Lamark theory of evolution

Achievements of Lamark

Lamarkisim Jean Baptiste de 

Lamarck Jean Baptiste de Lamarck was born in France on 1744. He presented the idea of evolution in 1809 in his book "philosophie zoologiqe. "

 

Lamarck Jean Baptiste de

 





His ideas about evolution were called as Lamarckism. 

Important Points of Lamarckism 

There were two important points of his theory. 

Use and Disuse of Organs: 

According to Lamarckism, in organisms certain changes are developed by use and disuse of body parts due to biological needs arising out of environmental changes. The organs greatly used develop in its size and organs disused tend to become weaker and can eventually disappear. This statement of Lamarck, that use or disuse can produce changes in .organs seems correct, e.g. the development of muscles in athletes and development of calf muscles in ladies who wear high heals. 

Inheritance of Acquired Characters: 

 Variations in phenotype due to use and disuse of organs are called acquired characters because these characters are developed during the life-time of an individual. Lamarck believed that these characters are heritable and are passed on to the offspring from their parents during reproduction. 

Giraffe's neck 

Lamarckism can be illustrated by an example of giraffe's long neck. Lamarck assumed that long necked giraffes evolved from short necked ancestors.
 He explained the Lamarckism in giraffes as follows: 
The short necked giraffe could graze on grasses. When the grass became scanty the giraffes tried to stretch their necks to reach the top of the trees to feed on leaves. As a result of these efforts the giraffe's necks became longer.The same trend continued and the neck went on becoming longer and longer. The giraffes with long necks were able to survive in such an environment.
Lamarck believed that acquired characters of long neck in giraffes were heritable and were passed on the next generation. 

Disuse of Organs:

 Lamarckism can also be described with the help of an example of snakes. Ancestors of modern snakes had four legs with which they used to move on land. In order to escape from enemies the snakes started hiding in holes and Creeping in narrow cracks in rocks. In this new mode of life, the legs were not used but muscles of their body were-used more for creeping. As a result of disuse of organ, legs were completely disappeared from the body and the snakes became elongated cylindrical in shape. This is how present day snakes have evolved. 

Objection of August Weisemann on Lamarckism:

 To prove the hypothesis of Lamarck August Weisemann in 1870, cut off tails of mice over many successive generations. In each generation the mice were born with the normal tails. 
According to Lamarckism, the acquired characters would be inherited. But this was not the case. 
Weisemann postulated that "somatic acquired characteristics resulting in phenotypic changes did not directly ’affect the germ cells (gametes) by which characteristics are passed on to other species." The variants that are better adapted to the environment are successful and continue to produce offspring with their adaptive characteristics.
 Over time the characteristics which provide adaptiveness or fitness come to accumulate in the population than those characteristics which decrease fitness.
 The greater adaptive success of the better adapted form constitute the natural selection. He means to say that better adaptive individuals are favoured by nature, to survive and reproduce as compared to individuals which are not fit for that environment.