Weismann barrier or theory

WEISMANN THEORY OF EVOLUTION,

1. Prepared By: Vipin Kumar Shukla Assistant Lecturer. Weismann Barrier or Theory

2. Introduction: Weismann was the first person to make a definite distinction between heritable changes and those which cannot be inherited. According to Weismann characters influencing the germ cell and egg dells. Germ cells are also called Biological cells that give rise to the gametes of am organism. He was a German evolutionary biologist . Ernst Mayr   ranked him as the second most notable evolutionary theorist of the 19th century, after  Charles Darwin.

3. Continued..……. His main contribution involved germplasm theory at one time also known as  Weismannism , according to which inheritance (in a multicellular organism) only takes place by means of the germ cells the gametes such as egg cells and sperm cells. Other cells of the body somatic cells do not function as agents of heredity. The effect is one-way: germ cells produce somatic cells and are not affected by anything the somatic cells learn or therefore any ability an individual acquires during its life. Genetic information cannot pass from soma to germplasm and on to the next generation. Biologists refer to this concept as the Weismann barrier.

4. Continued……. The idea of the Weismann barrier is central to the modern synthesis of the early 20th century, though scholars do not express it today in the same terms. In Weismann's opinion the largely random process of mutation, which must occur in the gametes (or stem cells that make them) is the only source of change for natural selection to work on. Weismann became one of the first biologists to deny Lamarckism entirely. 

5. Continued……. Weismann's ideas preceded the rediscovery of Gregor Mendel 's work, and though Weismann was cagey about accepting Mendelism, younger workers soon made the connection. Weismann is much admired today. Ernst Mayr  judged him to be the most important evolutionary thinker between Darwin and the evolutionary synthesis  around 1930–1940, and "one of the great biologists of all time.

6. Weismann's work on germ cells: Weismann's work on the demarcation between germ-line and soma can scarcely be appreciated without considering the work of (mostly) German biologists during the second half of the 19th century. This was the time that the mechanisms of cell division began to be understood. He said "new cell nuclei can only arise from the division of other cell nuclei". Van Beneden discovered how chromosomes combined at meiosis during the production of gametes, and discovered and named chromatin . Walther Flemming, the founder of cytogenetics, named mitosis.

7. Continued….. The discovery of mitosis, meiosis and chromosomes is regarded as one of the 100 most important scientific discoveries of all times, and one of the 10 most important discoveries in cell biology. Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by Van Beneden in Ascaris eggs. The significance of meiosis for reproduction and inheritance, however, was first described in 1890 by Weismann, who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained

8. Continued…… Thus the work of the earlier cytologists laid the ground for Weismann, who turned his mind to the consequences for evolution, which was an aspect the cytologists had not addressed. All this took place before the work of Mendel had been rediscovered.

9. Weismann experiment: The idea that germline cells contain information that passes to each generation unaffected by experience and independent of the somatic (body) cells, came to be referred to as the Weismann barrier, and is frequently quoted as putting a final end to the theory of Lamarck and the inheritance of acquired characteristics. What Lamarck claimed was the inheritance of characteristics acquired through effort, or will.

10. Continued……. The Great Mutilation Experiment: In 1891, a scientist by the name of August Friedrich Weismann conducted an interesting experiment with mice. He was trying to prove a theory called “Lamarckism” which still teaches “inheritance of acquired characteristics.”  The theory declares that birds living in the water grow webbed feet, etc . Weismann cut the tails off white mice in an attempt to prove that their babies would be born with short or no tails! Weismann was greatly disappointed when the babies that were born had tails the same length as other mice. But he continued his experiments.

11. Continued….. Eventually he cut the tails off 901 mice in 19 successive generations, yet each successive generation had full-length tails!  Today, he would go to jail for animal cruelty! Dr. Weismann and Jean Baptist Lamarck, the scientist known for the discovery of “Lamarckism” didn’t know that the inherited characteristics of animals was because of DNA coding and not habits or environmental circumstances.  That unique DNA coding is the work of a masterful Creator! Lamarck taught that giraffes had long necks because they were always stretching to get food from tall trees. Now we know that this just isn’t so!