LAMARCK’S THEORY OF ORGANIC EVOLUTION

Jean Baptiste Lamarck, a French biologist was one of the first biologists to put forward the theory of organic evolution in 1801. His idea of organic evolution was hinged on three theories.

1) The theory of need

This states that the development of a new part or organ by an organism (plant or animal) results from the need of that part or organ to the organism. For example, the early ancestors of snake had short bodies and legs. As the land was changing during its formation, many narrow places and dense vegetation were formed. For snakes to walk through narrow places and dense vegetation they started stretching their bodies to become elongated so that they could easily crawl through the narrow places and dense vegetation instead of walking which would be rather difficult.

 

2) The theory of use and disuse

This theory states that organ become well developed and active when being used but slowly degenerate, become functionless or disappear with disuse. For example, each generation of snakes continued to stretch their bodies resulting in the strengthening of their bodies. Their legs became useless and strengthless and finally disappeared since they would hinder crawling through narrow places and dense vegetation.

 

3) Theory of acquired characteristics by inheritance

This theory states that all characteristics (structures) or variations acquired by organisms during their life time are transmitted to the next generation by inheritance. For example, the snakes of today inherited their legless and elongated bodies from the ancestors that had lost the legs and short bodies through disuse.

The modern giraffe, Lamarck believed, evolved from a giraffe with short neck and short legs. When competitions for low grasses among the herbivorous animals became too keen, the short-necked and short-legged giraffe started to stretch its neck and forelegs so as to reach the leaves of trees. These characteristics of long neck and long forelegs were inherited by subsequent generations including the modern giraffe.

Some other beliefs of Lamarck were the development of webs between the toes of ducks and frogs. Lamarck believed that ducks and frog originally had no webs between their toes but as they continued to visit water frequently for food, therefore the need to develop webs for swimming became as necessity. The toes and the surrounding skin became stretched and more blood was supplied to the toes leading to the formation of webs for swimming. The webbed toes were then inherited by their offspring.

 

Testing Lamarck theory of evolution

Not all scientists at that time supported the Lamarck theory. Many biologists did carry out experiments to disapprove Lamarck theory of acquired inheritance. The most credible work was that of August Weissmann, a German zoologist. He wanted to investigate whether acquired characteristics (traits) were inherited. In his experiment, Weissmann, cut off the tails of mice (males and females) and allowed them to mate. The offspring of all the tailess mice had normal tails. He again cut off tails of the second generation and allowed them to mate again and the offspring of the second generation of the tailess mice had normal tails. Weissmann carried out his experiment for 21 generations. The offspring of the 21st generation had normal tails as the first generation. Non of the offspring had a cut tail throughout the 21 generations.

Weissmann then explained that the body of a living organism consists of two parts namely: somatoplasm (somatic part) and germplasm (reproductive part). The germplasm is developed in special organs which produces reproductive cells that bring about variations in organisms that are inherited by offspring. Germplasm is subject to no destruction or alteration and is outside the influence of the environmental factors.

The somatoplasm is responsible for all production of all the somatic (body) cells of an organism. The somatic cells are influenced by environmental factors resulting into acquired variations which are never inherited by offspring since such variations are never transferred to germplasm for inheritance. Therefore, Lamarck theory of acquired inheritance was set outside.

However, Lamarck theory of use and disuse is still plausible. Gradual changes take place from time to time due to use and disuse. For example, a weight lifter develops strong hand and chest muscles due to use and these muscles become weak and thin if put in cast of POP (Plaster of Paris) when there is bone fracture, thus preventing the use of the hand during the healing period. The wings of ostrich are small and weak and cannot be used for flight due to disuse.