Class 9 Science Note


The development of the organism with the time is called evolution. On the basis of similarities and dissimilarities, it could be assumed that the simplest organisms originated in the beginning and then they gradually modified into different forms of organisms. Thus the origin of complex forms of organisms from simpler form is called evolution. The environment plays a significant role on the evolution of organisms. During the distribution of common descendants, those lived in a similar type of environment underwent no modification on the body structure and reproduced the similar kind of offsprings while those who migrated to different type of environment adapted and underwent modification. In long run, such continuous change gave rise to new species.


Two theories are widely acclaimed

Theory of special creation: In this theory, God is the creator of everything.

Theory of organic evolution: In this theory, series of changes appeared in the organism which is still continuing today.

Evidences supporting the organic evolutions are:

Evidence from fossils

Evidence from morphology and anatomy

Evidence from embryology

Evidence from vestigial organ

Evidence from distribution of organism

Evidence from connecting animals


Theories of evolution

Lamarck's Theory

In 1908 AD, Lamarck put forward his views as follows:

a. Environmental influence on organisms

b. Variation in the structure of body due to use and disuse of organs

c. Transfer of acquired characteristics from one generation to another


According to Lamarck, plant and animals living in different habitat and environment also have differences in the body structures. The variation occurred in the body structures of an organism due to influence of environment is transferred from one generation to another. Such variation depends on the use and disuse of organs by the organisms. For example, the fish inhabiting in deep and dark bottom of the sea are blind 

due to the disuse of eyes. The organ which is not used disappears and the organs used most evolve. The vestigial organs in the human body are an example of this.

However the theory was not accepted universally. Some of the criticisms of Lamarck's theory are as follows:

a. The change in the structure of the boy by use or disuse of organs is not manifested practically.

b."The new organs can be formed according to the will of organisms", this has been untrue.

c. The symptom of growing very small or very large is not observed in the evolutionary trend of organisms.

d. The offsprings of a tail trimmed dog instead of being short tailed are actually like normal dogs.




Darwin's Theory of Evolution

In 1859 A.D., Charles Darwin published his views on evolution of organism. In his book "Origin of Species", he put forward the following concepts on evolution.


a. Over Population

Each organism has the tendency of reproducing a number of offspring. However, their number is found roughly constant instead of growing quite large.


b. Struggle for existence

There could have been scarcity of food and space if all offspring of organism were alive. Since there has always been competition for food, water, space etc. only the strong organisms which adapt themselves to the environment, exist and the weak ones become extinct. The weak animals cannot survive due to different diseases and the attack of their enemies. Hence only the animals strong enough can adjust themselves to the environment can survive and the rest will die.


c. Variation and heredity

The struggle for existence is also not universal for the survival of organisms. Because of variation in structure and functions of similar organisms, some are capable and fitter than others due to which they can survive longer.

d. Natural selection

How long an organism lives is determined by the nature. Similarly the animals which can adapt itself to the environment can survive. The organs or characters that are useful in the competition are further continued to future generation. Thus after a long time, a new species seems to be evolved. In this way, the selection of more progressive organisms is carried out by the nature. This is called natural selection. In other words, it is called the survival of the fittest.


e. Origin species

Since successful and advantageous variation in characters are inherited, the new organisms can survive linger in the changing environment. Sometimes, special characters are seen in the offspring, which gradually modify from generations to generations and ultimately they look different from their ancestors. If there were no ancestral characters then no new organisms would have evolved. In this way, new organisms have been evolving from a common ancestor.

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