(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 can interbreed freely. but are
reproductively isolated from members of other species.
Higher plants and animals Ihese are sexually
reproducing organisms. 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.
Therefore, the criterion of free interbreeding and
reproductive isolation cannot be applied to them.