Evolution means opening out or unfolding. It explains the formation of non living things evolution-of-man and living things.
The term organic evolution means continuous change or descent with modification or the developmental stages of an organism repeats the developmental stages of ancestors.
The organic evolution can be defined as the formation of complex organism from simple organism over a period of time. The raw materials for evolution are variations
THEORIES OF ORGANIC EVOLUTION
Many scientists proposed different theories to explain organic evolution. Lamarck, Darwin and Hugo devries tried their level best to explain organic evolution. Later scientists used the genetics to explain organic evolution. The different theories proposed by evolutionists are Lamarckism, Natural selection and Mutation theory.
The principles of Lamarck are
1. Influence of the environment
2. Use and disuse of organs.
3. Inheritance of acquired characters.
INFLUENCE OF THE ENVIRONMENT
The environmental factors are sunlight, temperature, humidity etc. These factors have influence on living organisms. Influence of the environment states that when the organisms of same species were living in different environments, they develop changes to suit their environment. The changes developed in the life time are called acquired characters.
Example:If we observe the sheep living in tropical (warmer) region have less hair. Where as the body of polar-sheep-wool polar sheep is covered by dense thick hair. The variation is mainly due to the difference in the environments. The dense thick hair on the body of polar sheep is mainly to protect from severe cold. The above example tells us that the thick hair on polar sheep is the acquired character
USE AND DISUSE Principle
It states that the organs which are used continuously, they develop better.
The ancestors of Giraffe were short necked antelopes. The ancestors stretched their neck continuously to grab the leaves. Hence neck length was gradually increased in every generation.
The ancestors of Duck were terrestrial birds. The terrestrial birds entered the water and continuously used the digits of hind limbs. Hence web was gradually formed.
B. Disuse principle:
It states that the organs which are not continuously used they gradually decrease in size, first become vestigeal and finally disappear.
The ancestors of Ratitae (flight less) birds were flying birds. As the flying birds did not use the wings, the wings gradually decrease in size and finally became vestigial . flightless-birds-ratitae
The ancestors of snakes were limbed lizards. As the limbed lizards did not use the limbs continuously, the limbs gradually decrease in size, first become vestigeal and finally disappeared.
The use and disuse of organs in animals lead to the formation of acquired characters.
ll. INHERITANCE OF ACQUIRED CHARACTERS Inheritance is the transmission of characters from one generation to successive generations. Life time developed characters are called acquired characters. These are developed due to influence
of the environment and use and disuse of organs. Inheritance of acquired characters states that the life time developed characters are transmitted to the progeny. Lamarck cited many examples to explain the inheritance of acquired characters.
A. Evolution of Giraffe:
The ancestors, short necked antelopes of Giraffe used the neck continuously to grab the food. Hence small increase in the neck length was occurred in every generation. This increase is called acquired character.
The acquired character is transmitted to the next generation. As the neck was used continuously, neck length was increased in every generation and the increased neck was transmitted to every generation, this lead to the formation of long necked Giraffe.
B. Evolution of Snakes:
The ancestors, limbed lizards of snakes continuously used the body and did not use the limbs. Hence in every generation body length was increased and limbs size was decreased. As the use of body and disuse of limbs occurred in many generations, body length was increased and limbs were disappeared. This lead to the formation of snakes.
C. Evolution of Ratitae Birds:
The ancestors of flight less birds (Ratitae)were flying birds. If we observe the distribution of flight less birds, they live in areas where there were no enmies. Hence flying birds did not use the wings. The wings gradually decreased in size and became vestigeal. This lead to the formation of Ratitae birds.These birds evidence evolution
Lamarck’s third principle, inheritance of acquired characters was highly criticised. But the remaining two principles (use and disuse of organs,influence of the environment) were accepted by many scientists.
Women wear Jewellary to the nose and earlobe. Gold smith bores (making holes) the nose and earlobe for wearing. Presence of holes in the nose and ear lobes is the acquired character. But the children of such women did not have hales.
A sports man develops well developed muscular body due to regular exercises. But his children will not inherit this acquired character.
A person may lose a limb in an accident, but his children are not limbless.
It was conducted by Augst Weismann to disprove the lamarck’s inheritance of acuired characters. He collected two rats and cut their tails. Weismann crossed such tail less (acquired character) rats and observed the progeny. He got the tailed progeny. He repeated the experiment (cutting of tails, crossing and observation of progeny) upto 22 generations. Weismann did not get not even a single rat with out tail. This showed him that the acquired character tail lessness is not transmitted to the progeny. So, the weismann is considered as a critic of Lamarck.
The above experiment resulted the weismann to propose Germplasrn theory.
It was proposed by weismann. The body of every individual contains two plasms. They are wisemen-germplasm-theory somatoplasm and germplasm. Somatoplasm is found in all organs except gonads. Germ plasm is found only in gonads. Germplasm theory states that the changes of germplasm are only transmitted to the progeny. But somatoplasmic changes like tail less ness, holes in the nose are never transmitted to the progeny.
It was conducted by Castle and Philips on Guinea pig. They transplanted the ovaries of homozygous black Guinea pig to the white female. Then Castle and Philips crossed such female white Guinea pig with white male. They got heterozygous black progeny. This indicates that the germplasm change of white female is transmitted to the progeny.
Even though the lamarckism was highly criticised, still some scientists belive lamarckism. They are called neo lamarckists. They are paul kammerer, Guyer, Smith and Mc Dugoll.
continue ---Natural selection