Scientists apply the biological species concept in determining organisms of the same species. Appearance is useful in identifying a species, but it is not a method of defining the species. Scientists are able to identify sexually reproducing organisms of the same species by crossbreeding them. The organisms should produce fertile and viable offspring (Wheeler & Meier, 2013).
Although some organisms have similar phenotypic characteristics, they will not produce a fertile offspring when they interbreed. For instance, a donkey and a horse have similar outward appearance but are genetically different. When they interbreed, they produce a mule, which is an infertile offspring. Other organisms have different physical appearances but belong to the same species. For example, ants belonging to the Pheidole barbata species differ in their appearance and size, but they are of the same species (Wheeler & Meier, 2013). Therefore, organisms can only breed with organisms from the same species and not with those from other species.
Evolutionary developments sometimes occur and organisms belonging of a certain species evolve into a different species in a process called speciation. New species develop from the existing ones; evolve along different genetic lines and with time, become so different that they lose their ability to interbreed. When this happens, organisms become reproductively isolated from each other and can no longer produce viable offspring since they belong to different species. The concept of viability and not physical appearance is the key factor in identifying individuals of the same species.
The biological species concept is limited only to sexually reproducing organisms in identifying organisms of the same species. Scientists have not derived a method of identifying asexual organisms such as amoeba. However, this is a developing area of study in evolutionary biology.
Wheeler, Q., & Meier, R. (Eds.). (2013). Species concepts and phylogenetic theory: A Debate. New York, NY: Columbia University Press.