CBSE Class 12-science Answered

(a)

Salient features of Darwin’ theory of evolution:

1. Darwin’s theory of evolution is based on natural selection.

2. He believed that minor variations cause evolution.

3. Darwinian variations are small and directional.

4. Darwin believed that evolution is a gradual process.

Salient features of Hugo De Vries’ theory of evolution:

1. Vries’ theory of evolution is based on mutations.

2. He believed that mutations cause evolution.

3. Mutations are random and directionless.

4. Vries’ believed that sudden mutations caused evolution

(b) Evolution is a very slow and gradual process that results in the generation of highly evolved and advanced species over a period of about thousands of years. Evolution occurs as a result of several processes such as mutation, natural selection, genetic drift etc. Each of these processes is a result of several other processes.

Example: Mutations occur due to errors that take place during replication. Natural selection is a process caused by the influence of the environment on the phenotypes of the individuals. Genetic drift is a process caused by the process of random matings, reproduction and chance events.