Theories of Evolution (Part-I)

Introduction: In continuation our discussion on evolution, so far we have discussed origin of life and the evidences to support the existence of evolution to give rise diversified organisms on earth. The mechanism of evolution is not known and several theories have been put forward to explain the process of evolution. These theories are as follows:  

1. Theory of inheritance of acquired characters:  This theory is proposed by French biologist Chevalier de lemarck in his famous book “Philosphie zoologique”. He proposed that organisms are not fixed and they evolve from the pre-exisiting organisms by modifications. The theory is proposed assuming three different assumptions:

• New Needs: Variation in environment, conditions and the over-all circumstance which affect the existence of organism, needs adaptation in organism to survive. As a result, organism has to put special efforts to fulfil its new needs for adaptation. In few cases, it just needs change in habit or behavior of the organism. New habit includes fresh or extensive use of certain organs or structure of the body or disuse of others.

 

• Acquisition of Characters: There are 3 ways to acquire the characters required to adopt into changed conditions:

Innate Tendency: There is innate tendency in each organism to acquire greater complexicity and perfection to perform functions. In this process of achieving perfection, organism is better and better adapted to the changed environment.


Use or disuse of organs: Use and disuse of organ affect their structure, shape and efficiency of functioning. More usage of a particular organ brings additional strength, size and more efficient. In contrast, disuse or underusage of organ gradually makes them weaker and smaller and finally they may disappear as well. Thus, differential usage of organ allowed the additional character in the body during the life span of an organism.


Environmental factors: Variation in environmental factors such as temperature, light, humidity, wind, ememies affects the living things and brings changes in life style and habits. The combined effect of use and disuse of organ and influence of environmental factors, results into the change in the body of organism and these characters are known as “acquired characters”.