Explain briefly with the help of an example how new species come up.

Hi Sunidhi,
 Species may come up due to the following reasons:

• Gene flow

It occurs between populations that are partly but not completely separated.
 

• Genetic drift

It is the random change in frequency of alleles (gene pair)in a population over successive generations.
 

• Natural Selection

The process by which nature selects and consolidates those organisms which are more suitably adapted and possesses favourable variations.
 

• Reproductive Isolation

It occurs when a particular sub population or deme is unable to interbreed due to totally different genetic compositions
 

• Geographic Isolation

It occurs when a physical barrier crops or is present between two demes. Ex. River, Mountain etc.

Hope it helps!!