Origin and evolution of land plants. Human Animal Evidence
The emergence of unicellular and multicellular algae, the emergence of photosynthesis: the emergence of plants on land (psilophytes, mosses, ferns, gymnosperms, angiosperms).
The development of the plant world took place in 2 stages and is associated with the appearance of lower and higher plants. According to the new taxonomy, algae are classified as lower (and earlier they were classified as bacteria, fungi and lichens. Now they are separated into independent kingdoms), and mosses, ferns, gymnosperms and angiosperms are classified as higher.
In the evolution of lower organisms, 2 periods are distinguished, which differ significantly from each other in the organization of the cell. During 1 period, organisms similar to bacteria and blue-green algae dominated. The cells of these life forms did not have typical organelles (mitochondria, chloroplasts, Golgi apparatus, etc.). The cell nucleus was not limited by the nuclear membrane (this is a prokaryotic type cellular organization). The 2nd period was associated with the transition of lower plants (algae) to an autotrophic type of nutrition and with the formation of a cell with all typical organelles (this is a eukaryotic type of cellular organization, which was preserved at subsequent stages in the development of the plant and animal world). This period can be called the period of dominance of green algae, unicellular, colonial and multicellular. The simplest of the multicellular are filamentous algae (ulotrix), which do not have any branching of their body. Their body is a long chain of individual cells. Other multicellular algae are dissected large quantity outgrowths, so their body is branched (at hara, at fucus).
Multicellular algae, in connection with their autotrophic (photosynthetic) activity, developed in the direction of increasing the body surface for better absorption of nutrients from the aquatic environment and solar energy. Algae have a more progressive form of reproduction - sexual reproduction, in which the beginning of a new generation is given by a diploid (2n) zygote, combining the heredity of 2 parental forms.
The 2nd evolutionary stage of plant development must be associated with their gradual transition from an aquatic lifestyle to a terrestrial one. The primary terrestrial organisms were psilophytes, which were preserved as fossils in the Silurian and Devonian deposits. The structure of these plants is more complex compared to algae: a) they had special organs for attaching to the substrate - rhizoids; b) stem-like organs with wood surrounded by bast; c) rudiments of conductive tissues; d) epidermis with stomata.
Starting with psilophytes, it is necessary to trace 2 lines of evolution of higher plants, one of which is represented by bryophytes, and the second by ferns, gymnosperms and angiosperms.
The main thing that characterizes bryophytes is the predominance of the gametophyte over the sporophyte in the cycle of their individual development. The gametophyte is everything green plant capable of self-feeding. The sporophyte is represented by a box (cuckoo flax) and is completely dependent on the gametophyte for its nutrition. The dominance of the moisture-loving gametophyte in mosses under the conditions of an air-ground lifestyle turned out to be inappropriate, therefore, mosses have become a special branch of the evolution of higher plants and have not yet produced perfect groups of plants. This was also facilitated by the fact that the gametophyte, in comparison with the sporophyte, had a dinner heredity (haploid (1n) set of chromosomes). This line in the evolution of higher plants is called gametophyte.
The second line of evolution on the way from psilophytes to angiosperms is sporophytic, because in ferns, gymnosperms and angiosperms, the sporophyte dominates in the cycle of individual plant development. It is a plant with a root, stem, leaves, organs of sporulation (in ferns) or fruiting (in angiosperms). Sporophyte cells have a diploid set of chromosomes, because they develop from a diploid zygote. The gametophyte is greatly reduced and adapted only for the formation of male and female germ cells. In flowering plants, the female gametophyte is represented by the embryo sac, which contains the egg. The male gametophyte is formed by the germination of pollen. It consists of one vegetative and one generative cell. When pollen germinates from a generative cell, 2 sperm are produced. These 2 male germ cells are involved in double fertilization in angiosperms. A fertilized egg gives rise to a new generation of plants - the sporophyte. The progress of angiosperms is due to the improvement of the reproduction function.
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Conclusions:
1. Study of the geological past of the Earth, the structure and composition of the core and all shells, flights spacecraft to the Moon, Venus, the study of the stars brings a person closer to the knowledge of the stages of development of our planet and life on it.2. The process of evolution was natural.
3. The flora is diverse, this diversity is the result of its development over a long period of time. The reason for its development is not divine power, but a change and complication of the structure of plants under the influence of changing environmental conditions.
Scientific evidence: the cellular structure of plants, the beginning of development from a single fertilized cell, the need for water for life processes, finding prints of various plants, the presence of "living" fossils, the extinction of some species and the formation of new ones.
on the topic: "Biocenoses and ecosystems"
PROPERTIES AND TYPES OF BIOCENOSES
Natural biocenoses are very complex. They are characterized primarily by species diversity and population density.
Species diversity- the number of species of living organisms that form a biocenosis and determine various nutritional levels in it. The number of species populations is determined by the number of individuals of a given species per unit area. Some species are dominant in the community, outnumbering others. If the community is dominated by a few species, and the density of the rest is very low, then the diversity is low. If, with the same species composition, the abundance of each of them is more or less even, then the species diversity is high.
In addition to the species composition, the biocenosis is characterized by biomass and biological productivity.
Biomass- total organic matter and the energy contained in it of all individuals of a given population or the entire biocenosis per unit area. Biomass is determined by the amount of dry matter per 1 ha or the amount of energy (J) 1 .
The value of biomass depends on the characteristics of the species, its biology. For example, in rapidly dying species (microorganisms), the biomass is small compared to long-lived organisms that accumulate a large amount of organic matter in their tissues (trees, shrubs, large animals).
biological productivity- the rate of formation of biomass per unit of time. This is the most important indicator life of an organism, population and ecosystem as a whole. There are primary productivity - the formation of organic matter by autotrophs (plants) in the process of photosynthesis and secondary - the rate of biomass formation by heterotrophs (consumers and decomposers).
The ratio of productivity and biomass is different in different organisms. In addition, productivity is not the same in different ecosystems. It depends on the size solar radiation, soil, climate. Deserts and tundra have the lowest biomass and productivity, and tropical rainforests have the highest. Compared to land, the biomass of the oceans is much lower, although it occupies 71% of the planet's surface, which is associated with a low nutrient content. In the coastal zone, biomass increases significantly.
In biocenoses, two types of trophic web are distinguished: pasture and detrital. AT pasture type In the food web, energy flows from plants to herbivores and then to higher-order consumers. Herbivores, regardless of their size and habitat (terrestrial, aquatic, soil), graze, eat green plants and transfer energy to the next levels.
If the flow of energy begins with dead plant and animal remains, excrement and goes to the primary detritivores - decomposers, partially decomposing organic matter, then such a food web is called detrital, or decomposition network. Primary detritophages include microorganisms (bacteria, fungi) and small animals (worms, insect larvae).
Both types of food web are present in terrestrial biogeocenoses. In aquatic communities, the grazing chain predominates. In both cases, the energy is fully utilized.
Ecosystem evolution
SUCCESSIONS
All ecosystems evolve over time. The successive change of ecosystems is called ecological succession. Succession occurs mainly under the influence of processes occurring within the community in interaction with the environment.
Primary succession begins with the development of an environment that was not previously inhabited: destroyed rock, rock, sand dune, etc. The role of the first settlers is great here: bacteria, cyanobacteria, lichens, algae. By isolating waste products, they change the parent rock, destroy it and promote soil formation. When dying, the primary living organisms enrich the surface layer with organic substances, which allows other organisms to settle. They gradually create conditions for an ever greater diversity of organisms. The community of plants and animals becomes more complex until it reaches a certain equilibrium with the environment. Such a community is called climax. It maintains its stability until the equilibrium is disturbed. The forest is a stable biocenosis - a climax community.
Secondary succession develops on the site of a previously formed community, for example, on the site of a fire or an abandoned field. Light-loving plants settle on the ashes, shade-tolerant species develop under their canopy. The appearance of vegetation improves the condition of the soil, on which other species begin to grow, displacing the first settlers. Secondary succession occurs over time and, depending on the soil, can be fast or slow, until finally a climax community is formed.
The lake, if the ecological balance is disturbed in it, can turn into a meadow, and then into a forest, characteristic of this climatic zone.
Succession leads to a progressive complication of the community. His food webs are becoming more and more branched, the resources of the environment are being used more and more fully. A mature community is most adapted to environmental conditions, species populations are stable and reproduce well.
ARTIFICIAL ECOSYSTEMS. AGROCENOSES
Agrocenosis- artificially created and maintained by man ecosystems (fields, hayfields, parks, gardens, kitchen gardens, forest plantations). They are created to obtain agricultural products. Agrocenoses have poor dynamic qualities, low ecological reliability, but are characterized by high yields. Occupying approximately 10% of the land area, agrocenoses annually produce 2.5 billion tons of agricultural products.
As a rule, one or two plant species are cultivated in an agrocenosis, so the relationships of organisms cannot ensure the stability of such a community. The action of natural selection is weakened by man. Artificial selection goes in the direction of preserving organisms with maximum productivity. In addition to solar energy, there is another source in the agrocenosis - mineral and organic fertilizers introduced by man. The main part of the nutrients is constantly taken out of the cycle as a crop. Thus, the circulation of substances is not carried out.
In the agrocenosis, as in the biocenosis, food chains are formed. Man is the essential link in this chain. And here he acts as a consumer of the first order, but the food chain is interrupted at this point. Agrocenoses are very unstable and, without human intervention, exist from 1 year (grain, vegetables) to 20-25 years (fruit and berry).
DEVELOPMENT OF BIOLOGY IN THE PRE-DARWIN PERIOD
The origin of biology as a science is associated with the activities of the Greek philosopher Aristotle (4th century BC). He tried to build a classification of organisms based on anatomical and physiological studies. He managed to describe almost 500 species of animals, which he arranged in order of complication. Studying the embryonic development of animals, Aristotle discovered a great similarity initial stages embryogenesis and came to the idea of the possibility of the unity of their origin.
Between the 16th and 18th centuries there is an intensive development of descriptive botany and zoology. The discovered and described organisms required systematization and the introduction of a single nomenclature. This merit belongs to the outstanding scientist Carl Linnaeus (1707-1778). He first drew attention to the reality of the species as structural unit living nature. He introduced a binary nomenclature of the species, established a hierarchy of systematic units (taxa), described and systematized 10,000 plant species and 6,000 animal species, as well as minerals. According to his worldview, K. Linnaeus was a creationist. He rejected the idea of evolution, believing that there are as many species as various forms was created by God in the beginning. At the end of his life, K. Linnaeus nevertheless agreed with the existence of variability in nature, faith in the immutability of the species was somewhat shaken.
The author of the first evolutionary theory Jean-Baptiste Lamarck (1744-1829) was a French biologist. Lamarck immortalized his name by introducing the term "biology", creating a system of the animal kingdom, where he first divided animals into "vertebrates" and "invertebrates". Lamarck was the first to create a holistic concept of the development of nature and formulated three laws of variability of organisms.
1. The law of direct adaptation. Adaptive changes in plants and lower animals occur under direct influence environment. Adaptations arise due to irritability.
2. The law of exercise and non-exercise of organs. On animals from the central nervous system environment has an indirect effect. Prolonged exposure to the environment induces habits in animals associated with the frequent use of organs. Its enhanced exercise leads to the gradual development of this organ and the consolidation of changes.
3. The law of "inheritance of acquired traits", according to which beneficial changes are transmitted and fixed in the offspring. This process is gradual.
Unsurpassed authority of the XIX century. in the field of paleontology and comparative anatomy was the French zoologist Georges Cuvier (1769-1832). He was one of the reformers of comparative anatomy and taxonomy of animals, introduced the concept of "type" in zoology. Based on rich factual material, Cuvier established the "principle of correlation of body parts", on the basis of which he reconstructed the structure of extinct animal forms. According to his views, he was a creationist and stood on the positions of the immutability of species, and considered the presence of adaptive traits in animals as evidence of the initially established harmony in nature. J. Cuvier saw the reasons for the change of fossil faunas in catastrophes that occurred on the surface of the Earth. According to his theory, after each catastrophe, the organic world was re-created.
MAIN PROVISIONS OF THE THEORY OF C. DARWIN
The honor of creating the scientific theory of evolution belongs to Charles Darwin (1809-1882), an English naturalist. Darwin's historical merit is not the establishment of the very fact of evolution, but the discovery of its main causes and driving forces. He introduced the term "natural selection" and proved that the basis for natural selection and evolution is the hereditary variability of organisms. The result of his many years of work was the book "The Origin of Species by Means of Natural Selection" (1859). In 1871, his other great work"Human Origins and Sexual Selection".
The main driving forces of evolution Ch. Darwin called hereditary variability, struggle for existence and natural selection. The starting position of Darwin's teaching was his statement about the variability of organisms. He singled out group, or specific, variability, which is not inherited and is directly dependent on environmental factors. The second type of variability is individual, or indefinite, which occurs in individual organisms as a result of uncertain environmental influences on each individual and is inherited. It is this variability that underlies the diversity of individuals.
Observing and analyzing one of the main properties of all living things - the ability to unlimited reproduction, Darwin concluded that there is a factor that prevents overpopulation and limits the number of individuals. Conclusion: the intensity of reproduction, as well as the limited natural resources and means of subsistence lead to a struggle for existence.
The presence of a spectrum of variability in organisms, their heterogeneity and the struggle for existence lead to the survival of the most adapted and the destruction of the less adapted individuals. Conclusion: in nature, natural selection takes place, which contributes to the accumulation of useful traits, their transmission and fixation in offspring. The idea of natural selection arose from Darwin as a result of observations of artificial selection and animal breeding. According to Darwin, the result of natural selection in nature was:
1) the emergence of devices;
2) variability, evolution of organisms;
3) formation of new species. Speciation proceeds on the basis of trait divergence.
Divergence- the divergence of characters within the species, arising under the influence of natural selection. Individuals with extreme traits have the greatest advantages in survival, while individuals with average, similar traits die in the struggle for existence. Organisms with evasive traits can become the ancestors of new subspecies and species. The reason for the divergence of characters is the presence of uncertain variability, intraspecific competition and the multidirectional nature of the action of natural selection.
Darwin's theory of speciation is called monophyletic - the origin of species from a common ancestor, the original species. Ch. Darwin proved historical development wildlife, explained the ways of speciation, substantiated the formation of adaptations and their relative nature, determined the causes and driving forces evolution.
EVIDENCE FOR EVOLUTION
biological evolution- historical process development organic world, which is accompanied by changes in organisms, the extinction of some and the emergence of others. modern science has many facts testifying to evolutionary processes.
Embryological evidence for evolution.
In the first half of the XIX century. the theory of "germ similarity" is being developed. The Russian scientist Karl Baer (1792-1876) established that early stages development of embryos, there is a great similarity between different species within a type.
The works of F. Müller and E. Haeckel allowed them to formulate biogenetic law:"ontogeny is a brief and rapid repetition of phylogenesis." Later, the interpretation of the biogenetic law was developed and refined by A.N. Severtsov: “in ontogenesis, the embryonic stages of the ancestors are repeated.” Embryos in the early stages of development have the greatest similarity. General traits of a type are formed during embryogenesis earlier than special ones. Thus, all vertebrate embryos at stage I have gill slits and a two-chambered heart. At the middle stages, features characteristic of each class appear, and only at later stages are features of the species formed. Comparative anatomical and morphological evidence of evolution.
The proof of the unity of origin is the cellular structure of organisms, a single plan for the structure of organs and their evolutionary changes.
Homologous Organs have a similar structural plan and a common origin, perform both the same and different functions. Homologous organs make it possible to prove the historical relationship of different species. The primary morphological similarity is replaced, in varying degrees, differences acquired in the course of divergence. A typical example homologous organs are the limbs of vertebrates, having overall plan buildings, regardless of their function.
Some plant organs develop morphologically from leaf primordia and are modified leaves (antennae, spines, stamens).
Similar bodies- secondary, not inherited from common ancestors, morphological similarity in organisms of various systematic groups. Similar organs are similar in their functions and develop in the process convergence. They testify to the same type of adaptations that arise in the course of evolution in the same environmental conditions as a result of natural selection. For example, similar animal organs are the wings of a butterfly and a bird. This adaptation for flight in butterflies developed from the chitinous cover, and in birds - from the internal skeleton of the forelimbs and feather cover. Phylogenetically, these organs were formed differently, but they perform the same function - they serve for the flight of the animal. Sometimes similar organs acquire a striking resemblance, such as the eyes of cephalopods and terrestrial vertebrates. They have the same general building plan, similar structural elements, although they develop from different rudiments in ontogeny and are in no way connected with each other. The similarity is explained only by the physical nature of light.
An example of similar organs are the spines of plants, which protect them from being eaten by animals. Spines can develop from leaves (barberry), stipules (white locust), shoots (hawthorn), bark (blackberry). They are similar only externally and in their functions.
Vestigial organs- relatively simplified or underdeveloped structures that have lost their original purpose. They are laid during the period of embryonic development, but do not fully develop. Sometimes the rudiments take on other functions compared to the homologous organs of other organisms. Thus, the rudiment of the human appendix performs the function of lymph creation, in contrast to the homologous organ - the caecum of herbivores. Rudiments of the pelvic girdle of a whale and the limbs of a python confirm the fact that whales originate from terrestrial tetrapods, and pythons from ancestors with developed limbs.
Atavism - the phenomenon of a return to ancestral forms observed in individual individuals. For example, zebroid coloring of foals, multi-mating in humans.
Biogeographic evidence for evolution.
The study of the flora and fauna of various continents makes it possible to restore the general course of the evolutionary process and to identify several zoogeographic zones with similar land animals.
1. The Holarctic region, which combines the Palearctic (Eurasia) and Neo-Arctic (North America) regions. 2. Neotropical region (South America). 3. Ethiopian region (Africa). 4. Indo-Malay region (Indochina, Malaysia, Philippines). 5. Australian region. In each of these areas, there is a great similarity between the animal and plant worlds. One area differs from others in certain endemic groups.
Endemics- species, genera, families of plants or animals, the distribution of which is limited to a small geographical area, i.e., it is a flora or fauna specific to a given area. The development of endemia is most often associated with geographic isolation. For example, the earliest separation of Australia from southern mainland Gondwana (more than 120 million years) led to the independent development of a number of animals. Not experiencing pressure from predators, which are absent in Australia, monotreme mammals, the first animals, have survived here: the platypus and echidna; marsupials: kangaroo, koala.
Flora and fauna of the Palearctic and Neoarctic regions, on the contrary, are similar to each other. For example, closely related are American and European maples, ash, pine, spruce. Of the animals, mammals such as moose, martens, minks, polar bears live in North America and Eurasia. The American bison corresponds to a related species - the European bison. Such a relationship testifies to the long-term unity of the two continents.
Paleontological evidence for evolution.
Paleontology studies fossil organisms and makes it possible to establish the historical process and causes of changes in the organic world. On the basis of paleontological finds, the history of the development of the organic world was compiled.
Fossil transitional forms - forms of organisms that combine features of older and younger groups. They help restore the phylogeny individual groups. Representatives: archeopteryx - a transitional form between reptiles and birds; foreigncevia - a transitional form between reptiles and mammals; psilophytes - a transitional form between algae and land plants.
paleontological series are composed of fossil forms and reflect the course of phylogenesis (historical development) of the species. Such rows exist for horses, elephants, rhinos. The first paleontological series of horses was compiled by V. O. Kovalevsky (1842-1883).
relics- species of plants or animals preserved from ancient extinct organisms. They are characterized by signs of extinct groups of past eras. The study of relic forms allows us to restore the appearance of extinct organisms, to suggest their living conditions and way of life. Hatteria is a representative of ancient primitive reptiles. Such reptiles lived in the Jurassic and Cretaceous period. The coelacanth fish coelacanth has been known since the early Devonian. These animals gave rise to terrestrial vertebrates. Ginkgoes are the most primitive form of gymnosperms. The leaves are large, fan-shaped, the plants are deciduous.
Comparison of modern primitive and progressive forms makes it possible to restore some features of the alleged ancestors of the progressive form, to analyze the course of the evolutionary process.
Abstract on the topic: "Biocenoses and ecosystems" PROPERTIES AND TYPES OF BIOCENOSES Natural biocenoses are very complex. They are characterized primarily by species diversity and population density. Species diversity - the number of living species aboutII. Embryological evidence (embryology studies the embryonic development of an organism).
1. Embryo similarity.
a) The structure of the chordate embryo consistently resembles the body of animals of other types:
ovum - protozoa;
gastrula - intestinal;
· roundworms;
Representatives of the subtype Cranial.
b) This indicates the common origin of all chordates.
2. Divergence of signs of embryos (embryonic divergence).
a) As the similarity between the embryos develops different types weaken.
b) First, the signs of the genus appear, and then the species.
· The initial similarity in the structure of the head of a child and a baby monkey gradually disappears.
3. Haeckel-Muller biogenetic law: each individual in its individual development (ontogenesis) briefly and concisely repeats the history of the development of its species (phylogenesis).
a) Examples in animals:
Vessels of the embryos of land vertebrates are similar to the vessels of fish;
The human fetus has gill slits.
· Butterfly caterpillars and beetle larvae are similar in structure to annelids.
Tadpoles of amphibians are similar to fish.
b) Examples in plants:
The bud scales in the bud of plants develop like leaves.
The petals of the buds are green at first, and then acquire their characteristic color.
· From the spores of moss, a green thread first appears, similar to filamentous algae (pre-sprout).
c) Amendments to the biogenetic law.
· In embryos, the repetition of phylogenesis may be disturbed due to adaptations to living conditions in ontogeny. Appear: embryonic membranes, yolk sac in fish eggs, external gills in a tadpole, cocoon in a silkworm.
Ontogeny does not fully reflect phylogenesis due to the appearance of mutations that change the course of development of the embryo (in the embryo of a snake, all vertebrae are laid at once, i.e. their number does not increase gradually; in birds, the five-fingered stage of limb development fell out, 4 fingers are laid in the embryo, and not 5, only 3 fingers grow in the wing).
In ontogenesis, the embryonic stages of development are repeated, and not adult forms (the Lancelet repeats in ontogeny the general stages with a free-swimming ascidian larva, and not with its adult, fixed form).
d) Modern ideas about the biogenetic law.
Severtsov showed that due to changes in development, some stages of embryonic development can fall out; there are changes in the organs of the embryo that were not in the ancestors; new species emerge; new signs are revealed (for example, tailed (newts) and tailless (frogs) amphibians descended from one ancestor: the newt larva is long, because it has many vertebrae, in the frog larva the number of vertebrae has decreased due to mutation; in the lizard embryo less number vertebrae than in the snake embryo due to developmental mutations).
III. Biogeographic evidence (Biogeography studies the distribution of animals and plants on Earth).
1. There are 5 zoogeographic zones that do not differ in classes and types of animals:
a) Holarctic;
b) Indo-Malaysian;
c) Ethiopian;
d) Australian;
e) Neotropical zone.
2. Zones differ in families, orders and genera.
a) In Australia, all mammals are marsupials.
b) The only representative of the beak-headed lizards, the tuatara, lives in New Zealand.
c) There are American and European species of maple, ash, pine.
3. Causes of similarities and differences in fauna and flora.
a) Isolation of areas.
· If the isolation occurred recently, then there are more similarities than differences: the Bering Strait was formed recently, so the fauna of Asia differs little from the fauna of America; Northern and South America united recently, so their faunas are different; Australia separated from the rest of the continents a long time ago, therefore it has a peculiar flora and fauna, evolution was slow, since Australia is relatively small; the fauna and flora of the islands and closed reservoirs are peculiar.
4. The current geographical distribution of animals and plants can only be explained from an evolutionary point of view.
IV. Paleontological (Paleontology studies fossil organisms, the conditions of their life and burial).
1. Change of fauna and flora on Earth.
a) Only invertebrates have been found in the most ancient strata.
b) The younger the reservoir, the closer the remains to modern species.
c) With the help of paleontological finds, it was possible to establish phylogenetic series and transitional forms.
2. Fossil transitional forms- forms of organisms that combine the features of older and younger forms.
a) Animal-toothed reptiles were found on the Northern Dvina (genus Inostrancevia). They had similarities with mammals in the structure of the following organs: skulls; spine; limbs located not on the sides of the body, as in reptiles, but under the body, as in mammals; teeth differentiated into canines, incisors and molars.
b) Archeopteryx- a transitional form between birds and reptiles, found in the layers Jurassic(150 million years ago).
· Signs of birds: hind limbs with tarsus, wings and feathers, resemblance.
· Signs of reptiles: a long tail, consisting of vertebrae; abdominal ribs; the presence of teeth; claws on the forelimb.
· He flew badly for the following reasons: the sternum was without a keel, i.e. pectoral muscles were weak; the spine and ribs were not rigidly supported, as in birds.
in) psilophytes- a transitional form between algae and land plants.
Descended from green algae.
Higher spore vascular plants - club mosses, horsetails, ferns - originated from psilophytes.
· Appeared in the Silurian, and spread in the Devonian.
· Differences from algae and higher spores: psilophytes - herbaceous and woody plants growing along the shores of the seas; had a branched stem with scales; the skin had stomata; the underground stem resembled rhizomes with rhizoids; the stem was differentiated into conductive, integumentary, and mechanical tissues.
3. Phylogenetic series- rows of some forms that successively replaced each other in the course of evolution (phylogenesis).
a) V.O. Kovalevsky restored the evolution of the horse by constructing its phylogenetic series.
· Eohippus, who lived in the Paleogene, was the size of a fox, had a four-fingered forelimb and a three-fingered hind limb. The teeth were tuberculate (a sign of omnivory).
· In the Neogene the climate became more arid, the vegetation changed, eogippus evolved through a number of forms: eogippus, merigippus, hipparion, modern horse.
Signs of eogippus have changed: the legs have lengthened; claw turned into a hoof; the support surface was reduced, so the number of fingers decreased to one; fast running led to the strengthening of the spine; the transition to roughage led to the formation of folded teeth.
Transitional (intermediate) forms- organisms that combine in their structure the features of two large systematic groups.
Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) features than later forms, but at the same time, by the presence of more progressive (in the sense of later) features than their ancestors. As a rule, the term "transitional forms" is used in relation to fossil forms, although intermediate species do not necessarily have to die.
Transitional forms are used as one of the proofs of the existence biological evolution.
History of the concept
In 1859, when Charles Darwin's work "The Origin of Species" was published, the number of fossil remains was extremely small, and transitional forms were not known to science. Darwin described the absence of intermediate forms "as the most obvious and weighty objection that can be raised against a theory", but attributed this to the extreme incompleteness of the geological record. He noted the limited number of available collections at the time, while at the same time describing the available information about available fossil specimens in terms of evolution and the operation of natural selection. Only two years later, in 1961. Archeopteryx was found, which represented the classical transitional form between reptiles and birds. His findings became not only a confirmation of Darwin's theory, but also a landmark fact confirming the reality of the existence of biological evolution. Since then, a large number of fossil forms have been found that show that all classes of vertebrates are related to each other, most of them through transitional forms.
With the increase in information about the taxonomic diversity of vascular plants in the early twentieth century, began research to find their possible ancestor. In 1917, Robert Kidston and William Henry Land discovered the remains of a very primitive plant near the village Rhynia in Scotland. This plant was named Rhynia. It combines features of green algae and vascular plants.
Interpretation of the concept
Transitional forms between two groups of organisms are not necessarily the descendants of one group and the ancestor of another. From fossils, it is usually impossible to accurately determine whether a certain organism is the ancestor of another. In addition, the probability of finding a direct ancestor in the fossil record certain form extremely small. It is much more likely to find relatively close relatives of this ancestor who are similar in structure to it. Therefore, any transitional form is automatically interpreted as a lateral branch of evolution, and not a "section of the phylogenetic trunk."
Transitional forms and taxonomy
Evolutionary taxonomy remained the dominant form of taxonomy throughout the 20th century. The allocation of taxa is based on various characters, as a result of which taxa are depicted as branches of a branched evolutionary tree. Transitional forms are seen as "falling" between different groups in terms of anatomy, they are a mixture of characteristics from the inner and outer baggage that has recently split.
With the development of cladistics in the 1990s. Relationships are usually depicted as a cladogram illustrating the dichotomous branching of evolutionary lines. Therefore, in cladistics, transitional forms are considered as earlier branches of the tree, where not all the features characteristic of previously known descendants on this branch have not yet developed. Such early members of the group are usually referred to as the main taxon (eng. Basal taxa) or sister taxon Sister taxa) depending on whether it belongs fossil organism to this baggage or not.
Problems of detection and interpretation
The lack of transitional forms between many groups of organisms has been criticized by creationists. However, not every transitional form exists in the form of fossils due to the fundamental incompleteness of the paleontological record. Incompleteness is caused by the peculiarities of the process of fossilization, that is, the transition to a petrified state. For the formation of a fossil, it is necessary that the organism that died be buried under a large layer sedimentary rocks. Due to the very slow rate of sedimentation on land, land species rarely go into a petrified state and are preserved. In addition, it is rarely possible to identify species that live in the depths of the ocean through rare cases of raising large massifs of the bottom to the surface. Thus, most known fossils (and, accordingly, transitional forms) are either species that live in shallow water, in seas and rivers, or terrestrial species that lead a semi-aquatic lifestyle, or live near coastline. To the problems mentioned above, one should add an extremely small (on a planetary scale) number of paleontologists who carry out excavations.
Transitional forms, as a rule, do not live on large territories and do not exist for a long time, otherwise they would be persistent. This fact also reduces the likelihood of fossilization and subsequent detection of transitional forms.
Therefore, the probability of finding intermediate forms is extremely small.
Examples among animals
Ichthyostegs are considered the oldest representatives of amphibians. They are considered a transitional link between lobe-finned fish and amphibians. Despite the fact that ichthyostegi had a five-fingered ending adapted to life on land, they spent a significant part of their lives as fish, had a caudal fin, a lateral line, and some other signs of fish.
Batrachosaurs, which existed in the Carboniferous and Permian periods, are considered as a transitional form between amphibians and reptiles. Batrachosaurs, although they spent their adult life on land (like reptiles), were closely associated with water bodies and retained a number of features inherent in amphibians, in particular, laying eggs and developing larvae in water, the presence of gills, and the like.
A large number of reptiles have been found that have developed the ability to fly, some of them had feathers, so they are considered as transitional forms between reptiles and birds. The most famous is Archeopteryx. It was about the size of a modern crow. The shape of the body, the structure of the limbs and the presence of plumage, similar to modern birds, may have flown. In common with reptiles was the special structure of the pelvis and ribs, the presence of a beak with conical teeth, three free fingers on the wings, subflexible vertebrae, a long tail with 20-21 vertebrae, the bones could not be pneumatized, the sternum without a keel. Other known transitional forms between reptiles and birds are Protoavis, Confuciusornis.
A large number of fossil forms of animal-like reptiles (synapsids, therapsids, pelycosaurs, various dinosaurs, etc.) found in many regions of the globe existed in the Jurassic and Cretaceous periods, combining signs of reptiles and mammals, reveal possible directions and ways of becoming various groups tetrapods, in particular mammals. For example, an animal-like reptile from the group of therapsids is a lycanops (Lycaenops) in terms of the development of the bones of the oral cavity, the differentiation of teeth into fangs, incisors, incisor teeth, and a number of other signs of the body structure, it resembles predatory mammals, although in other ways and way of life they were real reptiles.
One of the forms preserved in the fossil state is Ambulocetus. Ambulocetus natans("walking whale") - a transitional form between terrestrial mammals and cetaceans, which are second-aquatic forms. Outwardly, the animal resembled a cross between a crocodile and a dolphin. The skin should have a partially reduced coat. The animal had webbed paws; the tail and limbs are adapted as auxiliary organs of locomotion in water.
Examples among plants
The first terrestrial plants from the class of rhyniopsids, the families of rhynievies and psilophytes, living in the Silurian-Devonian, combined signs of green algae and primitive forms of higher plants. Their body was leafless, a cylindrical axial organ - a body in the upper part dichotomously branched at the tops with sporangia. The function of mineral nutrition of rhiniopsids was performed by rhizoids.
Fossil forms of seed ferns that flourished at the end of the Devonian combine features of ferns and gymnosperms. They formed not only spores (like ferns), but also seeds (like plant seeds). The conductive tissue of their stems in structure resembles the wood of gymnosperms (cycads).
Another precursor of seed plants has been identified from the Middle Devonian deposits. Runkaria (Runcaria heinzelinii) existed about 20 million years ago. It was a small plant with radial symmetry; had a sporangium surrounded by an integument and a cupule. Runkariya demonstrates the path of evolution of plants from spore to seed.
Transitional forms in human evolution
In our time, a large number of fossil remains have been found that reveal the evolutionary path of Homo sapiens from its anthropoid ancestors. Forms that can be more or less classified as transitional include: Sahelanthropus, Ardipithecus, Australopithecus (African, Afar and others), skilled man, working man, erect man, predecessor man, Heidelberg man and Cro-Magnons.
Among the forms mentioned, australopithecines deserve considerable attention. Australopithecus afaris, in terms of evolution, is between modern bipedal people and their four-legged ancient ancestors. The large number of skeletal figures of this Australopithecus clearly reflect bipedalism, to the extent that some researchers believe that this property arose long before the appearance of Australopithecus afarensis. Among the general features of anatomy, his pelvis is much more similar to these bones in humans than in monkeys. The edges of the ilium are shorter and wider, the sacrum is wide and located directly behind the hip joint. There is clear evidence for the existence of attachment sites for the knee extensor muscles, providing for the upright position of this organism. While the Australopithecus pelvis is not exactly human (noticeably wider, with the edge of the iliac bones oriented outwards), these features point to a fundamental rearrangement associated with bipedal walking. The femur forms an angle in the direction of the knee. This feature allows the foot to be placed closer to the midline of the body and is a clear indication of the habitual nature of bipedal locomotion. In our time, Homo sapiens, orangutans and coats have the same features. Australopithecus legs had thumbs, which makes it almost impossible to grab a foot of tree branches. In addition to the features of locomotion, Australopithecus was also significantly more brain than modern chimpanzees and the teeth were much more similar to those of modern humans than to apes.
Phylogenetic series
Phylogenetic series - series of fossil forms, interconnected in the process of evolution and reflect the gradual changes in their historical development.
They were investigated by the Russian scientist A. Kovalevsky and the English J. Simpson. They showed that modern one-toed ungulates are descended from ancient small omnivores. The analysis of fossil horses helped to establish the gradual process of evolution within this group of animals, in particular, how, changing over time, fossil forms became more and more similar to modern horses. Comparing the Eocene eohypus with the modern horse, it is difficult to prove their phylogenetic relationship. However, the presence of a number of transitional forms that successively replaced each other over large areas of Eurasia and North America made it possible to restore the phylogenetic series of horses and establish the direction of their evolutionary changes. It consists of a series the following forms(simplified): Phenacodus — Eohippus — Miohippus — parahippus — Pliohippus — Equus.
Gilgendorf (1866) described a paleontological series of gastropod molluscs from the Miocene deposits accumulated over two million years in lacustrine deposits of the Steinheim basin (Württemberg, Germany). It was found in successive layers of 29 different forms belonging to the planorbis series. (Planorbis). Ancient mollusks had a shell in the form of a spiral, and later - in the form of a turbocoil. The row had two branches. It is hypothesized that the turtle's change in shape was caused by an increase in temperature and an increase in calcium carbonate as a result of hot volcanic springs.
Thus, phylogenetic series represent a historical sequence of transitional forms.
At present, phylogenetic series are known for ammonites (Waagen, 1869), gastropods of the genus Viviparous (Viviparus)(Neymair, 1875), rhinos, elephants, camels, artiodactyls and other animals.
similarity of embryos. biogenetic law
The study of the embryonic and postembryonic development of animals made it possible to find common features in these processes and to formulate the law of germinal similarity (K. Baer) and the biogenetic law (F. Müller and E. Haeckel), which are of great importance for understanding evolution.
All multicellular organisms develop from a fertilized egg. The processes of development of embryos in animals belonging to the same type are largely similar. In all chordates, in the embryonic period, an axial skeleton is laid - a chord, a neural tube appears. The plan of the structure of chordates is also the same. In the early stages of development, vertebrate embryos are extremely similar (Fig. 24).
These facts confirm the validity of the law of germinal similarity formulated by K. Baer: "Embryos reveal, already from the earliest stages, a certain general similarity within the limits of the type." The similarity of the embryos serves as evidence of their common origin. Later, in the structure of the embryos, signs of a class, genus, species, and, finally, signs characteristic of a given individual appear. The divergence of signs of embryos in the process of development is called embryonic divergence and reflects the evolution of one or another systematic group of animals.
The great similarity of embryos in the early stages of development and the appearance of differences in later stages has its own explanation. The study of embryonic variability shows that all stages of development are variable. The mutation process also affects the genes that determine the structural and metabolic features of the youngest embryos. But the structures that arise in early embryos (ancient features characteristic of distant ancestors) play a very important role in the process of further development. Changes in the early stages usually lead to underdevelopment and death. On the contrary, changes in the later stages may be favorable to the organism and are therefore picked up by natural selection.
The appearance in the embryonic period of development of modern animal features characteristic of distant ancestors reflects evolutionary transformations in the structure of organs.
In its development, the organism passes through a unicellular stage (the zygote stage), which can be considered as a repetition of the phylogenetic stage of the primitive amoeba. In all vertebrates, including their higher representatives, a chord is laid, which is then replaced by the spine, and in their ancestors, judging by the lancelet, the chord remained all their lives.
During the embryonic development of birds and mammals, including humans, gill slits appear in the pharynx and their corresponding septa. The fact that parts of the gill apparatus are formed in the embryos of terrestrial vertebrates is explained by their origin from fish-like ancestors that breathed through gills. The structure of the heart of the human embryo during this period resembles the structure of this organ in fish.
Such examples point to a deep connection between the individual development of organisms and their historical development. This connection found its expression in the biogenetic law formulated by F. Müller and E. Haeckel in the 19th century: the ontogenesis (individual development) of each individual is a brief and quick repetition of the phylogenesis (historical development) of the species to which this individual belongs.
The biogenetic law has played an outstanding role in the development of evolutionary ideas. A. N. Severtsov made a great contribution to the deepening of ideas about the evolutionary role of embryonic transformations. He established that in individual development the signs are repeated not of adult ancestors, but of their embryos.
Phylogenesis is now considered not as a succession of sequences of a number of adult forms, but as a historical series of ontogenies selected by natural selection. Entire ontogenies are always selected, and only those that, despite the impact of adverse environmental factors, survive at all stages of development, leaving viable offspring. Thus, the basis of phylogeny is the changes that occur in the ontogeny of individual individuals.
paleontological evidence. Comparison of fossil remains from the earth's layers of different geological epochs convincingly testifies to the change in the organic world in time. Paleontological data give great material about successive links between various systematic groups. In some cases, it was possible to establish transitional forms, in others - phylogenetic series, that is, series of species that successively replace one another.
Fossil transitional forms:
BUT) archeopteryx- a transitional form between birds and reptiles, found in the layers of the Jurassic period (150 million years ago). Signs of birds: hind limbs with a tarsus, the presence of feathers, resemblance, wings. Signs of reptiles: a long tail consisting of vertebrae, abdominal ribs, the presence of teeth, bones on the forelimb;
B) psilophytes- transitional form between algae and land plants.
phylogenetic series. V. O. Kovalsky restored the evolution of the horse by constructing its phylogenetic series (Fig. 25).
The evolution of the horse covers a fairly large period of time. ancient ancestor the horse belongs to the beginning of the Tertiary period, while the modern horse belongs to the Quaternary period. Species of the genus Eucus were small forest animals 30 cm high. They had four toes each, which made it easier to walk and run on the marshy soil of forest swamps. Judging by the teeth, these animals ate soft plant foods. They belong to the Lower Eocene of North America. This form is followed by the Middle Eocene Orohippus, in which four toes were still developed on the forelegs. In the middle Eocene, epihippus appears, in which the fourth finger is reduced. In the Oligocene lived a descendant of the previous forms - mesogippus. He has only three fingers on his feet, and the middle finger is noticeably more developed than the others. The growth of animals reaches 45 cm.
Changes in the dental system begin to appear. The tuberculate front teeth of Eohippus, adapted to soft plant foods, turn into teeth with grooves. Evolution also affects the molars, they become more adapted to coarse steppe plant foods. In the Upper Oligocene, the mesogippus gives way to a whole series of forms: myohumgaus, and in the lower Miocene, para-hippus. Parahippus is the ancestor of the next stage of the horse series - merichippus. Merihippus were undoubtedly inhabitants of open spaces, and in different species of this genus there was a process of shortening of the lateral fingers: in some species the fingers were longer, in others they were shorter, approaching in last case to swift one-toed horses.
Finally, in Pliogippus, who lived in the Pliocene, this process ends with the formation new form, an ancient one-toed horse - plesippus. In shape and size, the latter is close to the modern horse known from the Pleistocene.
Originating in America modern form horse then penetrates into Eurasia among several species. Ultimately, all American horses died out, while European horses survived and then re-entered America. This time they were brought here by Europeans in early XVI in. Thus, the evolution of horses convincingly shows the process of evolution leading to the emergence of new species through the transformation of their ancestors.
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