Question

Species is the group of organisms which can interbreed under natural conditions, right? Then, In cross pollination (hybridisation) there is transfer of pollen grains between different species (eg. Pollution btw two species of cotton ) they are interbreeding then how they're in different species??

Answer 1

Under natural conditions, pollination does not occur between two different species.There are chemical mechanisms by which pollen from a different species is rejected by stigma.This prevents fertilisation and subsequent embryo formation.Because naturally interspecific fertilisation is not possible.That is why the two are different species.
If fertilisation which is brought by artificial means is not a natural one.So it is not considered a criteria for species definition.
As long as the two animals/plants are naturally not reproducing, they belong to different species.
If two organisms are naturally reproducing, they belong to same species.
Artificial reproduction and hybridisation are not considered while defining a species.