Darwin’s Theory of Evolution: Study Notes

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DARWIN’S THEORY OF NATURAL SELECTION

Charles Darwin explained the theory in his famous book origin of species by natural selection.

FEATURES:

Over Production:

• Living organisms have fast process of reproduction
• Cod fish produces five to seven millions eggs in seasons.
• A house fly lays one twenty eggs in every six time summer.
• Star fish lays millions of eggs in a year.
• A rabbit breeds at the age of 6 months, he produces six children every four time in one year.
• Elephant is the slowest breeders, he takes part in reproduction about sixty years in his age but he produces only six children.

Struggle for Existence:

• A logical result of over production is the severe competition for food and space and other necessities of life, what Darwin called struggle for existence.
• This struggle may be Intraspecific Struggle (between the same kinds of species) or Interspecific Struggle (between other kinds of species) or Environmental Struggle (against the severe climatic conditions such as light temperature rain etc).

Variation in Heredity:

Those organisms that show suitable variations continue to exist and reproduce (Favourable variations) while those that show unfavourable variations are eliminated.

Natural Selection or Survival of Fittest:

In the competition or struggle for existence if an individual is fit for environment it can survive, otherwise if an individual is unfit it is eliminated. This is natural selection.

Formation of New of Species:

• The individuals which have favorable variations live to maturity and transmit their variations to offspring.
• This process continues and variations accumulated to such extend that structure of body is modified. It is known as structural modification through which origin of new species take place.

OBJECTIONS:

• Darwin did not clearly differentiate between heritable and non-heritable variations.
• Darwin had no explanation for the presence of neutral variations which have neither useful nor harmful effects but exist generation after generation.
• Natural selection theory can’t account for the presence of vestigial organs.
• Natural selection has a key position in evolution but it operates after variations have appeared, so it accounts for the preservation rather than the origin of favorable variations.
• This theory explains the survival of fittest but does not explain the arrival of fittest.

DE VRIES (1848-1935) - Mutation theory

• He was the first who realized the role of mutation in evolution.
• He called mutation to discontinuous variations due to which permanent racial changes occurs. Mutations are sudden heritable changes. They occur in organisms in unpredictable manner they are caused by environment effects or cross breeding.
• De Vries suggested that mutation affected a change in the nature of germ cells while fluctuation was because of the effect of environment.
• The credit goes to De Vries for indicating that mutations are the source of variability on which natural select ion acts.

IMPORTANCE OF NATURAL SELECTION

Genetic Death: It is important from evolutionary point of view because if an individual die before reproduction, unfavorable genes eliminated from population.

Natural selection is responsible for both maintaining the constancy in species and changing them.

Natural selection is most destructive against those individuals that deviate most widely from the mean and favor those that align most closely to it. This kind of selection is called Stabilizing Selection.

Natural Selection And Population Genetics:

• As more or less genetically isolated unit of population is known as deme.
• The genetic constitution of demes i.e. the sum of total of all different genes is known as gene pool.
• The evolutionary future of an individual organism depends on its genetic constitution. Therefore, the evolution future of an individual again depend upon on its gene pool.

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