In this the inheritance of acquired characteristics theory post we have briefly explained about acquisition of characters, explanation of theory of inheritance of acquired characteristics, criticism of the theory of inheritance of acquired characteristics, and neo-lamarckism.
Theory of Inheritance of Acquired Characteristics
This theory of inheritance of acquired characteristics is proposed by French biologist Chevalier de lemarck in his famous book “Philosphie zoologique”. He proposed that organisms are not fixed and they evolve from the pre-existing organisms by modifications. The theory of inheritance of acquired characteristics proposed assuming three different assumptions.
Variation in environment, conditions and the over-all circumstance which affect the existence of organism, needs adaptation in organism to survive. As a result, organism has to put special efforts to fulfil its new needs for adaptation. In few cases, it just needs change in habit or behaviour of the organism. New habit includes fresh or extensive use of certain organs or structure of the body or disuse of others.
Acquisition of Characters
Acquisition of Characters: There are 3 ways to acquire the characters required to adopt into changed conditions:
Innate Tendency: There is innate tendency in each organism to acquire greater complexity and perfection to perform functions. In this process of achieving perfection, organism is better and better adapted to the changed environment.
Use or disuse of organs: Use and disuse of organ affect their structure, shape and efficiency of functioning. More usage of a particular organ brings additional strength, size and more efficient. In contrast, disuse or under usage of organ gradually makes them weaker and smaller and finally they may disappear as well. Thus, differential usage of organ allowed the additional character in the body during the life span of an organism.
Environmental factors: Variation in environmental factors such as temperature, light, humidity, wind, enemies affects the living things and brings changes in life style and habits. The combined effect of use and disuse of organ and influence of environmental factors, results into the change in the body of organism and these characters are known as “acquired characters”.
Inheritance of Acquired Characters: The characters acquired by one generation are transmitted to the new generation and subsequently newer characters are added in next generation to acquire perfection.
Explanation of theory
Lemarck in his book explained the evolution of various animals to elaborate the proposed hypothesis.
1. Giraffe: Lemarck uses his hypothesis to explain the appearance of Giraffe with long neck and forelimbs. As per the theory of inheritance of acquired characteristics, the giraffe is evolved from the short height deer like ancestor. These ancestors are living in the barren place with leaves on the trees available to them for eating. In order to reach the leaves on tree, it stretched its neck and forelimbs. As a result, these organs get elongated. This acquired character in first generation passed on to the subsequent generation and continuous stretching accumulates this character over the course of few generations to evolve giraffe with long neck and forearms.
Theory of Inheritance of Acquired Characteristics: Event in the evolution of giraffe from deer link ancestor
2. Other examples: Other examples of use/disuse to support the Lamarck theory of inheritance of acquired characteristics are black smith and rabbit. Blacksmith acquires large biceps muscle as they do rigorous hammering for wielding. Whereas, rabbit develop well developed pinna muscle to move the ear to receive sound waves from different direction to protect them selve from the enemy.
3. Aquatic Birds: Aquatic birds such as ducks need to go into the water for food and protection. To achieve this, they spread their toes to float on water. As a result of continues efforts, they have developed web between the web.
4. Snakes: it is considered that snake had lizard like ancestor with two pairs of fully developed legs. These ancestor needs to hide from the mammals and they prefer to stay in places with dense vegetation, holes and narrow spaces. In this process, they stretch their body to hide in a narrow space and did not use legs. Over the course of time, continuous stretching of the body made it cylindrical and they lost the legs.
5. Deer: It is believed that deer has acquired speed through continuous running in a process to protect himself from the enemies.
6. Cave animals: cave animals stay in low light environment and does not use eye and as a result they lost their vision over the course of time.
Criticism of the theory
Lamarck theory of inheritance of acquired characteristics received initial attention but it could not be able to explain several observations. The initial two assumptions are correct; (1) New needs are created by a change in the environment and (2) these characters are acquired by use/disuse of organ and environmental factors. The inheritance of acquired characters to the subsequent generation is arguable. There are evidences against the inheritance of acquired characters.
The major critism is gathered by experiment performed by August Weismann. According to the Weismann’s theory of continuity of germplasm, animal is made up of two types of cells; Somatic cells and germ cells. The nuclei present in germ cell are responsible for inheritance of characters whereas somatic cells contain nucei which respond to the environmental factors and use/disuse. Hence, acquired characters remain within the somatic cells. As a result this theory of inheritance of acquired characteristics supports the idea that acquired characters are not inherited. Weisman conducted a conclusive experiment on rat where he has cut their tails for 80 generations but it didn’t produce rats tailless.
Boring of ear (pinna) is practiced in women for thousands years but this characters never been inherited.
European women’s wear tight garments to maintain slender waist but their children have normal waist at birth.
Chinese women wear tight shoes to have small feet but their children have normal feet.
Pavlov has trained the mice to come for food on bell ring but he found that training is necessarily in each generations.
Eyes of a voracious reader do not grow in size nor they acquire improved eyesight with increasing age.
There are evidences to support the inheritance of acquired characters. For examples, effect of radiation and chemicals on germ cells and resulting change in the phenotype of the cell. The evidence for the inheritance of acquired character revived the Lamarck theory as Neo-Lamarckism. The modified Neo-Lamarckism has following postulates:
Germ cells are not always immune from the effect of environment. Germ cells may be affected directly by environment without any effect on the somatic cells. Germ cells may carry acquired character to the next generation.
Even considering these points, Lamarckism could not be able to provide satisfactory mechanism for evolution.