05 Origin and Evolution of life - part 05 - Darwinism

05 Origin and Evolution of life - part 05 - Darwinism

Darwinism : 

• Before darwinism, several theories were proposed to explain the process organic evolution. Few of them are explained below : 
• Lamarck (1809) published theory of origin of acquired characters. Which was then ruled out.

Darwnism (theory of origin of species by Natural Selection):

• Charles Darwin as a naturalist travelled extensively the world over from 27th December 1831 to October 1836 and returned to England. 
• Based on his collections of living and fossil material he published a book ``The origin of species by Natural Selection'' in 1859. 
• Before publishing his book, he was very much influenced by C. Lyell (Geologist) who pointed out that the natural forces that existed in the past are same as those existing at present. 
• Darwin observed that the natural forces resulted in the changes in the earth's crust along South America. 
• He observed variety of strange plants and animal in Galapagos islands and other nearby islands in the Archipealago in terms of variations between the tortoises and finches. 
• Similar observations were also made by Wallace. 
• He was also influenced by R. Malthus, an Economist, who worked on human populationmentioning that the reproductive potential of humans is greater than others. 
• This resulted in multiplication of human population without corresponding increase in the food supply. 
• This increase lead to competition and struggle for existence of human species.
• He was also influenced by plant/animal breeders who by using artificial selection, altered the characteristic of cultivated plant and domestic animals. 

Darwinism is based on five main postulates: 

1. Overproduction (Prodigality of nature) - 

• It is the natural tendency to produce more number of progeny in geometric Ratio, for perpetuation of the species. 
• He observed prodigality potential many species of plants and animals e.g. Salmon fish produces about 28 lakh eggs in a single season. 
• In a span of 750 years single pair of elephants would produce 19,000,000 elephants. 
• However the size of given species in a given area remains relatively constant because of fluctuations that occur seasonally. 

2. Struggle for existence - 

• Tendency of over production leads to the struggle for existence between the members of population for - 

1. Limited supply of food or 
2. To overcome adverse environmental conditions or 
3. For a space or to escape from enemies etc. 

3. Organic variations - 

• The variations speak for all kinds of differences that occur in morphology, physiology, nutrition, habit behavioural patterns etc. 
• Darwin recognized these variations as raw material for evolution. 
• Variations were observed among members of the same species and even in different species. 

4. Natural selection - 

• Organic variations can serve as evidence that for the some organisms have better adapted to survive under existing environmental conditions than the others. 
• In the struggle for existence organisms with favourable variations are selected by the nature while those with unfavourable variations perish. 
• According to Darwin, the principle by which useful variations are preserved by nature, is called `Natural Selection'. 
• H. Spencer named this process as `survival of fittest'. 

5. Origin of new species (speciation) - 

• As favourable variations are transmitted from generation to generation, successive generations become better adapted to environment. 
• Gradually these adaptation with few new modification become fixed in the life cycle and finally giving rise to a new species. 
• Evidences Darwinism include - 

(i) Evolution of long-necked Giraffe to pluck and eat more leaves from tall trees and woody climbers. This adaptation became fixed in the life for survival. The Giraffe borne tall could survive in famine heat areas. This adaptation was transmitted to their offspring. This is how, present long-necked Giraffe came to existence.
(ii) Black colour peppered moths evolved gradually as new species.
(iii) DDT resistance in mosquitoes-intensive DDT spraying destroyed all types of mosquitoes. However some mosquitoes developed resistance to DDT and survived the on slaught of DDT spray. Such resistant mosquitoes survived and reproduce giving rise to more resistant offspring.

Drawbacks and Objections to Darwnism 

1. He considered minute fluctuating variation as principal factors which are not heritable and not part of evolution. 
2. Did not distinguish somatic and germinal variation and considered all variations are heritable. 
3. Not explain the `arrival of the fittest'. 
4. Did not explain the cause, origin and inheritance of variations and of vestigial organs, nor he could explain the extinction of species.
5. According to natural selection new species are formed by gradual accumulation of useful variations. If it is so, then their should be intermediate forms. But in most cases intermediate form were not recognised. 
6. Moreover, Darwinism also could not explain existence of neutral flowers and the sterility of hybrids.