What is a species: Conflict between evolutionary continuity and taxonomic discontinuity

Abstract

What is a species? This is a question seemingly simple but difficult to be addressed clearly. If only by a simple definition, a “species” can be described as the basic category or unit for taxonomic classification of organisms (including animals, plants, and microbes). On the other hand, it is extremely difficult to define a “species” or “species concept” that are generally acceptable by all biologists. Substantial debates exist over the “species concept” and have lasted for more than a contrary. During this time span, many species concepts have been proposed by various groups of biologists who work in different disciplines. However, it is extremely difficult to have a clear definition for “species” and particularly for “species concept” that are universally agreeable. Among the proposed species concepts, only some of them are influential, including those: taxonomic species concept, Darwin’s species concept, biological species concept, genetic species concept, and phylogenetic species concept. Obviously, biologists who proposed their species concepts attempt to interpret what “species” are only by emphasizing the specific features of a species appreciable from their perspectives. For example, biologists who have proposed taxonomic species concept highlight the classification feature of a species; whereas those who have Darwin’s species concept stress the evolutionary process of a species. Disagreements always exist among biologists when define the species concept. Some biologists believe that a species is a natural unit, which is strictly followed for the classification of a species. However, other biologists do not believe a species to be a natural unit, instead, they emphasize more on the evolution and reproductive aspects of a species. The extreme groups even reject “species” as a natural units. They only accept individuals as the natural unit. Obviously, the dilemma is due to the diverged understanding and opinion of species. We know that species are the outcomes of evolution—biodiversity. From the practical viewpoint, it is necessary to categorize the evolution-resulted biodiversity using a stable classification system. Therefore, taxonomists prefer to use a discontinued unit to classify species. On the other hand, evolutionists emphasize more on the variation of organisms. Thus, the characteristics of evolutionary continuity and taxonomic discontinuity have stimulated such debates over a species—a natural unit or an artificial category. Species are outcomes of continued evolutionary process and categories of discontinued taxonomy. As such, one can recognize, understand, and define a species according to the objectives and demands to suit his or her research, without being troubled too much by a universally unified concept of species. Thus, to answer what is a species, we prefer the definition that a species is an individual collection that occurs in a particularly space/time, shares similar morphological and physiological features with a common ancestor. Members of the same species are able to interbreed and reproduce normal descendants.