DARWIN’S THEORY OF NATURAL SELECTION
- Charles Darwin, an English naturalist and one of the most dominant biologists of 19th century made an extensive study
of nature for over 20 years while on a voyage of world exploration on a famous ship H.M.S. Beagle. - During this journey, he explored the fauna and flora of a number of continents and islands such as Galapagos islands. There
he observed great variations among the organisms living there and called it “a living laboratory of evolution.” - Later, he proposed the theory of natural selection in his book “Origin of Species by Means of Natural Selection”.
Rapid multiplication- All organisms possess enormous fertility and they multiply in
geometric ratio, resulting in over-production. E.g., a cod fish lays
several hundred eggs at a time.
Inheritance of useful variations- Useful variations are passed on to the next generation
and non-useful are eliminated.
Survival of the fittest - Organisms better adapted to their surroundings survive and unfit
ones are destroyed.
Variations- Variations are differences among the individuals.
Variations helpful in adaptation of organism towards its
surroundings are passed onto the next generation.
Limited food and space- Resources such as food and space remain limited and
are not liable to increase with increase in population.
Struggle for existence- Intraspecific struggle between the individuals of same species for
similar requirements of food and shelter. - Interspecific struggle between the members of different species.
- Environmental struggle between the organisms and environ-
mental factors.
Formation of new species- Accumulation of useful variations generation after
Salient Features of Darwin’s Theory of generation leads to formation of new species.
Natural SelectionEvidences in Favour of Darwinism
- Higher rate of reproduction in all organisms.
- Limitation of food, space and other resources.
- Struggle for existence is seen in all organisms.
- Abundance of variations among individuals of a
population. - Mimicry and protective colouration in certain animals.
- Correlation between position of nectaries in flowers
and length of proboscis in pollinating insects.
Criticism Against Darwinism
- Darwin did not differentiate between somatic and germinal
variations and considered all variations as heritable. - Occurrence of organisms that remained unchanged for
several million years. - Occurrence of discontinuous variations.
- Arrival of the fittest cannot be explained.
- The effect of use and disuse and the presence of vestigial organs cannot be explained.
Branching descent and natural selection are two important postulates of Darwinism. Branching descent can be best explained
by convergent evolution of Australian Marsupials and placental mammals. Natural selection of different traits can be of
stabilising, directional and disruptive type.
Principle of Natural Selection
- The principle of natural selection arises from five important observations and three inferences. It was proposed by
Ernst Mayer in 1982. It demonstrates that natural selection is the differential success in reproduction and it operates
through interactions between the environment and inherent variability in the population.