(i) Similar morphologies (phenotype)
concept/basis: Organisms sharing the similar characteristics and morphologically alike are of the same species.
problems: This definition is applicable only if evolution does not exist. Considering the fact that species have evolved from a common ancestor, large morphological differences sometimes exist between very closely related populations; conversely, since organisms are known to adapt to the changing environment, genetically distinct populations may look very similar.
(ii) An interbreed of population
concept/basis: Organisms belong to the same species if they can mate and produce viable offspring
problems: This definition relies on reproductive behaviour, therefore cannot be applied to fossil species and asexual organisms.
Modern definitions: by comparing the genetic constituents of organisms, e.g. DNA base sequences, amino acid sequences.