(i) Species is a basic unit of classification, where individuals share common characteristics.
(ii) It is a natural population or group of natural population of individuals having similar morphology, anatomy, physiology and cytology.
(iii) Species is an assemblage of structurally similar individuals, which interbreed freely, but are reproductively isolated from members of other species.
Higher plants and animals These are sexually reproducing organisms. So, the third definition of species is applicable in case of higher plants and animals.
Bacteria The second definition of the species can be applied to bacteria. As bacteria do not reproduce sexually. So, the criterion of free interbreeding and reproductive isolation cannot he applied to them.