- It is also known as theory of natural selection.
- Charles Robert Darwin was (1809- 1882) an English naturalist published his famous book “ The origin of species by natural selection” in 1859.
- This theory is based on a mass of accurate observations and prolonged experiments.
- This theory led the whole scientific world to believe in the doctrine of evolution.
- He accompanied Adam Sedgewick on an exploration of North-Wales where he spent five years (1831-1836) on a voyage on the ship Beagle.
- He made extensive observations on animals and plants life and brought back a lot of material for further examination.
- Many kinds of natural balances, catastrophic changes and fossil remains of many communities were his studied material.
- He saw nature as dynamic force, constantly achieving and retaining equilibria and felt the need for all these factors to be put into causative pattern.
- He was impressed with infinite variety and wonderful adaptations found in the animals and plants.
- He found a living laboratory of evolution along South American coast.
- He observed great variation among the organism that lived on these islands.
- He found many species of given genus confined to a single island (Galapagos Island), yet resembling other species on the mainland.
- Various islands had different species of finches living on them, of different sizes and with different shapes of beak.
- From this, he concluded that finches were arrived from the mainland of South America and has undergone profound changes under the different conditions of the individual island.
- It showed that a single ancestral group can give rise to several different varieties of species.
- In 1858, Alfred R. Wallace (1823- 1923) concluded and supported the views of Darwin’s as well.
- Darwin’s theory of natural section is based on the following factors.
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A) Excessive rate of multiplication
- Every individual leaves behind a large number of progeny.
- The living organisms reproduce and increase their number in geometrical pattern if environmental check is absent.
- A salmon produces about 28,000,000 eggs in a season and a single maize plant yields hundreds of seeds.
- Similarly, a single pair of English sparrow would be the ancestors of over 275 billion individuals in 10 years.
- Even a pair of elephants produces 29 million descendants at the end of 800 years in the absence of environmental check though it is the slowest breeding animals.
- If it would not be checked then the number of animals would increase in such manner that the number would be greater than the available food or the space that the earth could accomodate.
- But this does not happen, the individuals in a population remain approximately constant under natural selection.
B) Struggle for existence
- The organisms will struggle with each other for food and space due to excessive rate of multiplication and limited resources.
- There will be more struggle between the individuals of same species i.e. intraspecific because of similar requirements and adaptations.
- The struggle also takes place between the individuals of different species (interspecific) and between the organisms and environment.
- Many individuals die or fail to survive while others survive during this struggle.
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C) Survival of the fittest (Natural selection)
- The successful variants survive and reproduce further while the rest are eliminated during the struggle for existence.
- This process is known as the survival of the fittest or the natural selection.
- It means that nature selects the fittest or those individuals which are best adapted to the condition of life and eliminates those which are not well adapted.
D) Variations and Heredity
- Variations exist in every plant and animals of nature due to which no two individuals of a species are ever exactly alike.
- Darwin noticed two types of variations one useful and another useless.
- He believed that the individuals with useful variations have a better chance of survival than those without them.
- But, he was unable to explain the cause of variation which he assumed as one of the innate properties of living things.
- Useful variations are preserved and inherited to the offspring.
- Thus, variations provide a raw material for natural selection.
- He noticed that the tortoises were clearly different from island to island, although the islands were only about fifty or sixty miles apart.
- In isolation, each population had evolved its own distinctive features, yet al the island tortoises showed basic resemblances.
- It showed that island tortoises shared a common ancestor with the mainland forms.
E) Inheritance of useful variations(Origin of new species)
- The successful variants survive and reproduce to give rise to the next generation.
- Thus, the useful and fittest variations are transmitted to the succeeding generations.
- After many generations, the individuals become so modified that they give rise to new species.
Evidences in support of Darwinism
- There existed a large number of animals in the past but due to scarcity of food, changed climate and struggle for their habitats only a few survived.
- Production of various domestic varieties by artificial selection is evident.
- Ancestors of horse were four-toed, from them modern horses have evolved by reduction of three toes, fibula and ulna.
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Objections to the theory of Natural Selection
- He believed that natural selection operates on small variations, which are heritable and lead to evolution after many generations.
- But, these variations are rarely inherited as their origin is due to environmental conditions. He did not consider the possibility of the origin of new hereditary variations, which are really responsible for the origin of species.
- He could not explain about the arrival of the fittest i.e. he could not explain the beginning of organs as he was focused with the survival of the fittest.
- Though the vestigial organs have no functions, they are being preserved and transmitted to another generation. But, the natural selection selects the useful organs for the struggle of existence.
- He indirectly accepted the Lamarckian idea of inheritance of acquired characters.
- In certain animals over specialization occurs in some organs such as huge antlers of deer, tusks of elephants which cannot be explained on the basis of continuous variation and natural selection.
Darwin’s theory of evolution (Theory of Natural Selection)