theories of evolution

1. Theories Of Evolution

2. There are 3 main theories of Evolution
 Theory of Special Creation
 Lamarckism
 Darwinism

3. Theory Of Special Creation
 Purely religious concept
 Proposed by religious scholars and educationalists

4. According to this concept
• All the organisms have been created in their current form
millions of years ago
• There had been no process of evolution throughout the life
on earth
Criticism
This theory had no scientific basis

5. Lamarckism
Proposed by Jean Baptiste de Lamarck in 1809

6. Main Postulates
 Living organisms or their component parts tend to increase in size.
 Production of new organ is resulted from a new need.
 Continued use of an organ makes it more developed, while disuse of an
organ results in degeneration.
 Acquired characters developed by individuals during their own lifetime are
inheritable and accumulate over a period of time resulting a new species.

7. Example of Lamarckism
 Development of present day long-necked giraffe by the gradual elongation of neck
 In response to deficiency of food on the barren ground in dry deserts
 These body parts were elongated so as to eat the leaves on the tree branches
 This increase in the length of their necks was heritable
 Thus the giraffes of today possess sufficiently long necks

8. Criticism
 Theory of “continuity of germplasm” states that environmental factors do affect only
somatic cells and not the germ cells
 Heart size does not increase generation after generation though it is used continuously
 Presence of weak muscles in the son of a wrestler was also not explained by Lamarck.
 So, Lamarckism was rejected.

9. Darwinism
Proposed by Charles Robert Darwin in 1859

10. Main Postulates
Geometric increase
The populations tend to multiply geometrically and the reproductive powers of living organisms (biotic
potential) are much more than required to maintain their number
Limited food and space
A population tends to increase geometrically, the food increases only arithmetically. So, 2 main limiting
factors limited food and space do not allow a population to grow indefinitely

11. Struggle for existence
• There starts an everlasting competition between individuals having similar
requirements
Every living organism desires to have an upper hand over others

12. Variations
This everlasting competition among the organisms has compelled them to change according to the conditions to
utilize the natural resources and can survive successfully
Natural selection or Survival of the fittest
• Nature selects only those individuals out of the population which are with useful continuous variations
• And are best adapted to the environment

13. Inheritance of useful variations
The selected individuals pass their useful continuous variations to their offsprings so that
they are born fit to the changed environment
Speciation
Evolution is a gradual process and speciation occurs by gradual changes in the existing
species.

14. Drawbacks
o Inheritance of vestigial organs
o Did not differentiate between somatic and germinal variations
o He did not explain the causes of the variations and the mode of transmission of variations