12th Zoology

The Theory of Natural Selection

The Theory of Natural Selection

During his period of study Darwin has made several observations
and collected facts. Through these facts he arrived at certain conclusions.
These conclusions constitute Darwinism. Thus Darwinism or the theory of
Natural selection includes the following elements.

1. Overproduction or Prodigality of nature

All living beings have an innate desire to reproduce and form their
own progeny. In this attempt they have the capacity to multiply in a
geometrical manner. Such an enormous reproductive potentiality can be
observed in all species of organisms. For example a single female salmon fish
can produce 28,000,000 eggs in a season. A common oyster of the Atlantic
coast may release as many as 80 million eggs in one season. At the time of
breeding the ovaries of a cod fish may contain 10 million eggs. (theory of natural selection)

If natural processes of reproduction are allowed without any check a single pair of
English sparrows can produce 275 billion descendents in 10 years. The
elephants are the slowest breeders. An elephant begins to breed at 30 years
of age. It goes on breeding till it is 90 years old. A female elephant can give
birth to 6 young ones during its life time. Under these conditions, a pair of
male and female elephants can cause the production of 19 million elephants
in 750 years. Thus, the reproductive capacity is an innate nature of living
organisms.(theory of natural selection)

2. Struggle for existence

According to Darwin while the population increases in geometric
ratio there is no corresponding increase in food production. This causes an
intensive struggle for living. The struggle happens for food, space for
living and for coping with environmental conditions. The struggle may be
intraspecific or interspecific. In an intraspecific struggle there is a
competition among the individuals of the same species. Such a struggle is
severe because the needs of the competing organisms are identical. Struggle
might happen with environment too. Conditions like heat, cold, drought,
storms, floods and other natural changes can affect organisms resulting in
struggle for existence. (theory of natural selection)

3. The universal occurrence of variations

The occurrence of variations is a characteristic feature of all groups
of animals and plants. The variations may be morphological , anatomical,
physiological or behavioral. (However during Darwin’s time the actual cause
and nature of variations were not known).

As a result of variations, no two animals would look alike. Even the
progeny of the same parents are not exactly alike in all respects. Eventhough
variations are universal all such variations need not be significant from an
evolutionary point of view. Certain variations that get established in the
population and get inherited continually are termed, heritable variations.
Such variations form the raw material for evolution.(theory of natural selection)

4. Survival of the fittest

While all living organisms face the struggle for existence, certain
organisms possessing adequate modifications are able to escape and survive. Such modifications are due to inherent variations. Hence favourable
variations make an animal or a plant to be successful in life. They survive as
fittest organisms in an environment which gets altered frequently. (theory of natural selection)

5. Natural Selection

According to Darwin ‘the fittest’ forms that are allowed to survive
are chosen by ‘Natural Selection’ (an imaginative concept which includes all
real aspects of the natural environment that supports the life of

The forces of natural selection will encourage only those that have
suitable variations as adaptive features, to survive. Darwin designated them
as ‘fittest’ forms. All other organisms having non adaptive or deleterious
modifications shall be disqualified. Natural selection will eliminate such
organisms from the populations. The selected group of modified
individuals will occupy the next level in the evolutionary ladder.
Darwin strongly believed that using the natural selection concept, all
evolutionary processes in the living world can be explained. As an
example he differed from the explanation provided by Lamarck while
explaining the lengthening of neck in giraffe. According to Darwin the
population of giraffes had individuals having varying neck lengths. Those that
had longer necks had more survival value, since they had more food and
remained healthy. Gradually natural selection encouraged them to
survive. Thus in course of time the average length of the neck increased. (theory of natural selection)

Related Topics in Zoology:

Bio Zoology All Important Topics

  1. Livestock and Management

  2. Important cattle breeds and their characteristics

  3. Milch breeds (or) Dairy breeds

  4. Dual purpose breeds & Draught breeds

  5. Exotic breeds of cattle

  6. Common diseases and control in Cattle – Contagious diseases

  7. Non-contagious diseases – Cattle

  8. Techniques adopted in cattle breeding

  9. Poultry Breeds

  10. Farming methods

  11. Poultry byproducts

  12. Fish Pond

  13. Edible Fishes Of Tamilnadu

  14. Medical Lab Techniques – Stethoscope

  15. Sphygmomanometer

  16. Haemocytometer

  17. Urine Sugar analysis

  18. ECG Electrocardiogram

  19. Computed Tomography CT

  20. Endoscopy (Laproscopy) techniques , Artificial Pacemaker

  21. Autoanalyser

Unit 7. THEORIES OF EVOLUTION Topic List Zoology

  1. Theories Of Evolution Introduction

  2. Lamarck Laws

  3. Neo Lamarckism

  4. Darwinism

  5. Theory of Natural Selection

  6. Objections to Darwinism

  7. Modern concept of Natural Selection

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