Darwinism Short Note | Evolution Class 12 Notes

In this article we will discuss about Darwinism:- Concepts of Darwinism, Evidences in Favor of Natural Selection and Criticism of the Natural Selection Theory

• Evolutionary idea contributed by Darwin is called Darwinism
• In 1831 Darwin got an opportunity to travel on H.M.S. Beagle for a voyage of world exploration.
• He explained the mechanism of evolution

CONCEPTS OF DARWINISM

• • Over Production
  • Struggle for survive
  • Variations
  • Natural selection
  • Speciation

1. OVER PRODUCTION

• Living organisms have capacity to produce more individuals to ensure continuity of the race
• This led to overcrowding
• Example
  • Oyster may produce over 60-80 million eggs per year
  • Single female salmon produces 28 million eggs in a season

2. STRUGGLE FOR SURVIVE

• Reproduction is high but there is no corresponding space, food and other factors essential for life
• This lead to competition for getting requirement of life
• This competition is called Struggle for survive
  Intraspecific struggle
• Competition between members of same species
• They need and requirements are same so competition is heavy
  Interspecific struggle
• Competition between members of different species
  Environmental struggle
• Competition between organisms and environmental factors

3. VARIATIONS

• Differences in character between individuals
• Variations may be harmful, neutral or useful
• Variations that are passed on from generation to generation
• variations arise due to changes in the genes or the chromosomes
• Small continuous variations are important for evolution

4. NATURAL SELECTION

• Organisms with favorable variation succeed in the struggle for existence
• They are fittest for survival and other animals unfit to survival
• Fitness and unfitness is decided by environment
• Process of selection of organisms with favorable variation is called natural selection

5. SPECIATION

• New species arise by the co-operation of all the principles
• Overproduction leads to struggle for existence
• Animals are provided with a bulk of variations
• Environment select the animals with favorable variation
• They are survive and during reproduction character transmitted to next generation
• A new species originated by accumulation of favorable variations from a number of generation

Evidences in Favor of Natural Selection:

1. Rate of Reproduction: Rate of reproduction is many times higher than the rate of survival in all organisms.
2. Limitation of Resources: Food, space and other resources are limited.
3. Struggle for Existence: Competition or struggle for existence is seen in all organisms.
4. Abundance of Variations: Variations are so abundant in nature that no two individu­als of a species are similar, not even the monozygotic twins (they possess some dissimilarities due to their environment).
5. Production of New Varieties of Plants and Animals by Artificial Selection: When man can produce various new varieties of plants and animals in a short period, nature with its vast resources and long time at its disposal can easily produce new species by selection.
6. Mimicry and Protective Colouration: They are found in certain animals and are products of natural selection.
7. Correlation between Nectaries of Flowers and Proboscis of Insects (Entomophily): The position of nectary in a flower and the length of proboscis in pollinating insects are wonderfully correlated.
8. Pedigrees of Some Animals: Pedigrees of horses, camels and elephants also sup­port the Natural Selection Theory.

Criticism of the Natural Selection Theory:

1. Inheritance of Small Variations: According to natural selection theory only useful variations are transmitted to the next generation, but sometimes small variations which are not useful for the possessor, are also inherited. It is beyond understanding that if the appearance of small wings in birds could help them in flying.
2. Over-Specialization of Some Organs: Some organs like tusks of elephants, antlers of deer have developed so much that instead of providing usefulness to the possessor, they often give hindrance to them. This theory cannot explain these facts.
3. Vestigial Organs: Why vestigial organs are present in some animals when they have no function? According to the Natural Selection Theory, vestigial organs should not be present.
4. Arrival of the Fittest: The theory only explains the survival of the fittest but, is unable to explain the arrival of the fittest.
5. Degeneration of Organs: The theory does not account for the degeneration of certain organs in animals.
6. Discontinuous Variations: The theory fails to explain the cause of sudden changes in the body. The main drawback of Darwin Is theory was lack of the knowledge of heredity and that is why he could not explain that how the variations are caused. Darwin himself was conscious of the inadequacies of his theory, when he remarked that, “I am convinced that natural selection has been the most important but not the exclusive means of modifications.”