Moreover, some concepts define species at one instant in time whereas others attempt to define species through geological time. Some address the process of speciation whereas others address the products of speciation. Historically, our concept of a species has changed and is still changing. Six major concepts have emerged from the pages of the biological literature. This concept involves subjectivity in the definition through the degree of difference.
Different species are organisms that look different.
Species identified under this concept and soley on the basis of morphological differences are called morphological species or morphospecies . This concept is fundamentally an application of the typological species concept . Some critics of the phenetic species concept view it as a numerical update of the morphological species concept. Sexual dimorphism Different morphological forms occur in the two sexes. Polymorphism Two or more morphological forms in a population.