Complete information on the Lamarckian theory of evolution as well as on Neo-Lamrckism

Lamarck was a French biologist. He was the first biologist to explain a definite theory of evolution. He was also first man to coin the term biology. He focused on the phylogenetic tree or tree of life to show the phylogenetic relationship among the organisms. The Lamarckian theory of evolution or inheritance of acquired characters is called Lamarckism. He published this theory in his book “Philosophie Zoologique” in 1809. According to his theory, the mechanism of evolution is controlled by following factors:

1. Influence of Environment:

The plants and animals in the past adapted themselves to the changed environmental condition. This resulted to give a new or degenerated structure.

Use and Disuse of Organs

According to this theory, when a particular organ is constantly used, it develops well. At the same time when an organ is not used for a long time, unit gets reduced and in due course it degenerates and disappears completely from the organism.

Example This of Use of Theory:

(i) Webbed Foot:

The web-footed aquatic birds probably evolved from the non web-footed birds. When non-web foot edaquatic birds used their toes propelling water from generation to generation, webs were formed between toes.

(ii) Bicep Muscles:

The bicep muscles of blacksmith are well developed due to constant hammering on metals. This changed structure is inherited as Lamarckian theory.

(iii) Giraffe:

According to Lamarck, the ancestral giraffes had short necks and the length of fore and hind limbs were uniform. The giraffes were grazing animals feeding on grass. At that time there was scarcity of grass. Therefore, the ancestral giraffes stretched their necks to reach the foliage of trees. This resulted slightly increase in the length of the neck. This changed structure is transmitted to next generation. The cumulative effect of changed character, generation after generation, gave rise the modern giraffe.

Examples of Disuse Theory:

(i) Eyes of Cave Animals:

In cave dwelling like Proteus, the eyes are not used for many generations. Hence, the eyes are degenerated.

(ii) Fightlessness in Birds:

The kiwi was flying bird in New Zealand. At the time there were no enemies on the land in the New Zealand. Hence, they did not attempt to fly. This happened generation after generation resulting degeneration of wings and loss flight.

(iii) Vestigial Organs:

The vestigial organs of man and other animals are the result of continuous disuse, e.g. coccyx, ear muscles of man.

(iv) Limbs of Snakes:

In the past snakes had limbs like lizards. They lost their limbs as they lived in burrows and thick bushes of jungles. These useless organs are gradually degenerated.

Inheritances of acquired characters- when the environment changes the new adaptive characters develop. This is called acquired character. The acquired characters are transmitted to off springs. This is called inheritance of acquired characters, e.g. the long neck of giraffe is an acquired character which is transmitted to the off springs generation after generation.

Criticism:

(1) The holes in the nose and ear of Indian woman are not inherited.

(2) The bicep muscles of blacksmith are not inherited.

(3) Lamarck did not explain the origin of new characters.

Neo-Lamarckism:

The modified Lamarckian theory in the light of recent finding is called Neo- Lamarckism. Many biologists like Mc Dougall, Cope, Giad, Nageli, etc. contributed to the idea of Neo- Lamarckism. They are called Neo- Lamarckians.

Experimental Evidence:

Mc Dougall’ s experiment:

Mc Dougall conducted learning experiments on rats and proved that offspring could learn quickly than parents and this learning habit is inherited generation after generation.

Tower’s Experiment:

When the potato beetles are exposed to extreme conditions of moisture and temperature, some modifications are seen. These modifications are inherited.

Agar’s Experiment:

The abnormal shape of carapace of water flea is inherited.