Evolution

The process of gradual change from simple living to complex is called evolution.

Theories of origin of life

Theory of special creation :

This theory believes that almighty god created every thing including earth, stars, plants, animals, microbes and human being. Genesis of Bible has proposed that God created the whole world around 4000 years ago in six days.

• First day - Materia prima, heaven and earth

• Second day - sky and water

• Third day - land and land plants

• Fourth day - sun, moon and stars

• Firfth day - fish and fowls (birds)

• Sixth day - land animals and man's

The first man was Adam. He was created from clay. The first woman was Eve who developed from the twelfth rib of Adam.

Hindu Mythology beleives that supreme power created a Trinity or group of three gods. Brahma, the creator god, formed humans from head, birds from chest, goat from mouth and plants from hair. The first man was Manu and the first woman was Shardha.

Theory of panspermia :

This theory believes that life came to earth from other planet in the form of spores.

Theory of abiogenesis (theory of spontaneous generation) :

This theory believes that life originated from non-living decaying object like rotten mud of river Nile.

Theory of Biogenesis :

According to this theory life originated from pre-existing life or omne vivum e vivo. Theory of Biogenesis was experimentally proven by three scientists - Redi, Spallanzani and Pasteur.

Redi's experiment - He put dead snake, fish, meat and eel in separate wide mouthed flasks, some without cover, other covered with fine muslin and parchment. After few days, he observed that maggots(larve) did not appear in covered flask but were present in uncovered flasks which were regularly visited by flies. From this experiment he concluded that if life originated from non-living rotten object than all the jar should have new flies. Therefore life originate from pre existing life.

Louis Pasteur experiment - He boiled broth in flasks having bent swan neck. No microorganisms were observed in broth after keeping for several days though broth was connected to air. It is because the dirt carrying microorganisms got settled in the bent part of the neck. When the neck was broken, colonies of microorganisms soon developed over the broth showing that microorganisms have come from air.

Theory of Chemogeny/organic evolution

This theory was proposed by Oparin a Russian scientist and Haldane a British scientist. According to this theory life originated from chemical. This theory states that- 'Abiogenesis first and Biogenesis ever since’. It means the origin of the chemical evolution of life happened first, followed by biological evolution.

According to the theory, about 4 billion years ago, there was no free oxygen on Earth (reducing state). The atmosphere had ammonia, methane and water vapours. These gases made all the simple and complex compounds. The compound of higher weight settle down at the base and lighter forms the ancient atmosphere ( nebular dust).

Formation of cloud and then rainfall might have occurred and these chemicals gradually got accumulated in the ancient oceans.

Continuous polymerization and condensation of these chemicals occurred that resulted into formation of amino acids, carbohydrates, fats and nucleic acid. The ancient oceans where filled up with these biomolecules and it was called pre biotic soup or Haldane soup.

Oparin in his laboratory saw that protein has a tendency to form coat over carbohydrates to which fats and nucleic acid gets attached such types of cell was called coacervate from where life might have originated.

Miller's and Urey's experiment

S. Miller and Urey at University of California conducted an experiment that upto some extent proves Oparin's theory of chemical evolution. They took an electric discharge tube fitted with electrodes. The discharge tube was connected to vaccum pump from where air was removed and gases like ammonia, methane, Hydrogen and water vapour was filled in. The condition was similar like that of primitive earth. When high voltage electricity was passed frequently, electric sparks occured and temperature becomes upto 800⁰C. Which was continued for about seven days. After which simple amino acids like glycine, arginine was found in discharge tube.

Oparin also stated that in the primitive earth due to continuous lighting and high temperature formation of biomolecules like amino acids took place and this experiment prooves the same.

Theories of Evolution

Lamarckism or theory of use and disuse or theory of inheritance of acquired characters

This theory was proposed by French biologist Jean Baptiste Lamarck in his book "Philosophie Zoologique". According to this theory the organ or body part which is continuously in use is called organ of use and which is not in use are called organ of disuse . Evolution of organ of use takes place and elimination of organ of disuse occurs and these characters are inherited.

Evidences

1. Evolution of long neck of Giraffe - in ancient time Giraffe was deer like animal that fed on surface vegetation. However, due to famine like conditions food became scare on the ground. To reach the trees they continuously streched their neck and forelimbs that cause enlargement of body parts . The trait was passed to the next generation and thus evolution of long necked Giraffe happens.

2. Evolution of limbless snake - Ancient snakes had limbs which were organ of disuse since the moved with ventral body scale , making limbs organ of disuse which gradually got reduced and ultimately limbless snakes evolved.

3. Webbed toe of aquatic birds

4. Long legged birds

5. Flight less birds

6. Cave dwellers

Theory of Natural selection or Darwinism

Charles Darwin after his 5 year of Voyage (1831-1936) in his own ship HMS Beagle returned back to England and studied a book written by Thomas Malthus " on the principle of population" in which he described the presence of limited resources, geometrical increase in population, struggle for existence and correction of population size through wars, famines, earthquake etc. Darwin published his own book " On the origin of species by means of natural selection".

Main points of this theory was

1. Unlimited food and space When the life originated there where unlimited food and space.

2. Exponential rise in population According to Darwinism, the populations tend to multiply geometrically and the reproductive powers of living organisms (biotic potential) are much more than required to maintain their number.

3. Limited food and space As population increased rapidly the food and space became limited.

4. Struggle for existence Due to rapid multiplication of populations but limited food and space, there starts an everlasting competition between individuals having similar requirements. In this competition, every living organism desires to have an upper hand over others. The struggle for existence may be of following types - a. Intra specific When struggle takes place between members of same species. b. Inter specific When struggle takes place between members of different species. c. With environment or Extra specific Between living organisms and adverse environmental factors like heat, cold, drought, flood, earthquakes, light etc. Out of these three forms of struggle, the intraspecific struggle is the strongest type of struggle as the needs of the individuals of same species are most similar.

5. Occurance of Variation In order to win the struggle for existence organisms brought about some changes or variation occurs.

6. Survival of fittest The organisms that wins the struggle for existence are called fittest and Nature selects only fittest organism, unfit are removed by nature.

7. Origin of species Group of fittest organism ultimately forms species. The process of formation of species is called speciation.

8. Origin of population Group of similar species which interbreed among themselves naturally forms population.

Criticism of Darwinism

The theory of evolution proposed by Charles Darwin, has emerged since its inception. While Darwinism has had a profound impact on the field of biology and is widely accepted by the scientific community, it is not without its critics. Some of the main criticisms of Darwinism include:

1. Incomplete Fossil Record: Critics argue that the fossil record does not provide sufficient evidence to support the gradual and continuous evolution of species as proposed by Darwin. They claim that there are significant gaps in the fossil record, making it difficult to establish a complete evolutionary lineage.

2. Lack of Transitional Fossils: Critics contend that the lack of intermediate or transitional fossil forms raises questions about the validity of Darwin's theory. They argue that if evolution occurred gradually, there should be numerous transitional fossils documenting the intermediate stages between different species. The absence of these fossils is seen as a weakness in Darwinism.

3. Irreducible Complexity: Some critics argue that certain biological structures and systems are too complex to have evolved gradually through natural selection. They claim that complex organs or biochemical pathways, such as the eye or blood clotting, could not have evolved in a step-by-step manner, as each intermediate stage would have been non-functional and therefore not subject to natural selection.

4. Origin of Life: Critics of Darwinism often raise questions about the origin of life itself. While Darwin's theory primarily addresses the diversification of existing life forms through natural selection, it does not provide a comprehensive explanation for the origin of the first living organism. Some critics argue that the emergence of life requires a separate explanation beyond Darwinian evolution.

5. Genetic Information and Complexity: Critics argue that the vast amount of genetic information and the complexity of biological systems cannot be adequately explained by random genetic mutations and natural selection alone. They suggest that other mechanisms, such as intelligent design or inherent complexity, must be involved in the development and organization of biological structures.

6. Lack of Predictive Power: Critics claim that Darwinism lacks strong predictive power and cannot accurately explain certain phenomena or make testable predictions. They argue that the theory relies heavily on post hoc explanations and retroactive interpretations rather than making precise predictions that can be tested and validated.

It is important to note that these criticisms do not undermine the broad consensus within the scientific community regarding the validity of evolutionary theory. The scientific community continues to refine and expand upon Darwin's ideas, incorporating new evidence and addressing these criticisms to further our understanding of the natural world.

Mutation theory of Evolution

This theory was proposed by Hugo de Vries in his book "The mutation Theory " . He self crossed Oenothera lamarckiana (Evening Primrose) for 4 to 5 generation. Plants he received were like that of parental and few shows Variation.

The different plants were also self-pollinated and when their seeds were sown, the majority of the plants were similar to the parents while a few were still more different plants and this continued generation after generation. These plants appeared to be new species named Oenothera gigantia.

The Mutation Theory of evolution, also known as the Mutationism or Saltationism, is a hypothesis that suggests that significant evolutionary changes occur primarily through sudden and large-scale mutations rather than gradual accumulation of small changes. Here's a brief explanation of the Mutation Theory:

1. Mutation is the raw material of evolution.

2. Mutation occurs all of a sudden

3. The same type of mutation can appear in a number of individuals of a species.

4. Mutation are inheritable

5. Usefull mutation are selected by nature and lithal mutation are eliminated

6. Accumulation of variation results in formation of new species or sometimes a single large step mutation or saltation results in formation of new species.

Criticisms and Rejection: The Mutation Theory faced significant criticism and was largely rejected by the scientific community in the early 20th century. The theory did not provide a satisfactory explanation for the mechanisms underlying large-scale mutations, and its predictions were inconsistent with empirical evidence and the principles of population genetics.

Modern Understanding: While mutations do contribute to evolutionary change, the Mutation Theory as proposed by early mutationists is generally not accepted as the primary mechanism for significant evolutionary transformations. Modern evolutionary biology emphasizes the role of mutations in generating genetic variation, which is then subject to natural selection and other evolutionary mechanisms.

In summary, the Mutation Theory of evolution posits that large-scale mutations, occurring abruptly and leading to distinct traits, are the primary drivers of significant evolutionary changes. However, this theory has been largely rejected by the scientific community due to inconsistencies with empirical evidence and the lack of a comprehensive understanding of the underlying mechanisms of large-scale mutations.)

Modern synthetic theory of evolution

This theory was proposed by a group of scientists like J.B.S Haldane, S. Wright, Stebbin, Ernest Mayer and R.A. Fisher.

This theory is compilation of theory of natural selection and mutation theory.

The main regions for variation are

1. Mutation: sudden change in genetic elements (explained in principle of inheritance)

2. Crossing over / Recombination: the process of mutual exchange of chromosomal segment between two non sister homologous chromatids is called crossing over.

3. Gene flow / migration: when few members of a population moves from one place to another and interbreed with the native species of that place is called gene flow that also results in variation.

4. Genetic drift : random fluctuations in large number of gene is commonly known as genetic drift. Genetic drift may occur due to some adverse climatic conditions like volcano, tsunami etc.

5. Natural selection: the concept of natural selection was proposed by Darwin and Alfred Wallace. According to them nature select only such varient which are fittest.