If for coexisting populations these halves are of the same color, then the hybrids are viable, if they are different, they are not viable. The lower half of the circle corresponds to prezygotic (that is, existing even before fertilization) isolation: mating of individuals of different lines is possible if they are of the same color, impossible if they are different. The scale of time (and, accordingly, evolution) is directed from top to bottom. Barriers that prevent the meeting of individuals of different genetic lines are shown as black vertical lines and are indicated by the word "Barrier". Red horizontal arrows show the possibility of contact between individuals of different lines. Black thin arrow down - crossing individuals of different lines. The red cross indicates the impossibility of hybridization.

Three cases are considered: a- classical allopatric speciation (gradual accumulation of differences in populations isolated from each other and the loss of the ability to interbreed); b- allopatric speciation that has begun is completed by “reinforcement” (although individuals are still able to mate, their hybrids are sterile or are characterized by reduced viability); c- the allopatric speciation that has begun changes its course due to a new system of barriers: the population of the "southern" line is subdivided by a new barrier, while part of it falls on one side of the barrier along with the "northern" line; it is in this part of the southern population (shown by a small circle with an index S') there is a selection aimed at isolating as quickly as possible from the "northern" population (this is possible due to behavioral mechanisms that ensure the meeting of marriage partners); in this case, the selection goes so far that individuals of the new line ( S') lose the ability to interbreed with individuals of the maternal southern line ( S), actually leading to the formation of a new species.

On fig. 4. just the variant of the displacement of reproductive traits in the frog population is illustrated in more detail A in the case of living next to a competing species. Selection for reproductive traits, aimed at enhancing differences from a competitor, leads to the fact that individuals of this part of the population ( B) may lose the ability to interbreed with the parent population and, in fact, give a new look. Thus, interspecies interactions can stimulate speciation within one of the species.

Finding in nature a shift in the reproductive traits of one species (which can lead to the formation of a new species) caused by interactions with other species is actually very difficult. And one of the complicating circumstances is the extraordinary speed of these changes. The results of research by Australian scientists are another confirmation that our impossibility to see the process of speciation with our own eyes is not due to the fact that this process is extremely long (as was believed in Darwin's time), but, on the contrary, is very fast.

The formation of species is an important stage in evolution. It begins in populations saturated with constantly emerging mutations, which, when freely crossed, form new genotypes and phenotypes (see the chapter "Fundamentals of Genetics and Selection"). This leads to a divergence of characters among the individuals of this population - divergences(55). The initial population forms a group of forms with varying degrees of trait deviations.

Organisms with altered traits are able to develop new habitats and increase their numbers. Individuals with extreme contrast deviations have the greatest chances to survive and leave fertile offspring. Intermediate forms compete more and die out faster. Thus, in the original population, new groupings arise, from which new populations are first formed, and then, with subsequent divergence, new subspecies and species. The principle of divergence explains the origin of the diversity of life forms. Similarly, Darwin explained the formation of genera, families, orders, etc.

There are two ways of speciation: geographical and ecological.

Geographic speciation associated with the expansion of the range of the original species or its division into isolated parts by various natural barriers (rivers, mountains, etc.). With the expansion of the range of the species, individuals of the populations meet with new soil and climatic conditions, with a different animal and plant world. In new unusual conditions, those individuals whose genotypes most correspond to these conditions will survive and leave offspring. This leads to a change in the gene pool, the formation of new populations, and in the future to the emergence of subspecies and species.

Isolation of populations, preventing free interbreeding, also leads to a change in the gene pool of populations, and then to the creation of new populations, subspecies and species. An example of geographic isolation is the centers of origin of cultivated plants (see the Breeding section). The separation of these foci from each other by ridges, deserts, and oceans contributed to their isolation and the autonomous formation of flora in them, which led to an exceptional diversity of related plants in them.

Ecological speciation associated with the settlement of new habitats (ecological niches) within the range of its species. At the same time, small groups of the same population may find themselves in environmental conditions unusual for them within the range of their species. New conditions will contribute to the identification and consolidation of new mutations and a change in the direction of natural selection, which will lead to a change in the gene pool, to an even greater isolation of populations, and then to the formation of new populations, subspecies and species adapted to new specific conditions.

An example of this is the five species of buttercups that have developed under different habitat conditions (56):

1. hard-leaved ranunculus - an aquatic plant;
2. ranunculus grows on wet soils;
3. golden buttercup - in meadows, gardens, along roads;
4. kassubian buttercup (Kashubian) - in forests and parks;
5. ranunculus poisonous - in very damp places.

Elementary evolutionary factors.

The evolutionary process that takes place in a population, leading to a change in the genetic structure of the population and directed by natural selection, is called microevolution. It begins in a population consisting of individuals with unequal genotypes. The totality of all genotypes in a population is called gene pool. When exposed to various elementary factors evolution, the gene pool of a population changes.

These factors can be the following:

1. the emergence of new hereditary changes - mutations and combinations leading to the emergence of new genotypes in populations;
2. population fluctuations, called population waves. They can arise in connection with seasonal changes (annual plants, insects), with food security (mass reproduction of rodents), with natural disasters (droughts, floods, fires). Population waves change the concentration of individual genes. During a population decline, some genes may disappear, and with a new increase in the population, other genes may increase their concentration;
3. geographical or biological isolation populations that create barriers to free interbreeding, which leads to differences in the gene composition of different populations and to their isolation.

All these changes in the gene pool are random in nature, they are multidirectional. The only selective and guiding factor in evolution is natural selection, which, under changing conditions, selects and increases the number of individuals whose genotype is more consistent with specific environmental conditions and reduces the number of individuals with a genotype that is less appropriate for this environment. The diversity of species is the result of the divergence of characters and the guiding creative role of natural selection.

Natural selection usually leads to a gradual complication and increase in the organization of living forms, their relative adaptability to the conditions of existence and the diversity of species.

Species participate in evolutionary processes in the formation of higher systematic groups. This process is called macroevolution, or supraspecific evolution. In macroevolution, the same processes take place as in speciation - the divergence of characters, the struggle for existence and natural selection.

As the conditions of life change, the direction of natural selection also changes. If groups of individuals of one widely dispersed species fall into different conditions or begin, for example, to hunt for different prey, then selection in these groups will go in different directions. This will lead to the formation of various devices in them. As a result, from one view through natural selection several new species are formed, i.e., the process of speciation will be carried out. To illustrate it, Darwin gives a diagram of divergence or divergence of signs.

In the scheme, capital letters (A, B, C, D, etc.) under the bottom line conventionally designate individual species of the same genus. Parallel lines from bottom to top (from I to XIV) symbolize the change of generations in time. Darwin conditionally accepts that a thousand generations pass from one line to the next. Dashed lines directed from bottom to top illustrate the historical fate of these generations at different stages of development. The greater the distance between dashed lines crossing one parallel line, the greater the difference between diverging groups of individuals in the corresponding generation (II line) than between points a5 and m5 (V line). This means that the initial discrepancy (difference) between two groups of descendants (a2 and m2) of the common original species A, which arose during 2000 generations, is less than the differences that developed after 5000 generations (a5 and m5). Therefore, says Darwin, we can assume that the groups a2 and m2 are two more varieties of the same common species, and the groups a5 and m5 will already be two new species that have a common ancestor (species A).

Thus, according to Darwin, new species arise through a series of intermediate steps: first, two (or more) varieties arise within the same species; these varieties, continuing to diverge in their characters, gradually become subspecies and, finally, new species. A variety is a step towards the formation of a new species. At the same time, according to the divergence scheme, as a rule, one old species gives rise to not one, but several species.

Divergent nature Speciation occurs because the initial difference between organisms within a species increases its abundance. Darwin illustrates this point with this example: the total number of wolves begins to grow as different families of this predator begin to hunt different prey. Some wolves "specialize" in livestock, others in wild animals. As a result, the total number of wolves is increasing. There are different directions of natural selection among them and as a consequence of this - divergence.

Let's return to the divergence scheme. In the course of evolution, new directions of selection also arise (dashed lines from species A and species I branch many times). Some of these lineages turn out to be dead ends: their descendants do not survive to the present (XIV line) and die out, replaced by more adapted species. Do not leave behind offspring (i.e. die out) and many of the original species. And some (line F in the diagram) survive to the present, almost without changing their features.

At the final stage of the process considered, the newly emerged species exhibit varying degrees of similarity. On line XIV, 5 groups of species closest to each other are clearly visible. The reason for this proximity, as can be clearly seen from the diagram, is the close relationship of such species. Systematist, combining closely related species into one genus, thereby reflecting the kinship of the common origin of these species.

childbirth in turn, they unite into families, families into orders, etc. The divergence scheme shows that in this case, too, the basis of such associations is the process of evolution itself. If closely related, the species will belong to the same genus; if more distantly, they will belong to the same family. Finally, very distant species will fall into different classes of the same type. This would mean that all species of the same type have in the end one common ancestor, only this ancestor is extraordinarily ancient.

So, the modern systems of plants and animals reflect a certain stage of evolution. At the same time, it is important to remember that the modern species described by taxonomists are real now, but historically temporary: once upon a time they were only subspecies; in some distant future they may become genera uniting groups of new related species; these new future species in modern times are only subspecies or varieties. Thus, the divergence scheme explains how evolution is the basis of modern taxonomy. At the same time, it shows that divergence inevitably leads to the emergence of a variety of organic forms in nature.

The divergence scheme helps to understand another important issue. The general increase in the diversity of organic forms greatly complicates the relationships that arise between organisms in nature. Therefore, in the course of historical development, as a rule, the most highly organized forms receive the greatest advantages. Thus, the general progressive development of the plant and animal world on Earth from lower to higher forms is carried out.

However, in cases where the conditions of life do not become more complicated, but remain practically unchanged, organisms are preserved without further complication.

Along with divergence, the result of evolution can also be the opposite result - convergence, or convergence of features. Convergence arises from the unidirectional action of natural selection in organisms systematically distant from each other, when these organisms live in similar conditions. An example of convergent similarity is the streamlined body shape of the shark (fish), ichthyosaur (extinct, neck aquatic reptile), and dolphin (aquatic mammal). The similarity of body shapes here is caused by a non-close family. but by the unidirectional action of natural selection in the same aquatic environment, where such a form is useful for both fish and dolphins.

Thus, on the basis of the action of natural selection, relative fitness is formed, new species are formed, the general diversity of organic forms in nature is growing, and the progressive development of the animal and plant world on Earth is carried out.

The formation of new species in nature is the final stage of microevolution. Under the influence of evolutionary factors, with the leading role of natural selection, there is a process of transformation of genetically open intraspecific populations into genetically stable species systems. Between individuals of different populations within a species, the process of crossing and leaving fertile offspring is possible. As long as there is a flow of genes between populations within a species, the species gene pool is a single system. As a result of isolation of populations, crossing between divergent forms ceases, there is no exchange of hereditary information, and populations or groups of populations become independent genetic systems.

A species is considered a qualitative stage in the evolutionary process, since it is the smallest genetically stable supraorganismal system in wildlife. In the course of speciation, two processes are mainly carried out: the emergence of adaptations to changing environmental conditions and isolation based on the isolation of new species. Based on the territory of distribution of the original species, two main ways of speciation can be distinguished: allopatric and sympatric.

Allopatric or geographic speciation is associated with the spatial isolation of divergent groups and can be carried out mainly by migration or dismemberment of the range by various barriers (rivers, mountains, soils, climate, etc.). When settling outside the range of the original species, populations fall into other environmental conditions, which due to microevolutionary processes can lead to the formation of new species. An example of species differentiation in the process of migration can be a complex set of populations and subspecies of the great tit species. The dispersal of this species from Europe to the East went in two ways: northern to the Far East and southern around the Central Asian Highlands. In the Far East, there are Eurasian and East Asian subspecies, which, when living together, do not produce hybrids. In the process of settling, a reproductive barrier arose between them.

Speciation by fragmentation of the range of the parent species can be traced in the example of the emergence of lily of the valley species. The forest lily of the valley was widely distributed in Eurasia several million years ago, but due to glaciation, its range split into several territories. To date, several new species have emerged.

Sympatric speciation occurs within the range of the parent species. Several of its ways can be distinguished: by polyploidy (in the tobacco genus, the initial number of chromosomes is 12, but there are forms with 24, 48, 72 chromosomes); by hybridization with subsequent doubling of chromosomes (interspecific hybrids are common among plants: raspberry, wormwood, mountain ash-kyzylnik), by seasonal isolation (Lake Sevan trout forms winter and spring races in terms of reproduction).

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Biology
a course of lectures for students studying in Russian Ryazan Authors-compilers: associate professor, Ph.D. Kalygina T.A.

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Recall why a population of organisms is considered the basic unit of evolution. Describe isolation as an elementary factor in evolution. What forms of isolation exist between populations of organisms in nature?

The main condition for the formation of new species of organisms is isolation. As a result, it stops the exchange of genes between individuals of the isolated and other populations. This leads to a gradual change in characters in individuals of an isolated population, which leads to its transformation into one or more new species (Fig. 166).

Rice. 166. Scheme of speciation: separate branches - populations

Consequently, the formation of new species of organisms, or speciation, is the process of transformation of individual genetically isolated populations of the original species into new species. Depending on the nature of the barriers that prevent the crossing of individuals, two methods of speciation are distinguished - geographical and ecological.

Geographic speciation. Associated with a change in the range of the original ancestral species. Various geographical objects act as barriers to interbreeding of individuals: expanses of land or sea, mountain ranges, deserts, etc. Geographic speciation is carried out in two ways: by dispersal of individuals of a population to new territories or by dividing the former habitat of a population into separate isolated parts. As a result, geographical subspecies of the original species are formed, which become the ancestors of independent new species of organisms.

An example of geographic speciation by dispersal of individuals to new habitats is the appearance of two species of gulls: herring and black-billed gulls (Fig. 167). The ancestral form of these two species was one species of gulls that existed several hundred thousand years ago in the region of the modern Bering Strait (indicated by a cross in the figure). From it, by way of settlement to the east and west, several geographical subspecies of gulls were formed (areas of subspecies are indicated in the figure), from which two new species were formed.

Rice. 167. Geographical speciation of two types of gulls: herring and clovers

An example of geographical speciation by dividing the former range of a species into several isolated parts is the appearance of three species of lilies of the valley (Fig. 168). The original ancestral type existed several million years ago in the broad-leaved forests of Eurasia. In connection with glaciation, the single range of this species was torn into several parts. The lily-of-the-valley has survived only in forest areas that have escaped glaciation: in the center and south of Europe, in the Transcaucasus and in the south of the Far East. From these surviving populations, three independent species of lilies of the valley subsequently formed, differing in the size of the leaves and the color of the corollas of flowers.

Rice. 168. Geographical speciation of three species of lilies of the valley

ecological speciation. It is associated with a change in the living conditions of the original ancestral species. Differences in living conditions of isolated populations act as barriers to interbreeding. As a result, ecological subspecies are formed, which become the ancestors of new species of organisms.

An example of ecological speciation is the appearance of several species in the genus Buttercup, growing in places with different humidity (Fig. 169).

Rice. 169. Ecological speciation in the genus Buttercup

Rice. 170. Egg shell coloration in ecological subspecies of the common cuckoo

Thus, the formation of new species of organisms follows the scheme: populations of the original species of the organism >> geographical, or ecological subspecies >> new species of organisms

With the formation of new species, evolution does not stop. It leads to the emergence of more and more new species of plants, animals and other organisms, forming supra-specific systematic groups - genera, families, orders, orders, classes, divisions, types.

Lesson learned exercises

1. What is visualization?
2. What is the main factor for the formation of new types of organisms?
3. According to what scheme do new types of organisms form from the original ancestral species?