Theories Of Evolution

There are 3 main theories of Evolution

Theory of Special Creation

Lamarckism

Darwinism

Theory Of Special Creation

Purely religious concept

Proposed by religious scholars and educationalists

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

LamarckismProposed by Jean Baptiste de Lamarck in 1809

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.

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

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.

DarwinismProposed by Charles Robert Darwin in 1859

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

Struggle for existence• There starts an everlasting competition between individuals having similar

requirements

Every living organism desires to have an upper hand over others

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

Inheritance of useful variationsThe selected individuals pass their useful continuous variations to their offsprings so that

they are born fit to the changed environment

SpeciationEvolution is a gradual process and speciation occurs by gradual changes in the existing

species.

Drawbackso 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