Theories of Biological Evolution - Lamarck’s Theory of Evolution

Lamarck’s theory (Theory of inheritance of acquired characters):

• The first logical theory of evolution was proposed by French naturalist Jean Baptiste de Lamarck (1744-1829).
• Book: Philosophie Zoologique (1809)
  

Basic concepts of Lamarckism: 

1. Internal vital forces: Due to the presence of some internal vital forces all organisms have the tendency to increase in the size of their organs or entire body.  
   
2. Effect of environment and new needs: Environment influences all types of organisms. Changing the environment gives rise to new needs. New needs or desires produce new structures (doctrine of desire/ appetency) and change the habits of the organism.  
   
3. Use and disuse of organs: If an organ is constantly used over generations, it would be better developed whereas disuse of organ results in its degeneration (vestigial organs). 
   
4. Inheritance of acquired characters: During the lifetime of an organism, new characters develop due to internal vital forces, effects of the environment, new needs or use, and disuse of organs. All these acquired characters are inherited from one generation to another. By continuous inheritance through several generations, the variations are accumulated up to the such an extent that they can give rise to new species. 

Examples:  

1. Long neck and forelimbs of giraffe: Lamarck gave the example of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly over the years came to acquire long necks.  
   
   
2. Aquatic birds stretched their toes and developed the web.  
3. Snakes lost their legs, Lamarck had said that the evolution of life forms had occurred but was driven by the use and disuse of organs. Nobody believes this conjecture anymore. 

Lamarck had said that the evolution of life forms had occurred but was driven by the use and disuse of organs. Nobody believes this conjecture anymore. 

Criticism of Lamarckism:

• The first concept states that organisms tend to grow in size, but this is not universally true. For example, trees appear to be primitive among angiosperms, and shrubs, herbs, and grasses have evolved from trees whose size was reduced during evolution.
• The second concept is incorrect. Can we grow wings and fly like birds? 
• The third concept is somewhat true, such as blacksmiths' well-developed bicep muscles and flightless birds' less developed wings. However, there are numerous objections to this concept, such as the fact that the eyes of a student/reader do not increase in size and power with age, and that the constantly beating heart maintains a constant size across generations. 
• The fourth concept is incorrect because acquired characteristics are not inherited. 

Weismann's Theory of Continuity of Germplasm:  

• Weismann cut off the tails of rats for as many as 22 generations and allowed them to breed, but tailless or reduced tailed rats were never born. 
• On the basis of this experiment, Weismann proposed the theory of continuity of germplasm.
• According to this theory - Two types of protoplasms are present in an organism, germplasm, and somatoplasm. There is a continuity of germplasm and the variations influencing the germ cells are only inherited but the somatoplasm is not transmitted to the next generation, hence it does not carry variations to the next generation.  
• Boring of ear pinna and nose in Indian women is never inherited to the next generations. 
• Chinese women used to wear iron shoes in order to have small feet, but they still have normal feet.  

Neo-Lamarckism: 

• According to Neo Lamarckism, "Changing environment may create some physical and chemical changes in somatoplasm of organisms, which may affect their germplasm also and such acquired characters can inherit." Many experiments were done to support their theory of Lamarck which are not convincing and satisfactory.