- 1 Organic Evolution of Living Organisms – State the Four Evidences that Support the Theory of Organic Evolution
- 1.1 A. Lamarckism
- 1.2 Lamarckian Idea on Evolution
- 1.3 Examples in Support of Lamarckism
- 1.4 Criticism of Lamarckism
- 1.5 Evidence in Favour of Inheritance of Acquired Characters
- 1.6 Neo-Lamarckism
- 1.7 B. Darwin’s Contribution to Biological Evolution
- 1.8 Darwin’s Theory of Natural Selection or Darwinism
- 1.9 Limitations of Darwinism or Theory of Natural Selection
- 1.10 C. Modern Synthetic Theory of Evolution
Environmental biology is one of the critical Biology Topics that involves understanding how humans impact the environment and how to address environmental issues.
Organic Evolution of Living Organisms – State the Four Evidences that Support the Theory of Organic Evolution
The theory explaining the mechanism of evolution by Jean Baptist de Lamarck in 1809 is popularly known as Lamarckism. It is the first theory on evolution. Though the outline of the theory came to light in 1801, the details of the theory were published in his book, “Philosophic Zoologies” in 1809.
A. Weismann, proposed the theory of Germplasm, by which he discarded the theory of Lamarck. A. Weismann, conducted an experiment, in which he cut the tail of newborn mice generation often generation but could not get tailless mice or any shorter tail mice.
Lamarckian Idea on Evolution
Lamarckism is based on four basic propositions of Lamarck and those may be enumerated in the following way:
(i) Internal vital force:
All living organisms have internal vital forces and due to this living organisms and their component parts tend to increase in size.
(ii) Effect of environment and new need:
There is a constant effect of the environment on the organisms in nature. A change in the environment causes a change in the organisms. Due to environmental change new needs or desires result in the production of new structures and habits in organisms.
(iii) Use and disuse of organs:
Lamarck opined that if an organ is used constantly it would be developed in a better way. On the other hand disuse of an organ leads to its degeneration.
(iv) Inheritance of acquired characters:
Character acquired during the lifetime of an organism due to internal vital force, the effect of the environment, new needs, and the use and disuse of organs are inherited by the next generations. After several generations, the variation accumulates to develop new species.
Examples in Support of Lamarckism
(i) Evolution of Giraffe:
The ancestors of Giraffes had small necks and forelimbs like horses and they had to take vegetation from the branches of the trees because in their habitats there was no surface vegetation. Due to this habit of collecting vegetation from the upper branches of the trees they had to stretch their necks and forelimbs and because of this continuous effort the neck and forelimbs were elongated in generations. The change in characters was also inherited subsequently and in the long run, the giraffe with a long neck and elongated forelimbs evolved.
(ii) Webbed feet of duck:
The aquatic bird duck has webbed feet. This is an effect of the environment and an achievement due to new needs.
(iii) Disappearance of limbs in snakes:
The modern-day snakes had lizard-like ancestors having two pairs of limbs. Due to disuse the limbs were degenerated and limbless snakes appeared from lizard-like ancestors.
(iv) Flat fishes:
In seawater bottom dwelling flat fishes has both eyes on the same side of the head. This happens due to environmental effects, and pressure. At the embryonic stage, the eyes remain on either side of the head. But later on, both eyes are shifted to one side.
(v) Flightless birds:
The ancestors of flightless birds had wings for flight. But the flightless birds due to continuous disuse lost the wing power for flight. The environment would give them enough protection and supply plenty of food materials.
(vi) Retractile claws of carnivorous mammals:
The ancestors of carnivorous mammals like tigers and lions had ordinary claws. But the retractile claws were the later achievements. For swift running, the claws were the hindrance and this caused the development of retractile claws.
There are many examples supporting Lamarck’s propositions. The achievement of deer, cave dwellers, hydrophytes, etc. is a few such examples. In spite of these, there are many criticisms of Lamarckism.
Criticism of Lamarckism
1. That the living organism has a vital force and due to this living organism and its body parts tend to increase in size is objected by many. Actually, the existence of vital force has no scientific bases to accept.
2. Further the proposition that the environment influences the organism is no doubt, but due to environmental pressure new need arises, and due to that a new structure is developed and cannot be accepted. So it is doubtful that for a new need, a new structure is developed. For this also there is no visible evidence.
3. The use and disuse theory is correct to some extent.
4. The proposition of inheritance of acquired character is disputed on the following observations:
- The powerful muscles of wrestlers are not transmitted to their offspring.
- Boring pinna and nose in Indian women are never inherited in their girl children.
- Circumcision of the penis in Muslims is not inherited by the next generation.
- Dull progeny of Nobel Prize winners cannot be explained by the inheritance of acquired character.
- Chinese women wear iron shoes in order to keep their feet small, but their children are born with normal feet.
- Further, Mendel’s laws of inheritance and Weismann’s theory of continuity of germplasm also speak against the inheritance of acquired character.
Weismann proposed that the living body has two entities mainly germplasm and somatoplasm. The character which is influenced by germplasm may only be transmitted to the next generation. Therefore, a somatoplasmic character (Protoplasm of the somatic cell) cannot be transmitted to the next generation. In spite of this evidence, there is some evidence in favor of the inheritance of characters.
Evidence in Favour of Inheritance of Acquired Characters
(i) Formation of germ cells from somatic cells:
In vegetative reproduction in plants and during generation in some animals somatic cells can give rise to germ cells. This is against the continuity of germplasm of Weismann.
(ii) Environmental effect on germ cells:
Potato beetles during development were exposed to extreme temperature and humidity by Tower. It was found that though the beetles did not show any change their offspring faced cover variations and this character was found to be transmitted to the next generation. Therefore, environmental change can directly affect the germ cells.
(iii) Effect of mutagenic agents:
Aurbach et al obtained a number of mutations and chromosome aberrations in Drosophila by applying mustard gas. In the same way with the help of high-energy radiations as well as mutagenic chemicals sudden heritable mutations may be obtained in animals.
(iv) Curture in agar:
Water flees cultured in agar with green flagellates develop some abnormalities. The parthenogenetic eggs of the flees, if cultured in ordinary water and allowed to produce individuals, produce the same abnormality.
From the above examples, it becomes clear that the environment may directly affect the individuals to produce heritable change. Therefore, in spite of objections, the Lamarckian hypothesis is effective in certain cases. To support the Lamarckian concept a modified form of Lamarckism was proposed. This is called Neo-Lamarckism.
Neo-Lamarckism is a modification of the original Lamarck theory. In light of modern knowledge, some scientists modified the original theory. It gives stresses the direct effect of a changed environment on living organisms. The environmental effect that influences the germ cells may be transmitted to the next generation. The supporters of Neo-Lamarckism are known as Neo-Lamarckian. According to Neo-Lamarckians adaptations are a universal phenomenon. Due to adaptation new structures or habits may be developed in some organisms and the characteristics may be hereditary. In this regard, Lamarckism cannot be fully rejected. It is true to some extent.
B. Darwin’s Contribution to Biological Evolution
The theory of Natural Selection was published in 1858. Though Charles Darwin is considered the proposer of this theory, this was proposed by two scientists Charles Darwin and Alfred Russel Wallace. As a way to evolution, natural selection had come to the realization of both these Scientists and Naturalists separately. Wallace in a letter to Darwin expressed his ideas on evolution. Then Darwin observed that the ideas of Wallace were very much consistent with that of his own ideas. For this reason, Darwin published the theory of Natural Selection under joint authorship. However, Darwin discussed the theory of Natural selection in detail in the book entitled On the Origin of Species by Means of Natural Selection published in 1859.
Darwin’s Theory of Natural Selection or Darwinism
Darwin stated that new species could be originated through natural selection. Therefore, Darwin’s natural selection reveals the mechanism of the origin of species. Darwin postulated his concept of the mechanism of evolution in the most rational and logical way. He observed certain incidences in nature and made deductions out of these observations.
|1. Living organisms reproduce at an enormous rate in G.P.||The struggle for existence.|
|2. Limitation of food and habitat may increase in A.P.|
|1. Competition (for food and shelter)||Survival of the fittest and natural selection.|
|2. Appearance of variation.|
|1. Survival of the fittest through the gradual accumulation of variations and attainment of adaptability.||Origin of new species.|
G.P. = Geometrical Progression, A.P. = Arithmetic Progression
1. Prodigality of Reproduction:
Darwin was moved by the fact that every organism reproduces in geometrical progression and so that it may produce innumerable progeny within rather a short period of time. Several examples may be cited in this regard. A female salmon lays about 2,80,00000 eggs in a breeding season. Roundworm Ascaris sp. liberates about 7,00000 eggs/day. One mussel releases about 11,40,00000 eggs. The elephant exhibits the slowest reproductive rate. The average lifetime of an elephant is 100 years. Within the reproductive age (extends from 30-90 years) a pair of elephants may produce six cubs during their lifetime. If all the cubs remain alive and reproduce at the same rate then in 750 years number of elephants may come to 1,90,00000.
2. Limitation of Food and Space for Living:
The living space on Earth for all types of animals and plants is limited because the size and shape of the Earth are beyond any scope of extension. In this consideration, if we imagine the reproduction of organisms at a prolific rate then the earth will not be able to provide the space for living to all organisms. In the same way, food for the survival of the organisms is also limited. Though by human effort production of food substances may be increased to some extent in A.P. but that never becomes consistent with the reproductive rate of organisms. Therefore, reproduction leading to excessive progeny production may cause starvation.
3. Struggle for Existence or Competition for Survival:
Every living organism has an eternal effort to remain in surviving condition. Therefore, the innumerable progenies produced through reproduction face tremendous competition for limited food and shelter. Never has the space for living and food materials become equally available for all. As a result, the organisms compete among themselves for obtaining food and shelter. However, this competition may be of two types namely intraspecific struggle and interspecific struggle. In case of intraspecific struggle, the members of the same species show competition for food and shelter but in case of interspecific struggle, the members of different species compete for food and shelter. Besides, every organism has to struggle with its environment through the achievement of adaptation, and then only an organism becomes successful in this competition.
4. Survival of the Fittest:
According to the rule of competition, always the fittest achieves success. Therefore, in the competition for food and shelter, the successful member of a species gets the opportunity for survival. Hence, the surviving member may be considered the fittest. Darwin termed this phenomenon as Natural Selection. This means that nature selects the fittest individual for survival.
5. Origin of Variation:
In the case of natural selection, how a successful member in the competition is selected by nature, explains that Darwin stated the origin of variation. According to him the competing member who can accumulate some new attributes may achieve success. This additional attribute of a successful member is termed by Darwin as a variation. However, he opined that any type of variation may not help the organism in achieving success and only the favorable variations come to its use. Darwin also felt that for evolution small favorable variations are important.
6. Origin of Species:
By accumulating new variations when a member of a species gets natural selection, it becomes different from its fellow members in many respects. In this way in the long run a new species may be originated. Thus, in the course of time origin of new species may be possible. Darwin turned it into the origin of new species by means of Natural Selection.
Limitations of Darwinism or Theory of Natural Selection
Darwinism has limitations for which it faces criticism. Its limitations are the followings:
- Darwin did not mention anything about how variation J may have originated.
- Darwin gave importance to small variations but in most cases, such variations are not heritable. Therefore, they are not useful for evolution.
- According to Darwin, through natural selection positive alteration in species is possible. But there are such events also in evolution where the alteration in species members appears to be in reverse order.
- Darwinism does not state anything about the principles of inheritance.
- According to the theory of natural selection, the unit of evolution is individual. But according to modern concepts, evolution cannot occur at the individual level rather unit of evolution is population.
- According to Darwin through the accumulation of variations the member of a species may be overspecialized which sometimes becomes the cause for the extinction of a species.
The modern synthetic theory is also called Neo-Darwinism.
Differences between Lamarckism and Darwinism:
|Origin of life||Permanent spontaneous generation.||Derived from the ancestral form.|
|Diver of evolution||Complexification over time.||Natural selection.|
|Modifications||Adaptation to the environment.||Spontaneous variations are transmitted to the progeny.|
|Species extinction||No. Unless humans.||Yes.|
C. Modern Synthetic Theory of Evolution
In spite of the limitations of the theory of natural selection, many evolutionary biologists tried to modify the original proposition of Darwin in order to establish the Darwinian concept of evolution. The modified form of the theory of natural selection is known as Modern Synthetic Theory. This is based on the ideas of heredity, isolation, mechanism of origin of variation, etc. This is also called Neo-Darwinism.
After the publication of the theory of natural selection, it created a sensation throughout the countries of the world and almost all the scientists gave enough importance to this concept of evolution. However, there were also several critics of this theory and therefore, it took a modified shape in subsequent periods by his followers. The current views of natural selection are regarded as Modern Synthesis of Natural Selection or Modern Synthetic Theory. Principally the scientists who brought the modification to the theory of natural selection were Huxley (1942), Dobzhansky (1937), Muller (1949), Fisher (1958), Wright (1968), Mayr (1963 & 1970), and Stebbins (1966-76).
Modern Synthetic theory of evolution has been developed in recent times by the accumulation of scientific achievements by different researchers in the field of population genetics, the molecular mechanism of mutation, isolating mechanism, and others. Dobzhansky (1937) emphasized the role of genetic changes in population in the process of evolution. Huxley (1942) and Mayr (1943) explained the mechanism of origin of variations. Haldane, Fisher, and Sewall Wright (1970) established mathematical evidence for the role of natural selection in the evolution of populations. Stebbins (1973) and others have taken all these findings together and explained that many factors cumulatively work together for the evolution and origin of species. So a synthetic theory has been developed. The synthetic theory recognizes five basic processes of evolution namely gene mutation, chromosomal mutation, recombination, natural selection, and reproductive isolation.
This modern theory of evolution does not consider any single view rather it is a synthesis of many theories of many scientists. In the present concept, evolution is regarded as a small and slow change in the genetic makeup of the population of a small group rather than a change at the individual level. Before the appearance of synthetic theory in the late sixties, the most prominent and accepted concepts of evolution were natural selection and mutation. Later, it was conceived that to bring about evolution or the origin of species, a number of factors are involved, natural selection and mutation are two of those. Other factors are recombination, isolation, migration, and genetic drift. All these factors work together in the population of a species to give rise to a new species.
Wagner (1868) emphasized the role of geographic isolation in the formation of species. Dobzhansky (1937) termed this process an isolating mechanism. Darwin (1859) and his followers like Huxley, Weismann, etc. strongly believed that natural selection is the only vital force that operates among the different varieties of a species and the fittest survive by the merits of useful variations which are inherited through generations to evolve into a new species.
de Vries (1905) strongly opposed the role of natural selection as a prime factor for evolution. Instead, he advocated for mutation which is the only agent for a change of a species by means of change in its genes. He did not believe anything other than genetic changes that eventually lead to the origin of species. Sewall Wright (1930) postulated the concept of genetic drift which is an evolutionary force operating in a small population by which a new mutation may be fixed or lost irrespective of its selective or adaptive value.
Stebbins (1973) synthesized all those factors for evolution and opined that the five most important factors operate together in the population of a species to give rise to a new one, and these are gene mutation, chromosomal mutation, recombination, natural selection, and reproductive isolation.
Differences between Darwinism and Neo-Darwinism:
|1. It denotes the evolution of species by means of natural selection as proposed by Darwin.||1. It is the modern version of Darwin’s theory of evolution by natural selection with the incorporation of genetic findings.|
|2. It is the original theory given by Darwin.||2. It is a modification of modem theory based on Mendelian genetics and the modern synthesis of natural selection.|
|3. Accumulation of phenotypic variation acts as a major driving force in evolution.||3. Accumulation of genetic variation is the major driving force in evolution.|
|4. It fails to explain the origin of variation.||4. It gives importance to mutation, recombination, natural selection, and reproductive isolation causing the development of variation.|
|5. The survival of the fittest and removal of the unfit is seen in the process of evolution.||5. The differential amplification of the fittest genes and genotypes drives the process of evolution.|
|6. It does not give any importance to the process of isolation.||6. It gives importance to the process of isolation as a factor in evolution.|