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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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

9. 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.

10. 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.

11. 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.

12. 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!