(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.