Evolution : A slow, gradual, continuous and irreversible changes through which the present day complex forms have descended from their simple pre-existing forms of the past.
Principles of Darwin's Theory of Natural Selection : Charles R. Darwin was a British biologist. He postulated the 'Theory of Origin of Species by Natural Selection'.
Principles of Darwinism :
(i) Over production or prodigality of production : All organisms have a natural tendency to over produce. If this tendency is not checked, then even a single species of a plant or animal will occupy the entire space available on the earth.
(ii) Struggle for existence : Organisms multiply in geometric ratio but space and food remain constant leading to competition for survival. Increase in number of species leads to a competition called struggle for existence. The struggle for existence may be intra-specific, inter-specific or environ-mental.
(a) Intra-specific struggle : It is the competition among the individual of the same species. This type of struggle is very severe, because of the need and approach of all competing organisms is precisely same.
(b) Inter-specific struggle : It is the struggle between the organisms of the different species living together, individuals of one species compete with other species for food, shelter and breeding places.
(c) Environmental struggle : It is struggle of all living froms against adverse environmental conditions i.e., against natural calamities like extreme cold, heat, drought, storms, earthquakes, etc.
(iii) Variations and heredity : The differences which occur between the closely related organisms are called variations. It is universal law of nature. Variation may be favourable or unfavourable. According to Darwin the useful variations are preserved and passed on to their offsprings. They play an important role in evolution.
(iv) Survival of the fittest or natural selection :
According to Darwin, in the struggle for existence the fittest individuals survive and reproduce while the unfit individuals survive and reproduce while the unfit individual die out without reproducing. The nature selects the organisms which are provided with favourable variation and these are fittest to survive, hence, the theory is known as natural selection.
Origin of new species : As a result of struggle for existence, variability and inheritance, the organisms tend to become better adapted to their environment. Nature selects organisms with favourable variations and survive. The favourable characters are transmitted to the next generation. In the succeeding generations also natural selection operates. Hence, in each and every generation, there is slight betterment of the already developed characters.
These different forms of organisms are identified as new species. Thus, according to Darwin, a new species originates by the gradual accumulation of favourable variations for a number of generations.
Objections to Darwin's Natural Selection Theory :
(i) Natural selection theory explained "Survival of the fittest" but not "arrival of the fittest."
(ii) He was not clearly aware of hereditary principles.
(iii) Certain useless characters like vestigeal organs are also inherited. He cannot explain the inheritance useless variations.
(iv) He was unable to differentiate variations as hereditary and environmental variations.
