Derwinism (Theory of Natural Selection)

Derwinism (Theory of Natural Selection)

Definition :  Darwinism explains the mechanism of evolutionary changes by natural selection by Darwin.

Introduction :  Darwin’s theory of Natural selection published in 1859 – “ The origin of Species by Natural Selection”. This theory explains the origin of species by means of natural selection and preservation of favorable races in the struggle for existence Darwinism is based on certain facts of nature from which deductions have been made.

(1)        Overproduction :

• Production of a progeny by organisms is an innate desire amongst all living beings for the continuity of lrace.
• According to Darwinism, the populations tend to multiply geometrically and the reproductive powers of living organisms are much more than required to maintain their number.

e.g.  A fish lays an enormous number of eggs,  a cod lays about a million eggs in its lifetime. If all these eggs were to develop, then the entire ocean would be occupied by only a single species of fish. This indicates that every species of animal and plants have a mechanism to control over productivity and keep on an efficient check. Darwin recognized this natural check as – a struggle for existence.

(2)        Struggle for existence :

Although multiplication of organisms occurs geometrically, the food and space are almost constant. As a result, the struggle automatically follows for existence. This struggle may be of three types

•  Interaspacific Struggle  – between individual of the same species – every organism has similar structure and habits, so the competition is absolute. It is the most severe check as each one needs are similar.

• Interespecific struggle – between organisms of different species living together. Here, one species may become the food for the other species. Example: A lizard feeds on ants and is preyed upon by snakes, whereas a kite fees on both lizards and snakes. Therefore, individuals of different species compete for the same need.

• Expraspecific struggle – “Struggle with the environment”. Environment conditions of climate and physical surroundings play a selective force on living organisms – Those organisms which are able to withstand excess of moisture, drought, heat, cold, earthquakes etc can survive.

(3)        Variations and heredity under nature :

No two individuals of a species are exactly alike because competition has led them to change according to the conditions, utilize the resources and survive. These differences even in closely related individuals such as two brothers, are called variation

Variations form the basic pre-requisite and progressive factor for evolution, because without variations, no change would occur and evolution would be impossible. But all variations are not significant from the evolutionary point of view. Some changes occur only Somatically and are not inherited. Only those variations that can be inherited to the offsprings are evolutionarily significant. Such variations are called heritable Variations. (changes that occur in genes or the Chromosomes of the germ plasma are heritable) Darwin observed that useful variations were selected and inherited – leading to evolution.

(4)        Survival of the fittest :

After experiencing the various struggles for existence and acquiring useful and heritable variations, only those individuals could survive who could face the hardships and rigours of the environment. Those individuals or races which could not tolerate these hardships were the least fit and were the first to perish (die).

Those that survived showed high selective value and in course of time showed various adaptive modifications, to adapt/suit themselves in the changed conditions of life. The survival of the fittest is the result of natural selection which enforces adaptation.

Here, Darwin took the example of Lamarca giraffe. The giraffe showed variations in the length of neck and legs as grass on the ground was scarce, so the animals were forced to eat leaves on tall trees. Here, animals with longer necks and legs had on advantage over others, so they could get food easily and had better chances of survival.

(5)        Origin of a new species :

In due course of time, as a result of the struggle for existence, the best –fitted and the most suitable individuals survive and become adjusted to the surroundings. These adaptations are thus preserved and accumulated in the individuals of the species and ultimately lead to the formation of a ‘New species’ from the old one As natural selection continues, the descendants, after a few generations become marked distinct from their ancestors. In this way, another new species will appear.