Speciation is the process of the emergence of one or more new species based on a previously existing one.
New species can arise in conditions of spatial isolation of populations, that is, from populations occupying different geographical areas. This speciation is called allopatric(from rp. allos - different, patria - homeland), or, more often, geographical. As a result of long-term separation of populations, genetic isolation may arise between them, which persists even if they end up together. Allopatric speciation is a rather long process. An example is the presence of three subspecies of the great tit - Eurasian, South and East Asian. These subspecies occupy clearly distinguishable habitats, although along the periphery of their habitats, South Asian tits are still interbreeding with other subspecies (this indicates incomplete speciation). Similarly, when the vegetation cover changed in the Quaternary period, the range of the May lily of the valley was divided into five independent geographical areas located at a considerable distance from each other, in which the European, Transcaucasian, Far Eastern, Sakhalin-Japanese and North American races were formed, differing in a number of significant characteristics. Subsequently, these races formed independent species of lily of the valley. The May lily of the valley (European race), which survived in the south of Europe, spread again throughout Europe.
Another way of speciation is sympatric speciation(from the gr. syn - together). This includes cases when the emerging new species is located within the range of the parent species. Sympatric speciation, therefore, is not associated with territorial separation of populations during the period of creation of genetic isolation. An example of sympatric speciation is the formation of seasonal races of the great rattle. In unmown meadows in nature, the rattle blooms all summer long. But when they began to regularly mow the grass in midsummer, the rattles, which bloomed at that time, were unable to produce seeds. By natural selection associated with human activity, only those plants that bloom before or after mowing were preserved and left seeds. This is how subspecies of the great rattle arose, isolated by flowering time. (This mode of speciation is often called ecological speciation.) Sympatric speciation also includes cases of the emergence of new species based on polyploidy and distant hybridization. So, different types of potatoes have chromosome sets 12, 24, 48, 72; chrysanthemums - 9, 18, 27, 36, 45, ... 90. This will give reason to believe that these species were formed from the original by a multiple increase in chromosomes. Such processes are well modeled in laboratory conditions by delaying chromosome segregation in mitosis (as a result of exposure to colchicine). Polyploids, as a rule, are more viable and competitive and can displace the parent species. In addition to plants, polyploidy as a method of sympatric speciation is also known in some animals (echinoderms, arthropods, annelids and etc.). In nature, distant hybridization between species can also occur with subsequent doubling of chromosomes in the genome. Along the banks of the Aldan River, for example, a small population of the rowan-cotoneaster plant grows, originating from an interspecific hybrid between rowan and cotoneaster. It is believed that more than 1/3 of all flowering plant species are of hybridogenic origin. It has been experimentally proven that this is the origin of the species of plum, wheat, tobacco, cabbage, cotton, bluegrass, pickle grass, raspberries, rutabaga, wormwood, irises, etc.
Speciation is an evolutionary process in which individual populations existing species living organisms form new species. In living nature, speciation occurs everywhere and always. However, this is usually a fairly long process that cannot be directly observed. Thus, the formation of a new species can take millions of years.
As a result of speciation, the number of species on Earth is constantly increasing. However, many species become extinct for one reason or another (due to change climatic conditions, as a result of human activity, etc.). Therefore, over the entire history of the Earth, the number of species of living organisms that arose on it, according to some estimates, exceeds a billion, but the number of living species is estimated at around 2 million.
There are two main methods of speciation, that is, how exactly a new species is formed from a previously existing one. One mode of speciation is called geographic (or allopatric), the other is called biological (or ecological, or sympatric).
When geographical method speciation one of the populations of a species finds itself in slightly different living conditions and isolated from other populations of the same species. Isolation prevents the exchange of genes, and new conditions force the population to follow its own evolutionary path. Over a series of generations, individuals develop new characteristics adapted to the existing environment. In this case, such changes may occur in the genotype that exclude the possibility of crossing with individuals of the original species of this population. As a result, a new species is formed on the basis of this population.
A classic example of a geographic mode of speciation is Darwin's finches. It is assumed that some groups of finches living in South America, one way or another ended up on different Galapagos Islands. Moreover, each group followed its own evolutionary path.
Biological mode of speciation usually occurs in a shorter period of time than geographical, and is typical in to a greater extent for plants than for animals. In biological speciation, a new species is formed as a result of a random change in the genotype of an individual. At the same time, it can no longer interbreed with other individuals of the original species. This, for example, occurs in plants as a result of polyploidy (a multiple increase in the number of chromosomes). The mutant plant can then reproduce vegetatively or through self-pollination, effectively founding a new species. Polyploidy is not the only way of biological speciation.
Methods of speciation
A new species can also be formed as a result of other chromosomal rearrangements.
Typically, biological speciation results in the original species splitting into species occupying different ecological niches. Therefore, it is also called ecological speciation.
In the territorial aspect in space, a new species can arise from one or a group of adjacent populations located on the periphery of the range of the original species. Such speciation is called allopatric (from the Greek alios - other, patris - homeland). In other cases, a new species may arise within the range of the original species, as if within a species; this path of speciation is called sympatric (from the Greek sym - together, patris - homeland). In the phylogenetic aspect (in time), a new species can arise through gradual changes in the same species over time, without any divergence of the original groups. This speciation is called phyletic.
A new species can arise by splitting a single ancestral species (divergent speciation). Finally, a new species can arise from the hybridization of two existing species - hybridogenic speciation. Let us briefly describe these main pathways of speciation.
Allopatric speciation(sometimes called geographical) is illustrated by the above examples of the emergence of the species in great gulls and in the group of Australian flycatchers. In allopatric speciation, new species can arise through fragmentation, the disintegration of the range of a widespread parent species. An example of such a process is the emergence of lily of the valley species (see Chapter 6). Another method of allopatric speciation is speciation during the dispersal of the original species, during which peripheral populations and their groups increasingly distant from the center of dispersal, intensively transforming in new conditions, become the ancestors of the species. Examples similar to speciation in the group of large gulls are also known for other birds, some reptiles, amphibians, and insects.
Allopatric speciation is based on some form of spatial isolation, and this path of speciation is always relatively slow, occurring over hundreds of thousands of generations. It is over such long periods of time that in isolated parts of the population of a species those biological characteristics are developed that lead to reproductive independence even when the primary isolating barrier is violated. Allopatric speciation is always associated with the history of the formation of the species range.
Sympatric speciation. In sympatric speciation, a new species arises within the range of the original species.
The first method of sympatric speciation is the emergence of new species with a rapid change in the karyotype, for example, with autopolyploidy. Groups of closely related species (usually plants) with multiple chromosome numbers are known (see Fig. 6.28). So, for example, in the genus of chrysanthemums (Chrysanthemum) all forms have a chromosome number that is a multiple of 9, 18, 27, 36, 45, ..., 90. In the genera of tobacco (Nicotiana) and potatoes (Solanum), the main, initial number of chromosomes is 12 , but there are forms with 24, 48, 72 chromosomes. In such cases, it can be assumed that speciation occurred through autopolyploidy - through doubling, tripling, quadrupling, etc., of the main set of chromosomes of the ancestral species. The processes of polyploidization are well reproduced experimentally by delaying the segregation of chromosomes in meiosis as a result of exposure, for example, to colchicine. It is known that polyploids can also arise in natural conditions. The resulting polyploid individuals can produce viable offspring only when crossed with individuals carrying the same number of chromosomes (or through self-pollination). Within a few generations, if polyploid forms successfully pass the “control” of natural selection and turn out to be better than the original diploid, they can spread and coexist with the species that gave birth to them (Fig. 13.4) or, as happens more often, simply displace it.
Rice. 13.4. An example of the occurrence of reproductive isolation during polyploidization: the tetraploid plant species Dicanthium annulatum, which lives throughout Hindustan, undoubtedly arose from a diploid ancestral form, now occupying a small disjunct area (according to N. Ross, 1962)
Polyploid forms, as a rule, are larger and are able to exist in more severe physiographic conditions. That is why in the highlands and the Arctic the number of polyploid plant species increases sharply (Fig. 13.5). Among animals, polyploidy plays a much smaller role in speciation than in plants, and in all cases is associated with the parthenogenetic method of reproduction (for example, in echinoderms, arthropods, annelids and other invertebrates).
Rice. 13.5. Distribution of polyploid flowering species (as a percentage of total number flora species) in different parts of Eurasia (according to different authors from N.V. Timofeev-Resovsky et al., 1977)
The second method of sympatric speciation is through hybridization followed by doubling the number of chromosomes - allopolyploidy (see below).
Finally, the last, sufficiently studied method of sympatric speciation is the emergence of new forms as a result of seasonal isolation. The existence of pronounced seasonal races in plants is known, for example in the rattle Alectorolophus major (see Chapter 10), the early-flowering and late-flowering forms of which are completely reproductively isolated from each other, and if the selection condition remains, it is only a matter of time before these forms acquire rank new species. The situation is similar with spring and winter races of migratory fish; it is possible that these forms are already different species, very similar morphologically, but isolated genetically (sibling species).
A feature of the sympatric path of speciation is the emergence of new species that are morphophysiologically close to the original species. Thus, with polyploidy, the size increases, but the general appearance of the plants remains, as a rule, unchanged; with chromosomal rearrangements the same picture is observed; with ecological (seasonal) isolation, the emerging forms also usually turn out to be morphologically poorly distinguishable. Only in the case of the hybridogenic emergence of species does a new species form appear, different from each of the parent species (but also having characteristics characteristic of the two original species).
Phyletic speciation. With phyletic speciation, the species, changing as a whole over generations, turns into a new species, which can be distinguished by comparing the morphological characteristics of these groups.
Phyletic speciation includes stasigenesis - the development of a species over time with a gradual change in the same ecological niche, and anagenesis - the development of a species with the acquisition of some new fundamental adaptations that allow it to form a completely new, wider ecological niche. An example of stasigenesis can be the development of the main trunk of the Upper Pliocene mollusks of the genus Giraulus (see Fig. 6.5).
Rice. 13.6. An example of phyletic speciation in a series of fossil European elephants (Elephas planifrons - E. meridionalis) according to the lamellar index (the amount of enamel on the teeth) (from V. Grant, 1980)
It is clear that in this case only a comparison of morphological characteristics is possible, since the results of phyletic evolution can only be studied using paleontological material (Fig. 13.6). At the same time, there always remains the possibility that at some stage of evolution other groups could diverge from a single phyletic trunk and phyletic speciation could actually turn out to be divergent. Therefore, in its “pure form,” phyletic evolution is apparently possible only as an idealized and simplified reflection of the evolutionary process in one of the segments of the life of a species (phratry).
Note that it is impossible to draw boundaries between individual species in a phyletic series of forms - it will always be conditional (see Chapter 12).
Divergent speciation(cladogenesis). Charles Darwin considered this type of speciation to be the most common (the only drawing in “The Origin of Species” is devoted to this particular type of speciation). Examples of this type of speciation - the emergence of new species as a result of the splitting of a single ancestral form - are the emergence of several species of whitefish around the Irish Sea (see Fig. 6.10) and the divergence of Darwin's finches on the Galapagos (see Fig. 6.12) and the evolution of North American fruit flies of the group pseudoobscura - persimilis (see Fig. 6.27).
Hybridogenic speciation(synthesis or syngenesis). This type of speciation is common in plants: according to some estimates, more than 50% of plant species are hybridogenic forms - allopolyploids. Let's just point out a few. Cultivated plum (Prunus domestica) with 2n = 48 arose by hybridization of sloe (P. spinosa, 2n = 32) with cherry plum (P. divaricata, 2n = 16) with subsequent doubling of the number of chromosomes. Some species of pickleberry, raspberry, tobacco, rutabaga, wormwood, iris and other plants are also allopolyploids of hybridogenic origin.
An interesting case is the emergence of a new sympatric species in Spartina townsendii (2l = 120) based on hybridization with subsequent doubling of the number of chromosomes of the local English look S. stricta (2n = 50) and introduced in the 70s of the XIX century. from North America S. alternifolia (2n = 70).
Knowledge base
Now the range of this species is intensively expanding due to the reduction of the range of the local European species. Another example of a hybridogenic species that arose sympatrically is the mountain ash (Sorbocotaneaster), which combines the characteristics of mountain ash and cotoneaster and was distributed in the mid-50s in the forests of southern Yakutia along the banks of the middle reaches of the river. Aldan (K.M. Zavadsky). As a result of hybridogenic speciation, complexes of species (or so-called semi-species) related to each other by hybridization - syngameons (V. Grant) can especially often be formed. In the case of such hybrid complexes, it is sometimes difficult to detect clear boundaries between individual species, although species as stable genetic systems are quite clearly distinguished.
All four main forms of speciation over time are shown schematically in Fig. 13.7.
Rice. 13.7. The main forms of phyletic (in time) speciation (from N.N. Vorontsov, 2001)
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Question 1. Name the main forms of speciation. Give examples of geographic speciation.
Depending on whether a species arises as a result of some isolating mechanisms - spatial or other - two forms of speciation are distinguished: 1) allopatric (geographic), when species arise from spatially separated populations; 2) sympatric, when species arise in a single territory.
An example of geographic speciation is the emergence different types lan-dying from the original species that lived millions of years ago in broad-leaved forests Europe.
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The invasion of the glacier tore apart the single habitat of the lily of the valley for several hours. It has been preserved in forest areas that escaped glaciation: in the Far East, southern Europe, and Transcaucasia. When the glacier retreated, lily of the valley again spread across Europe, forming a new species - a larger plant with a wide corolla, and in the Far East - a species with red petioles and a waxy coating on the leaves.
Such speciation occurs slowly; to complete it, hundreds of thousands of generations must change in populations. This form of speciation involves physically separated populations diverging genetically, eventually becoming completely isolated and distinct from each other due to natural selection.
Question 2. What is polyploidy? What role does it play in the formation of species?
Polyploidy is a type of mutational change in the body, in which there is a multiple increase in the number of chromosomes. It is most characteristic of plants, but is also known among animals.
Polyploidy is one of the possible ways of speciation, and in populations inhabiting the same geographical area and not separated by barriers.
Question 3. Which species of plants and animals known to you arose as a result of chromosomal rearrangements?
The emergence of new species through chromosomal rearrangements can occur spontaneously, but more often occurs as a result of crossing closely related organisms. For example, a cultivated plum with 2n = 48 arose by crossing sloe (n = 16) with cherry plum (n = 8) with a subsequent doubling of the number of chromosomes. Many economically valuable plants are polyploids, for example potatoes, tobacco, cotton, sugar cane, coffee, etc. In plants such as tobacco, potatoes, the initial number of chromosomes is 12, but there are species with 24, 48, 72 chromosomes.
Among animals, polyploids are, for example, some species of fish (sturgeon, spined loach, etc.), grasshoppers, etc.
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EVOLUTIONARY TEACHING. MICROEVOLUTION (SPECIATION)
Microevolution – evolutionary changes within a species
Population– elementary unit of evolution
Individual– object of natural selection
Mutations– elementary evolutionary material
View– a qualitative stage of the evolutionary process
MICROEVOLUTION— a set of evolutionary processes occurring within separate or adjacent populations of a species:
Disequilibrium of individual genotypes and alleles in populations
Changes in population genetic structure
Accumulation of differences between populations
Formation of new species
EVOLUTION FACTORS:
Phenomena or processes that change the genetic structure of populations:
1. Hereditary variability
2. Change in gene balance
3. Insulation
4. Natural selection
Hereditary variability
Mutational
– Undirected changes
– In a heterozygous state they may not appear
Combinative
– New combinations during meiosis and fertilization
– Promotes the spread of mutations
Provide high level hereditary diversity natural populations
Delivers material for natural selection!
Non-directional action!
Changes in gene balance
Sharp changes in the frequency of occurrence of rare alleles not associated with natural selection
Migrations
Founder effect
– Sharp fluctuations in the number of organisms in natural populations
Delivery material for natural selection!
Non-directional action!
Insulation
The emergence of any barriers to free interbreeding within a species
Spatial (geographical)
Biological
– Ecological
– Ethological
– Genetic
Non-directional action
Strengthens genetic differences between populations
Natural selection
Affects the phenotype adapting it to existing conditions.
Directed action!
Prerequisites for natural selection:
– Redundancy of offspring
– Struggle for existence
Intraspecific
Interspecific
With environmental factors
Stabilizing
– Maintaining the average value of the characteristic
– In stable conditions
Moving
– Shift of the average value of the characteristic
– In changing conditions, when settling new territories
Disruptive
– Against medium forms, securing extreme ones
The result is the emergence adaptations.
But! They relative And correspond to specific environmental conditions
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SPECIATION
Evolutionary transformations within a species (at the population level), leading to intraspecific divergence of characters (diversity)
The result of microevolution is the formation of new species from populations
Speciation pathways:
Gradualistic
– Allopatric(geographical)
Based on geographic isolation
Dispersal, habitat disintegration
Territorial isolation
– Sympatric(ecological)
Within the same area
Based on biological isolation (for example, seasonal)
Saltation
– Genetic isolation (more often in plants)
Polyploidy
Hybridization
ALLOPATRIC (geographic) speciation
Speciation in different areas- due to geographic (spatial) isolation
Reason such isolation may include obstacles that impede the migration of animals or the dispersal of plant seeds or long distances separating populations
Geographic isolation occurs when the original range of a species is divided by various natural barriers.
As a result, separated populations cannot interbreed freely with each other, resulting in different subspecies.
1. Flower girls turned out to be the first birds to inhabit the Hawaiian archipelago. Lack of competition with other species caused rapid adaptive radiation: they formed various species, differing in food preferences and, in accordance with them, in the shape of the beak (parrotlet, treebill, sicklebeak)
Adaptive radiation- the emergence of several species from one ancestor, associated with the development of adaptations to various environmental conditions
2. The differences between the species living on neighboring islands prompted Charles Darwin to come up with the idea of the origin of species, and since then birds have been named Darwin's finches (large ground-billed, warbler, thick-billed arboreal).
3. Example of a subspecies - brown trout
- Migratory fish of the salmon family.
- Length up to 1 m, weighs up to 13 kg (Caspian salmon - up to 51 kg).
- It lives in the coastal waters of the seas of Europe, including the Black, Caspian, Baltic and Aral seas.
- It goes to rivers to spawn. Valuable object of fishing and breeding.
- Freshwater forms of brown trout.
4. Example of a subspecies - brown hare
- In the 1930s, several dozen brown hares captured in Bashkiria were acclimatized in the south Western Siberia- in the Barabinsk forest-steppe.
- Huge distance Ural Mountains and the arid steppes of the lower Volga region and the Caspian region, unsuitable for the life of the Rus, led to the complete geographic isolation of the West Siberian population.
- Divergence occurred, and in 1956, Siberian hares had to be identified as a separate subspecies.
SYMPATRIC (ecological) speciation
It begins with the division of a primarily single population into two or more groups of organisms, which then continue to diverge.
This may occur as a result ecological specialization .
1. Ecological isolation is observed when discrepancy between the habitats of different forms of the same species or several related species, for example forest and meadow pipits .
2. African Lake Victoria, which was formed 12 thousand years ago, is home to more than 500 species cichlid fish , differing from each other in morphology, lifestyle, behavior and other characteristics
3. Example of a subspecies: big tit . Feeds on large insects.
4. Example of a subspecies: blue tit . It only hammers the stems of herbaceous plants and hunts for small insects in bark crevices and buds.
5. Example of a subspecies: Muscovy tit . They search the terminal branches of trees for food. They feed on small insects.
6. Example of subspecies: large rattle plant . Regular mowing of the grass in mid-summer led to the formation of two ecological races of this plant, differing in flowering times: the spring race has yellow flowers, and the autumn race has orange flowers.
Describe the 2 main modes of speciation
Example of subspecies: beetle willow leaf beetle : There are two ecological races - “willow” and “birch”. Beetles and larvae of the willow race are able to feed only on willow leaves; the birch race can feed on both birch and willow.
SALTATION speciation
In recent decades, data have been accumulating on the third method - saltation speciation, associated not with the divergence of populations, but with hybridization of closely related species .
New species may arise as a result polyploidization – multiple increase in the number of chromosomes.
Isolation of hybrids from parental species is due to polyploidy hybrids
Established for some species of lizards, fish and flowering plants.
cultural plum - the result of crossing sloe and cherry plum, with the subsequent doubling of the number of chromosomes in hybrids.
Chromosomal speciation is possible in those groups of animals that are capable of parthenogenesis.
Closely related species that arose in this way are found, for example, in salamanders genus Ambistoma.
In rodents, it is not uncommon for closely related species to differ by the number and shape of chromosomes. For example, at mole vole (Ellobius talpinus) there are 16 forms that are externally indistinguishable from each other, but differ in the number of chromosomes (from 32 to 54).
Forms of speciation
A) - conversion of existing species (phyletic speciation).
B) fusion of two existing species A and B and the formation of a new species C (hybrid speciation)
IN) due to divergence (by division) one ancestral species into several independently evolving species. This is the path that evolution has followed.
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MICROEVOLUTION– these are evolutionary transformations within a species (at the population level), leading to intraspecific divergence of characters (diversity) and SPECIATION.
Occurs based on:
§ under the influence of natural selection
§ when various insulation barriers arise.
The time scale of microevolution leading to the formation of new species for different systematic groups– hundreds, often thousands of years.
Remember why a population of organisms is considered the basic unit of evolution. Describe isolation as an elementary factor of 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, the exchange of genes between individuals of the isolated population and the rest of the population stops. This leads to a gradual change in characteristics in individuals of an isolated population, which leads to its transformation into one or more new species (Fig. 166).
Rice. 166. Scheme of speciation: individual branches - populations
Consequently, the formation of new species of organisms, or speciation, is the process of transforming individual genetically isolated populations of the original species into new species. Depending on the nature of the barriers preventing the interbreeding 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 physical objects act as barriers to the crossing of individuals: land or sea spaces, mountain ranges, deserts, etc. Geographic speciation is carried out in two ways: the resettlement of individuals of a population to new territories or the division of the previous habitat of the population into separate isolated parts. As a result of this, geographical subspecies of the original species are formed, which become the ancestors of independent new species of organisms.
An example of geographic speciation through the dispersal of individuals into 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 a single species of gull that existed several hundred thousand years ago in the area of the modern Bering Strait (indicated by a cross in the figure). From it, through settlement to the east and west, several geographical subspecies of gulls were formed (areas of the subspecies are indicated in the figure), from which two new species were formed.
Rice. 167. Geographic speciation of two species of gulls: herring and black-billed gulls
An example of geographic 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. Due to glaciation, the single habitat of this species was torn into several parts. Lily of the valley was preserved only in forest areas that escaped glaciation: in the center and south of Europe, in Transcaucasia and in the south 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 flower corollas.
Rice. 168. Geographic speciation of three species of lilies of the valley
Ecological speciation. Associated with changes in the living conditions of the original ancestral species. Differences in the living conditions of isolated populations act as barriers to the crossing of individuals. 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. Coloring of egg shells in ecological subspecies of the common cuckoo
Thus, the formation of new species of organisms follows the following scheme: populations of the original species of organism >> geographic or ecological subspecies >> new species of organisms
Evolution does not stop with the formation of new species. It leads to the emergence of new and new species of plants, animals and other organisms, forming systematic groups above species - genera, families, orders, orders, classes, divisions, types.
Exercises based on the material covered
- What is speciation?
- What factor is the main one for the formation of new species of organisms?
- According to what scheme do new species of organisms form from the original ancestral species?
After watching this video lesson, everyone will be able to get an idea of the topic “The process of formation of species - speciation.” From it you will learn how, as a result of the action of evolutionary factors, changes in the gene pool accumulate in a population, which leads to the formation of new species. The teacher will talk about the central event of evolution - the process of speciation.
Evolution is a continuous process. New species are being formed even now as you read these lines. Most active speciation occurs in large bodies of water such as Lake Baikal or Lake Victoria. So, literally in the eighties, a new species of cichlid fish was found. It differed from its predecessors in the size and shape of its jaw. And in Baikal, the evolution of many species of plants and invertebrates is observed. How does evolution happen and is it possible to get a new species of animal at home? You will learn about this in our lesson.
As a result of the action of evolutionary factors, changes in the gene pool accumulate in the population. This leads first to the formation of new populations, then subspecies, and ultimately to the formation of new species.
That is, we can say that speciation is the central event in evolution, it is the main result of the evolutionary process.
Now scientists distinguish two ways of speciation:
1. Geographical (allopatric)
2. Biological (sympatric)
In geographic speciation, new species arise as a result of the appearance of spatial and territorial barriers. Obstacles can be of very different properties. These are purely geographical concepts such as mountains, rivers, and some man-made obstacles, such as the construction of large roads or plowing fields for agricultural needs.
By the way, it was geographical speciation that Charles Darwin explained by the appearance of such large quantity species of Darwin's finches that live on the islands of the Galapagos archipelago (see Fig. 1).
Rice. 1
Darwin believed that a number of finches could have been brought to these islands during storms, storms and strong winds.
But since the conditions were suitable for their life, the finches settled on these islands, while they were isolated both from the population on the neighboring islands and from the original South American population. This led to the fact that changes began to accumulate within such populations, which led to the formation of new species.
Speciation occurs in a similar way during the dispersal of the species (). At the same time, extreme populations accumulate a lot of changes in their gene pool and already differ in genetic composition from the original population, where they actually originated. This is exactly how several species of sturgeon fish were formed.
We can say that geographic speciation also occurs when the range of a species disintegrates, during the so-called mosaic range. Now here are several species of dandelions that arose from the once ancestral species of dandelion, which occupied the entire European continent (see Fig. 2).
The common dandelion now occupies forest edges, roadsides, and some weedy places, and you know this plant very well.
Rice. 2
Dandelion - Kok-saghyz grows in arid places, so pay attention to the shape of its leaves: its leaves are slightly thinner and more cut.
Pink dandelion has mastered the highlands of the Tien Shan. It is very similar to the common dandelion, but differs from it in the color of its petals.
Note that geographic speciation occurs very slowly. A change of hundreds of thousands of generations is necessary for the process of formation of a new species to take place. The second way of speciation is biological speciation.
Biological speciation occurs within the range of a single species and is based on biological isolation.
The ways in which biological isolations arise are different. For example, one of them may be polyploidy, when polyploid organisms arise within one population as a result of mutation. These organisms are viable and competitive, but in their genetic composition they differ from the rest of the population and cannot interbreed with them.
So within one population another population arises, which then forms into a separate species. Ecological isolation may also underlie biological speciation.
Ecological isolation is, for example, separation in breeding lines within one species. This is also some kind of seasonal isolation in part of the population, from the whole population.
All this leads to the accumulation of changes, resulting in the emergence of a new species. I would like to note that the lines of biological speciation are much shorter than those of geographic speciation. But they have the same result - the formation of new species.
Bibliography
- Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology. General patterns. - M.: Bustard, 2009.
- Pasechnik V.V., Kamensky A.A., Kriksunov E.A. Biology. Introduction to general biology and ecology. Textbook for 9th grade. 3rd ed., stereotype. - M.: Bustard, 2002.
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Homework
- What are the main pathways of speciation?
- What was the phenomenon of “Darwin’s” finches?
- How do you explain the concept of “mosaic habitat”?
- Compare biological and geographic speciation.
The formation of species is an important stage in evolution. It begins in populations saturated with constantly occurring mutations, which, when freely crossed, form new genotypes and phenotypes (see the chapter “Fundamentals of Genetics and Selection”). This leads to divergence of characteristics among individuals of a given population - divergence(55).
Organisms with altered characteristics are able to colonize new habitats and increase their numbers. Individuals with extreme contrasting deviations have the greatest opportunities to survive and leave fertile offspring. Intermediate forms compete more and die out faster. Thus, new groups arise in the original population, 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. Darwin explained the formation of genera, families, orders, etc. in a similar way.
There are two methods of speciation: geographical and ecological.
Geographic speciation is associated with the expansion of the range of the original species or its division into isolated parts by various natural barriers (rivers, mountains, etc.). When the range of a species expands, individuals of populations encounter new soil and climatic conditions, different animals and flora. In new unusual conditions, those individuals whose genotypes are most consistent with these conditions will survive and leave offspring. This leads to a change in the gene pool, the formation of new populations, and subsequently to the emergence of subspecies and species.
Isolation of populations, which prevents free crossing, also leads to changes in the gene pool of populations, and then to the creation of new populations, subspecies and species. An example of geographic isolation is centers of origin cultivated plants(see section “Selection”). The separation of these centers 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 colonization of new habitats (ecological niches) within the range of its species. At the same time, small groups of one population may find themselves in environmental conditions that are 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 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 evolved in different habitats (56):
- buttercup is an aquatic plant;
- pimple buttercup grows in moist soils;
- golden buttercup - in meadows, gardens, along roads;
- buttercup Kassubian (Kashubian) - in forests and parks;
- poisonous buttercup - in very damp places.
Elementary evolutionary factors.
The evolutionary process occurring 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.
- Such factors may be the following: emergence of new hereditary changes
- - mutations and combinations leading to the emergence of new genotypes in populations; fluctuations in population numbers, called population waves
- . They can occur due to seasonal changes (annual plants, insects), food availability (mass reproduction of rodents), and 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 population increase, other genes may increase in concentration; geographical or biological isolation populations that create barriers to free interbreeding, leading to differences in genetic composition different populations
and to their isolation.
All these changes in the gene pool are random in nature, they are multidirectional. The only selecting and directing factor of evolution is natural selection, which, under changed conditions, selects and increases the number of individuals whose genotype is more suitable for specific environmental conditions and reduces the number of individuals with a genotype less appropriate for this environment. The diversity of species is the result of the divergence of characteristics and the directing 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.
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