Theories of Evolution: Theories pull together observations, laws, hypotheses and inferences into coherent explanations for the great mysteries of the universe. The theory draws ideas or observations together and makes sense of them. The theory of evolution is like other scientific theories.

Evolution also helps us make sense of the natural world in which we live. Different scientists gave different theories that support evolution. From time to time, a number of hypotheses have been put forward to explain the mechanism by which a very large number of species of animals and plants emerged on the face of this earth.

Lamarck’s Theory of Inheritance of Acquired Characters

Lamarck, in his famous book “Philosophie zoologique” in 1809, proposed a mechanism that a lineage of organisms could change gradually over many generations as offsprings inherited structures that had become larger and more developed as a result of continuous use or conversely had become smaller due to less use.

Postulates of Lamarckism
Postulates of Lamarck’s theory:-

1. New need: The change in environmental factors like light, temperature, medium, food, air, etc. or migration creates new needs for the organisms. To fulfil these new needs, living organisms have to put efforts which results in a change of habit and habitat of the organism.
2. Acquisition of characters: Characters are acquired by the organisms to meet the new needs by following ways-
   a. Use and disuse of organs: The new habits involve greater use of certain organs and disuse of other organs. The continuous use of certain organs makes them active and strong, while disuse makes them inactive, reduced, and ultimately they disappear.
   b. Effect of the environment: Variation due to environmental factors such as light, humidity, food, pressure, wind, temperature, etc., may produce changes.
   The changes brought about by the use and disuse of organs and by the influence of environmental factors in an individual during its lifetime are called acquired characters.
3. Inheritance of acquired characters: Lamarck believed that every generation acquires new characters, and these are transmitted to the new generation.

Evidence in support of Lamarckism

1. The giraffe is a mammal with a very long neck and long forelimbs. They evolved from ancestors with short legs and short necks. These ancestors lived in the barren land. They used to stretch their neck upward to reach the available food in the form of leaves on the tall trees, which made their neck slightly elongated and passed the same feature to the next generation.
2. Snakes are limbless with long cylindrical bodies. This is believed to happen due to continuous disuse of limbs and stretching of the body to accommodate themselves in holes and to protect from powerful mammals.

Criticism of Lamarckism

1. Evidence against the inheritance of Acquired Characters:
   According to the theory of Germplasm, inheritance only takes place by means of the germ cells, i.e. the gametes such as egg cells and sperm cells.
   Other cells of the body, i.e. somatic cells, do not function as agents of heredity. Germ cells produce somatic cells and are not affected by anything. The somatic cells learn any ability the body acquires during its lifetime.
2. Wrestler’s muscles are not inherited, which has been acquired during their lifetime.
3. Pierced ears and nose are not inherited.

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Neo-Lamarckism

The evidence for the inheritance of acquired characters revived the otherwise discarded Lamarckism. The revival of Lamarckism in a modified form is called Neo-Lamarckism.
1. Neo-Lamarckism states that:
2. Germ cells are not always immune from the effects of the environment. Germ cells may be affected directly by the environment without any effect on the somatic cells.
3. Germ cells may carry acquired or somatic variation to the offspring.

Darwin’s Theory of Evolution

Darwin’s theory of evolution is based on the following factors:

1. Rapid multiplication: Population increases geometrically. For E.g. Paramecium reproduces every 24 hours if the conditions are optimum. Bacteria reproduce every 20 min. But if this rate continues, then within 2-5 yrs, the entire earth would get covered with bacteria or Paramecium.
2. Limited food resources and space availability: Food resources and space available for these organisms is limited. An increase of population requires more food and more space. But food increases in an arithmetic manner. Also, space remains constant.  So, the carrying capacity of the environment is limited, and it does not allow the population to grow beyond its limit. Hence population size remains constant.
3. Struggle for existence: Due to limited natural resources, there occurs interspecific and intraspecific struggle. The environmental struggle also exists.
4. Variation in a population: There occurs continuous and discontinuous variation. Continuous variation is slow and caused due to recombination. Discontinuous variations are due to sudden mutation. Much of the variation is heritable.
5. Natural selection and survival of the fittest: The individuals having more favourable variations will survive and reproduce. The environment would gradually weed out organisms with unfavourable variations but preserve those with favourable variations.
6. Inheritance of useful variations: The surviving individuals would pass on their useful variations to the next generation. So, the offspring of a fit individual would also be fit.
7. Origin of new species: After a number of generations, the variations become so prominent that their possessors turn into a member of a new species. New species thus arise by gradual modification of the older ones.

De Vries Mutation Theory

1. De Vries mutation theory states that new species arise from pre-existing ones in a single generation by a sudden appearance of marked discontinuous, inheritable genetic variations called mutations. Whatever changes take place are due to sudden changes in genetic makeup that Vries called a mutation.
2. De Vries said that evolution is a discontinuous and jerky process and not continuous and gradual, which was held by Lamarck and Darwin.
3. He said mutation is the raw material of evolution.
4. He worked on the Evening Primrose plant. He formulated his theory on the basis of observations he observed from his experiments. He observed in each generation, there were few plants with wide differences from parent plants, and many of them were having small variations from parent plants. Variations were like the size of the plant, the colour of the flower, position of the flower, etc.

Features of mutation theory
i. Mutations are the raw material for evolution.
ii. It is a discontinuous process.
iii. It appears suddenly.
iv. It appears again and again.
v. Mutation is changes in the genetic information, so they are inheritable.

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Summary

From the above discussion, we can conclude that different scientists gave their opinion about how evolution occurred. Lamarck gave the theory of inheritance of acquired characters. According to him, new needs lead to different characters being acquired. These characters are transferred from one generation to another. Darwin gave the theory of natural selection. According to him, there is a struggle for the existence and survival of the fittest. Then De Vries came and put forward the Mutation theory in 1901. He said mutation is a discontinuous process and not continuous. Mutations are changes in genetic information, so they are inheritable.

Frequently Asked Questions (FAQs) About Theories of Evolution

Let’s look at some of the commonly asked questions about theories of evolution:

Q.1. What are different theories of evolution?
Ans: Different theories of evolution are:-
1. Lamarck’s theory of the inheritance of acquired characters or Lamarckism.
2. Darwin’s theory of natural selection or Darwinism.
3. De Vries mutation theory.

Q.2. What was Lamarck’s theory of evolution?
Ans: Lamarck gave the theory of inheritance of acquired characters. According to him, new needs lead to the generation of new characters which are inherited from the offspring and gradually, new species are evolved.

Q.3. What is Darwin’s theory of evolution?
Ans: Darwin gave the idea of natural selection and survival of the fittest. The individuals having more favourable variations will survive and reproduce. The environment would gradually weed out organisms with unfavourable variations but preserve those with favourable variations.

Q.4. What is the theory of use and disuse?
Ans: The new habits involve greater use of certain organs and disuse of other organs. The continuous use of certain organs makes them active and strong, while disuse makes them inactive, reduced, and ultimately they disappear.

Q.5. Who invented mutation?
Ans: Hugo De Vries coined the term mutation.

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