Theories of organic evolution:
Organic evolution refers to the progression of life forms from simpler to more complex as a result of environmental changes and other factors. Evolution describes changes in the shape and behavior of organisms as they pass down through generations. It refers to the changes that occur within a population’s ancestry from generation to generation. Several hypotheses have been proposed to support and explain the evolutionary process. Several ideas have been proposed to support and explain the evolutionary process. A few of these sources are listed below:
- Lamarck, Evolution, and the Inheritance of Acquired Characters
- Darwinism or theory of Natural Selection
- Neo Darwinism or Modern synthetic Theory of evolution
Introduction:
]Jean Baptiste de Lamarck, a French biologist, proposed the first hypothesis of evolution in 1801. Lamarck published his ideas of evolution I famous journal “philosophie Zoologique” in 1809. His theory is popularly known as theory of inheritance of acquired characters or theory of use and disuse of organ.
Facts of Lamarckism:
- Tendency to grow: each and every organism has natural tendency to grow. The internal forces of life tend to cause an organism to become larger. This increase in size up to a certain limit which is determined by the life itself.
- Formation of new organs: The formation of a new organ or a part of the body is the result of the need or want to which has arisen and continues to be felt by the organisms.
- Use and disuse of organs and structures: The efficiency and development of an organ or system is directly proportional to its use i.e., continue use of organ makes them functional and strong and thus leads to its better development, which continuous disuse makes the organs undeveloped, weakened and finally disappeared.
- Inheritance of acquired characters: Generation preserves all structural changes that an organism has acquired across its lifespan as a result of direct or indirect environmental impacts and is transmitted to its offspring’s. Even in the offspring, these modifications become pronounced depending upon continuous use or disuses of the organ or systems. After certain generations the offspring produced become totally different from the original parent. The organism with new characteristics, therefore, established a new species.
Example:
- Giraffe: In support of his views, Lamarck put forward the classical example of giraffe. The long-necked and long-limbed giraffe developed from a short-necked and short-limbed giraffe, similar to the goat. Due to shortage of grasses, short necked and short limbed giraffe feeding on leaves on long trees. The continuous use of neck and limb made the giraffe long necked and long-limbed giraffe of today. According to this theory, this increase in length of neck and forelimbs was an acquired character and it was passed on to the offspring. This process persisted for generations, finally giving rise to the modern giraffe, which has long necks and forelegs. This example demonstrates the impact of regular use of organs.
- Snakes: The limbs are absent in snakes due to effect of disuse of organ. There is no use of such organs due to creeping habits and lived in burrows. The snakes were originally four-legged animals.
- Horse: The ancestors of modern horse left the soft ground of jungles and took to live on dry land. The change in habit was accompanied by change in length a structure of legs which were suited for fast running over hard ground.
Criticism of Lamarckism:
- Mutilation is not inherited. If the limb or finger is mutilated or cut it doesn’t disappear in offspring.
- Eyes that are used continually and constantly develop defects instead of being improved.
- Boring of ears and nose in women has seen continued as a tradition form century but their progeny does not show any trace of holes in ears and nose.
- The deadliest blow to Lamarck’s theory came from Weismann. He cut the tails of mice continuously for about 20 generation but size of tall was same as of the original parents.