Transitional form

Transitional form- an organism with an intermediate state that necessarily exists during a gradual transition from one biological type of structure to another. Transitional forms are characterized by the presence of more ancient and primitive (in the sense of primary) traits than their later relatives, but, at the same time, the presence of more progressive (in the sense of later) traits than their ancestors. As a rule, when speaking about intermediate forms, they mean fossil species, although intermediate species do not necessarily have to become extinct. Many transitional forms are known, illustrating the origin of tetrapods from fish, reptiles from amphibians, birds from dinosaurs, mammals from theriodonts, cetaceans from land mammals, horses from a five-toed ancestor, and humans from ancient hominids.

Theory of evolution

Examples

Examples of transitional forms include Ambulocetus - the “walking whale” (a transitional form between cetaceans and land mammals), Tiktaalik and Ichthyostega (a transitional form from fish to amphibians), Mesohippus (between early equids and the modern horse).

Transitional forms in the evolution of Homo Sapiens

Currently, many transitional forms are known between Homo sapiens and his ape-like ancestors. For example, the remains of Sahelanthropus, Ardipithecus, Australopithecus (African, Afar and others), Homo habilis, Homo working, Homo erectus, Homo precursor, Heidelberg man, Neanderthal and early Homo sapiens have been discovered.

Dispute about the gradualness of evolutionary transformations

Transitional forms are the object of study in the debate between gradualists and punctualists. Gradualists believe that evolution is a gradual, continuous process. Punctualists, or proponents, believe that species remain the same for a long time, but changes occur in a very short period of time. Transitional forms are an argument on both sides. Punctualists argue that most species appear suddenly and change little throughout their history. Gradualists note that among vertebrates there is not a single indisputable example of a very rapid transition from one species to another; most transitions are generally poorly documented by fossils. There are also many well-described intraspecific variations. For example, changes in Middle Cenozoic rabbits, Miocene rodents, hipparions, Eocene artiodactyls, and the transition between genera of Eocene primates are described.

Missing Links

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Progress in the study of human evolution. Increase in the number of hominin species known to science over time. Each species is depicted as a rectangle showing the boundaries within which skull volume varied and the species' place in the fossil record. One can see the gradual filling of the gaps between types.

Many transitional forms between living organisms have not yet been found. These are commonly referred to as missing links. The rarity of transitional forms is explained by the incompleteness of the fossil record. Proponents of the theory of punctuated equilibrium also point to the rapidity of evolutionary transformations. However, paleontologists are constantly finding new fossils, filling in the gaps (for example, in 2004, Tiktaalik was found - another transitional link between fish and amphibians)

Incomplete fossil record

Not every transitional form exists as a fossil due to the fundamental incompleteness of the fossil record. The incompleteness is caused by the peculiarities of the process of fossilization, that is, the transition to a fossilized state. For a fossil to form, the dead organism must be buried under a large layer of sediment. Due to the extremely slow rate of sedimentation on land, purely terrestrial species rarely become fossilized and persist. In addition, it is rarely possible to discover species living in the depths of the ocean, due to the rarity of large areas of the bottom rising to the surface. Thus, most known fossils (and, accordingly, transitional forms) are either species living in shallow waters, seas and rivers, or terrestrial species leading a semi-aquatic lifestyle, or living close to the coastline. The patterns of burial processes of the remains of living beings are studied by a special branch of paleontology - taphonomy.

Phylogenetic series

Phylogenetic series are series of species that successively replaced each other in the process of evolution of various groups of animals and plants.

They were first discovered by V. O. Kovalevsky, who showed that modern one-toed ungulates descend from ancient five-toed small omnivores.

Creationism

Proponents of creationism claim that no transitional forms have been found. The scientific community considers such statements to be false and deliberately misleading.

Notes

Literature

Darwin, Charles. Origin of species by means of natural selection: In 2 books. - M.: TERRA - Book Club, 2009. - ISBN 978-5-275-02114-1
Carroll R. Paleontology and evolution of vertebrates: In 3 volumes - Mir, 1992. - 280 p. - ISBN 5-03-001819-0

Links

The myth that “the missing link has not been found...” on the portal Anthropogenesis.ru

Wikimedia Foundation. 2010.

Transitional forms. Despite the fact that, as a rule, there are deep gaps between large natural groups of animals, plants and microorganisms caused by the extinction of intermediate forms, in a number of cases we find transitional forms. The existence of forms that combine in their structure the characteristics of different types of organization and therefore occupy an intermediate systematic position is determined by the general relatedness of the organisms. With such a relationship between separate large branches of the tree of life that are far removed from each other, small branches of an intermediate nature may exist (Fig. 6.23).

Rice. 6.23. Examples of currently existing forms: A – peripatus (Peripatus laurocerasus), bearing characteristics of arthropods and annelids; B – euglena (Euglena viridis), combining the characteristics of animals and plants; B – horseshoe crab (Limulus polyphemus), occupying an intermediate position between modern typical arthropods and fossil trilobites; G – horseshoe crab larva, similar to a trilobite larva (according to I.I. Shmalgauzen, 1969)

One example of transitional forms between tunicates (the most primitive group of chordates) and vertebrates is the genus of lancelets, which are characterized by all the main characteristics of chordates, but they are slightly developed (L.O. Kovalevsky).

The existence of intermediate forms in the modern organic world is evidence of the unity of organization of large trunks of the tree of life and the unity of their origin.

Microsystematics. Starting with the classic works of P.P. Semenov-Tyan-Shansky, in the case of well-studied groups of species, it became possible to identify their intraspecific structure - subspecies, races and other groupings (microsystematics). Such a study is important for understanding the features of microevolution. It is now clear that complex hierarchical relationships between populations and their groups can exist in a species system.

At the same time, the identification of the true phylogenetic relationship of such groupings, necessary for determining intraspecific taxonomic categories, turns out to be at the same time the restoration of their microphylogeny - the paths of historical development of individual parts of the species population and the species as a whole (Fig. 6.24).

Rice. 6.24. The microphylogenesis of Drosophila of the Willistoni group can be reconstructed on the basis of crossing experiments, analysis of chromosome structure and biochemical polymorphism. The hierarchy of phylogenetic forms in this case should be described as follows: population – subspecies – semispecies – species – superspecies (according to B. Spassky et al., 1971)
Drosophila willistoni is represented by two geographically isolated subspecies (13, 14), the species D. equinoxialis is also represented by two subspecies (8, 9) with even deeper reproductive isolation between them: the species D. paulistonim consists of six subspecies (population groups 1 and 2, 3 and 5 live sympatrically, there is complete reproductive isolation between them in nature, but individuals of these groups from different areas can interbreed in the laboratory). Representatives of all six species do not interbreed either in nature or in the laboratory (rare laboratory hybrids are always sterile). At the same time, the differences between D. willistoni and the other five species are greater than between the other five species

Population morphology. A. Wallace in the 19th century. gave variation series of values of the studied characteristics for small groups of individuals. With the spread of population thinking (see Chapter 7), morphological studies have also proven to be a useful weapon for studying ongoing microevolutionary processes. Population-morphological methods make it possible to capture the directions of natural selection by changing the nature of the distribution of trait values in a population at different stages of its existence or when comparing different populations (Fig. 6.25, A, B).

Rice. 6.25. Asymmetry in the distribution of values of a quantitative trait can show the direction of the pressure of natural selection (according to A.V. Yablokov, 1966).
The number of cells in certain areas of the wing of the dragonfly Lestes sponsa

Morphological methods make it possible to identify among a homogeneous group of traits those that are under greater or lesser pressure from selection (Fig. 6.26), although in this case the adaptive value of the trait itself can only be revealed by additional ecological and physiological studies. One of the difficulties in applying morphological methods to the study of the process of microevolution is the difficulty of distinguishing the action of modern evolutionary factors from variability determined by a broad norm of reactions. Thus, the method of morphophysiological indicators (S.S. Schwartz), which at one time became widespread in our country, allowing one to distinguish the physiological state of animal populations and the directions of its adaptive restructuring, often turned out to be compromised by the fact that fleeting, short-term and reversible indicators were taken as evolutionarily significant indicators changes.

Rice. 6.26. Values of the coefficient of variation that sharply fall out of the “flow of variability” of a group of traits can indicate the relationship of a given trait to selection pressure. In cranial measurements of the harp seal (Pagophilus groenlandica), the size of the auditory bone differs by a decreased value of the coefficient of variation, and the size of the nasal bone by an increased value. The first trait is extremely important and is strictly controlled by selection during all periods of ontogenesis, the second is of little significance (according to A.V. Yablokov, 1966)

The study of the microevolutionary process using morphological methods is the content of a new direction in modern morphology - population morphology, a discipline closely related to population genetics and ecology.

Why aren’t intermediate, fossil transitional forms found in the earth?

ATTENTION!!! THIS MATERIAL HAS BEEN REVISED, ADDED AND INCLUDED IN THE BOOK “Creation or Evolution? How old is the Earth? TO READ GO TO PAGES --> ,

When discussing interspecific changes, one cannot fail to note the most striking evidence of their absence. Today in museums around the world there are millions of fossilized fossils of once living creatures, but among them there are no intermediate forms. In his famous book On the Origin of Species, Darwin wrote: “If, in fact, species are descended from each other, gradually developing, then why are we not confronted with an innumerable number of transitional forms? Why is everything in nature in its place and not in chaos? Geology was unable to put forward a step-by-step process, did not discover transitional forms, and perhaps in the future this will be the most compelling argument against my theory.”

Indeed, if we imagine that evolution took place in the life of the Earth, our planet should simply be “overwhelmed” with the remains of intermediate forms. After all, according to neo-Darwinian scientists, the transformation of living beings lasted for millions of years. However, today we have a huge number of fossilized fossils of extinct animals belonging to known genera, as well as those still living on Earth: dragonflies, bees, ants, sharks, frogs... But not a single representative of an intermediate species!

Of these millions of fossilized remains, only the bird Archeopteryx is proposed by neo-Darwinists to be considered as a transitional form between reptiles and birds. Her fossil remains show that she had claws on her wings and teeth in her mouth. However, the supporters of Darwin's theory themselves do not dare to insist that they are right, since the remains of modern birds were found in the same layers as Archeopteryx, and even in deeper time slices. In addition, Archeopteryx has no scales at all, which should be at least in small quantities if this animal were an intermediate form. Moreover, today we know fossils of extinct birds with teeth and living birds with claws on their wings (hoatzin, turaco), with which they hold onto branches. So, according to the facts, Archeopteryx is only an extinct species of bird.

Rice. Reconstruction of Archeopteryx

In the same book, Darwin writes: “If numerous species belonging to the same class began to exist at the same time, it would be a fatal blow to the theory of evolution from a common ancestor by natural selection.” Modern scientists, having examined fossil remains in detail, have become convinced that life appeared on Earth suddenly. In the deepest layer of the earth, called the “Cambrian”, the remains of snails, trilobites, sponges, worms, starfish, swimming crustaceans, etc. were found. It is therefore evident that all these species, distinct from each other, existed at the same time and have no common ancestor from which they were descended. In geology, this phenomenon is called the “Cambrian explosion.”

Not only are there no fossil transitional forms, but even logically their existence and further development is simply impossible. Let's say a worm has eyes, first an apple, then a retina, a pupil, then their connection with the brain... But until the organ of vision is fully developed, the worm is a freak. And the eyes did not have protective functions, for example, tears, film, eyelids, etc. That is, through this underdeveloped eye, any infection could enter the body and kill the creature. Or imagine that amphibians began to grow wings. Until they grew, until their bones became light, until the muscles necessary for flight became stronger, how did these half-birds live? They did not yet know how to fly, but they already ran poorly - their wings hampered them and their shortening legs did not allow them to accelerate in pursuit of food and hide from their larger fellow predators.

By the way, it will also be difficult for evolutionists to answer the question why, just as supposedly millions of years ago, amoebas, fish, amphibians, and monkeys exist in nature? Why did they stop in their evolutionary development? And if we admit that they are still in the process of evolution, then the question immediately arises about the strange absence of living intermediate forms between them. That is, to our great surprise, we do not see either living intermediate forms or dead fossils. But, for example, the latter should amount to trillions and even sextillions, being accumulated in the bowels of the earth over billions of years.

Only in the form in which a living being lives on Earth today, its body is an example of harmony and beauty. Each representative of the fauna is unique in its own way and performs a strictly defined function in the life of the planet, serving as a cleanser, pollinator, food, orderly, or other...

Speaking about vertical evolution, it is appropriate to talk about chromosomes. According to the logic of evolutionists, the number of chromosomes should increase from simple species to more complex ones. However, this is far from the case. People have 46 chromosomes, a fly has 12, a lizard has 46, a crayfish has 200, a fir tree has 48, a monkey has 48, a mouse has 46, a cat has 38, wheat has 42. Please note: monkeys and cancer have more chromosomes than humans. Obviously, their number does not fit into the evolutionary scheme. Then how to explain this?

Also, evolutionists cannot explain why human DNA is closer to chimpanzee DNA and not to orangutan DNA? After all, humans share 28 morphological characteristics with orangutans, while chimpanzees share only 2. Moreover, for some reason, the DNA of humans and chimpanzees is more similar to each other than the DNA of chimpanzees and orangutans!

The answer to these questions is simple - DNA and chromosomes were not created by chance vertically, but by the Creator individually according to His wise plan. That is why they are so ingeniously and uniquely designed. And their complexity and genius are precisely confirmed by the fact that the “visual” similarity of DNA from the point of view of scientists, when tested, gives different, even opposite results. That is, for us DNA still remains a mystery, which humanity, despite its apparent literacy, cannot comprehend. That's why we see only unsuccessful attempts by scientists to create or remake at least one living creature, changing the structure of its DNA.