Darwinians Theory of Evolution

Charles Darwin, an English naturalist, did not believe in Lamarckism and in 1858 proposed a new concept of evolution in his book Origin of Species by Natural Selection. He made a voyage (sea journey) for 5 years by traveling on HMS Beagle with a view to studying diversity in world flora and fauna. Charles Darwin visited many islands including Galapagos and found great diversity among the organisms growing over there. Later on he concluded that evolution of few species is governed by following chief factors:

Over – Production of Offsprings

Darwinians Theory of Evolution marked that every living organism wants to produce more and more progeny for the continuity of its race. An oyster produces 60-70 millions eggs in a year. A housefly lays 120 eggs, six to seven times in a summer. Similarly the slowest breeder, elephant, can produce about 19x106 elephants out of a single pair within a period of 750 years, if all the young ones survive. But it does not happen.

Struggle for Existence

The over production results in competition within the individual as the space and food supply are limited according to Darwinians Theory of Evolution.

Variation and Heredity

Darwinians Theory of Evolution says that the everlasting competition compels the organisms to change their habitual characters with a view to utilizing maximum natural resources. The changes are called variation. Evolution is not possible without variations. These variations can be: Meristic, Sustantive, Continuous, Discontinuous, Somatogenic and Blastogenic.

Meristic: Numerical variations in body organs e.g., a six fingered man.

Sustantive: Qualitative variation e.g., in shape, size or colour.

Continuous: Small changes can be inherited.

Discontinuous: Discrete changes (now called mutations) and are heritable.

Somatogenic: Characters acquired due to environment (e.g. change of complexion in tropics), these are not inherited.

Blastogenic: These are the germinal variations occurring in germ cells and are heritable.

Natural Selection (Survival of the Fittest)

According to Darwinians Theory of Evolution, the harmful and even neutral variations are eliminated. The variations which are useful (fit) for the environment helps the organisms to survive.

Origin of Species

Darwinians Theory of Evolution suggested that the useful variations which can be transmitted to succeeding generations (heritable) will give rise to new species.

Criticism

Darwinians Theory of Evolution could not distinguish between blastogenic and somatogenic as well as heritable and non-heritable variations. It also failed to explain the mechanism of origin of variations and their transmission in progenies. Natural selection can not be the sole cause of evolution, because it simply preserves the favorable variations but does not create them. Darwin proposed the theory of pangenesis which stated that every somatic cell is a germ cell and can produce gemmules (pangenes) capable of regulating the characters.

Note: The word survival of the fittest coined by Herbert Spencer and Darwinians Theory of Evolution termed as Natural Selections.

Neo-Darwinism (Modern Synthetic Theory)

The modified form of Darwinians Theory of Evolution of natural selection is called Neo-Darwinism. The Neo-Darwinism includes Haeckel, Wallace, Huxley, Weismann and infact all the workers (including Mendel). The ideas of above scientists have been synthesized by G.L. Stebbins (1970) in the form of a modern theory. This states that evolution of a new species is the result of combination (synthesis) of forces like mutation, variation, heredity, natural selection and isolation. It has been concluded that: The prime factors causing variations are Gene mutation, Chromosomal aberrations, Polyploidy, etc. In sexually reproducing organisms, crossing over at the Meiosis leads to random assortment (independent assortment of Mendel of Genes leading to variations in progeny). The nature selects the fittest ones, i.e. those having better survival value. According to M. Wagner, the selected individuals get separated (drifted or isolated) from the related species under the influence of structural, physiological, seasonal or geographical barriers, imposed by the nature. The isolated individuals fail to interbreed with the main population i.e. become reproductively isolated (Dobzhansky) and lead to the evolution of a new species.

Patterns of Evolution

The process of organic evolution may proceed along the following lines:

Parallel Evolution: Where closely related organisms independently undergo similar changes in similar environmental conditions.

Convergent Evolution: Where more distantly related organisms undergo similar changes in response to identical environmental conditions, e.g. a fish, a reptile and a mammal (dolphin), all similarly adopted for aquatic life.

Progressive Evolution: Where simple organisms, through elaboration of parts give rise to complex ones.

Retrogressive Evolution: Reverse of progressive.