Name of TeacherDr Brijesh Pathak Designation Assistant Professor

Name of Teacher-Dr. Brijesh Pathak Designation- Assistant Professor Address-University College, Dhilwan, Tapa Subject Name-Botany (Evolution and Theories) B. Sc. (Medical)-Semester II

Concept of Evolution • The term ‘evolution’ literally denotes the process of continuous change. • The concept of evolution is based on detailed comparisons of structures of living and fossil forms. • The concept of evolution is also based on the sequences of appearance and extinction of species in past ages • The concept of evolution is based on the physiologic and biochemical similarities and differences between species. • It also depends on analysis of the genetic constituents of present day plants and animals.

• One of the major trends in the evolution of most plants and animals has been towards increased adaptation to some particular environment, and this has frequently involved increased specializations and complexity of structures and their functions. • Empedocles (495 -435 B. C. ) often regarded as the father of the evolutionary concepts. • The four basic elements are- air, earth, fire and water. • All changes in the world were associated with different mixtures of these four elements. • Aristotle (384 -322 B. C. ) a great biologist as well as philosopher, worked out an elaborate theory of gradually evolving life forms.

Theory of Inheritance of Acquired Characters or Lamarckism: Jean Baptisete De-Lamarck in 1809, known as Lamarckism. This was published in his book “Philosophic Zoologique”. 1. Environmental Influence: It brings about the new needs of organisms in order to survive in a changed environment, which are responsible for structural and behavioral modification in the organisms. 2. Use and Disuse of Organs: Lamarck believed that any particular part of the body, which is put into more frequent use, has a tendency to develop stronger and better while any part of the body, which is put into less or no use, leads to its degeneration. e. g. the ancestors of giraffe were short necked antelopes. • When their ancestors transferred to an area where there were only trees and no grass. • The practice of stretching the neck and fore limbs gradually continued from generation after generation resulting in increased length of the neck and fore limbs of giraffes as we see them today.

3. Inheritance of Acquired Characters: All those changes which have been acquired by the organisms during its life time due to the influence of the environment is preserved by the generation and is transmitted to off springs. Criticism to Lamarck's Theory: Weismann in (1904) rejected Lamarck’s theory by introducing “Theory of germ plasm”. Under his experiment, he removed the tail of mice for about twenty-two generation. As per him, the changes that occur in germplasm are only transmitted to the progeny but changes in somatoplasm, like non-availability of tail in case of mice, are never transmitted to the progeny.

(B) Theory of Natural Selection or Darwinism: Charles Darwin presented this theory in his book “Origin of Species by Natural Selection” in 1859. The major points of this theory are: 1. Living organisms have enormous fertility and reproduction can take place at a very faster rate but the number of survivors remains roughly constant under the given environmental conditions. 2. Struggle for existence among organisms continues looking to the fact that environment have limited amount of food, habitat and other resources. There remains the intense competition to ensure living among organisms and it continually keeps check on the number of living organisms. 3. The difference among two generation of organisms is called variation, which are transferred to off springs to provide them better survival capacity.

4. The best-fit and adaptive organisms can only survive as they carry useful variations in structure and functions. Survival of the fittest takes place naturally hence this is called “Natural Selection”. 5. Useful variations continue from one to another generation and the next generation again gets some new variations in order to become fittest in the given environmental set. This is called continual selection. 6. Origin of new species takes place as a result of natural selection, environmental changes, variations and struggle of existence etc. Criticism of Darwin’s theory of Natural Selection Richard Owen and Adam Sedgwick. 1. Darwinism does not account for vestigial organs and connecting links. 2. It also did not differentiate between somatic and germinal variations. He could not explain the arrival of the fittest.

(C) Neo Darwinism: The modern version of the Darwinian theory of evolution is known as Neo-Darwinism. • According to this, various factors like variation, mutation, natural selection, and genetic drift through isolation of species play a crucial role in the formation of new species. • As per this theory, the difference found in closely related living beings is called variation which is found in an organism either due to changes in environmental conditions (non heritable variation) or change in the genetic constitution (heritable variation).