Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12
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Theories of Biological Evolution - Darwin’s Theory of Natural Selection

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Topics

description

  • Darwinism or Theory of Natural Selection
  • Objections to Darwinism
  • Neo Darwinism

notes

Darwinian Theory of Evolution:

  • Charles Robert Darwin was born on 12th Feb. 1809 in England.  
     
  • Darwin travelled by H.M.S. Beagle ship, which left on 27 Dec. 1831 and returned on 02 Oct. 1836 through S. America, S. Africa, Australia & Galapagos Islands.  
  • Darwin was influenced by two books -
    (i) "Principles of population" of Malthus
    (ii) "Principles of geology" of Charles Lyell  
  • Alfred Wallace, a naturalist who worked in Malay Archipelago had also come to similar conclusions around the same time and he sent his conclusions to Darwin in form of a chart. 
  • This theory was later on explained by Darwin in his book ‘On the origin of species by means of Natural Selection’ (1859). 

Basic concepts of Darwinism: 

Branching Descent and Natural Selection are the two key concepts of Darwinian Theory of evolution. Natural selection is based on certain observations which are factual.  

(i) Over production:   

  • All organisms have the capability to produce an enormous number of offspring or organisms (multiply in geometric ratio). 
  • Hence, theoretically population size will grow exponentially if everybody reproduced maximally (this fact can be seen in a growing bacterial population) but the fact is that population sizes, in reality, are limited. 

(ii) Struggle for existence: 

  • Natural resources are limited and populations are stable in size (except for seasonal fluctuation) means that there had been competition for resources. Only some survived and grew at the cost of others that could not flourish. This is called the struggle for existence. 
  • It is of three types -  
    (a) Intra-specific struggle: It is competition among the individuals of the same species for the same needs like food, shelter, and breeding. (Most acute type of struggle)  
    (b) Inter-specific struggle: It is the struggle among the individuals of different species for food and shelter. It is the most potent force for organic evolution.  
    (c) Environmental struggle: This struggle is between the organisms and their environment. All organisms struggle with cold, heat, wind, rain, drought, flood, etc.  

(iii) Variations and heredity:  

  • Members of a population vary in characteristics (in fact no two individuals are alike) even though they look superficially similar i.e., the population has built-in variation in characteristics. 
  •  Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc.) are called adaptive or useful variations while others are called as non - adaptive or harmful variations.  
  • The novelty and brilliant insight of Darwin was, he asserted that variations, which are heritable and which make resource utilization better for few (adapted to habitat better) will enable only those to reproduce and leave more progeny.  

(iv) Natural selection/ Survival of the fittest: 

  •  Individuals with more adaptive variations are "better fit" than individuals with less adaptive variations. Hence, those who are better fit in an environment would be selected by nature and leave more progeny than others. Darwin called it natural selection and implied it as a mechanism of evolution. 
  • Fitness is the end result of the ability to adapt and get selected by nature. 
  • The fitness, according to Darwin, refers ultimately and only to reproductive fitness. 
  • It is observed that all adult individuals of a population don't have equal chances of mating; some males with better phenotypes are preferred by females. This is called Sexual selection. 

(v) Origin of New species:  

  • As a result of heritable variations and natural selection, there would be a change in population characteristic and hence new forms appears to arise. 
  • Theory of Pangenesis-According to this theory all organs of an individual produce Pangenes, which are minute particles carrying information about the organs. The pangenes travelling through the bloodstream will ultimately reach the gametes so that each gamete will have pangenes for each of the different organs. After zygote formation, the pangenes tend to form the same organs from which these pangenes were produced.  

Criticism of Darwinism: 

  1. The main drawback of this theory is that Darwin didn't have the knowledge of genetics and he had no satisfactory explanation for the cause, origin and inheritance of variations. 
  2. This theory only explained the survival of fittest but was unable to explain the arrival of the fittest. 
  3. Darwin was unable to explain why in a population only a few individuals develop useful variations and others have harmful variations. 
  4. Criticism of Darwinism was based on sexual selection. Why do only females have the right of selection for mating? 
  5. Darwin couldn't explain the existence of vestigial organs. 6. Darwin was unable to differentiate the somatic and germinal variations.
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