The biological species concept (BSC) is one that many biologists are familiar with. This concept has been used as a taxonomic tool to define what constitutes as a species for over 75 years (Bobay and Ochman 2018). The biological species concept uses the principle of interbreeding to define a species as a group of individuals that are reproductively isolated and can produce fertile offspring. For instance, a horse and a donkey can reproduce to have a mule offspring. However, the horse and donkey are considered to be different species because their mule offspring is sterile and cannot reproduce to pass on its genes into the gene pool. There are many other species concepts that can be used to define a species, such as the subspecies concept. The subspecies concept also defines a species as a group of individuals that can interbreed successfully and produce fertile offspring. However, there is the addition of the subspecies, a subordinate of species, which are individuals within a species that share similar variations with each other and not with the rest of the individuals within the larger species group or are in geographic isolation. Many believe that a replacement of the biological species concept by the subspecies concept is necessary because BSC has outlived its usefulness in taxonomic definitions. However, many others, including myself, do not believe this to be the case.
A study on the North American marten was performed in 1958 in which it was concluded that “partitioning of the species into subspecies was completely arbitrary (Hagmeier 1958).” Even though this study was done 60 years prior, the conclusion still holds true. The phenotypic variations used to group the members of a species into a so-called subspecies may not be valid because it is expected that variations exist among individuals in a population due to one of the conditions that must exist for natural selection to occur. That is, organisms have phenotypic variations, even within the same species. Therefore, characteristics that can be used to separate organisms into a subspecies are very limited, if there are any at all.
“Unless subspecies capture elements of recent evolutionary history, the concept has limited utility in taxonomy and little relevance in modern practical conservation (Braby et. al 2012).” The biological species concept is more practical than the subspecies concept when it comes to evolutionary history because it emphasizes reproductive isolating mechanisms. As previously mentioned, a subspecies includes members of a species that are geographically isolated. However, geographic isolation is often due to chance and chance may dictate for individuals that are isolated to come into contact again and reproduce fertile offspring. Therefore, the subspecies definition would not be practical to utilize as geographic isolation and barriers are constantly changing.
A main argument against the utilization of the biological species concept is that this concept cannot be applied to organisms that reproduce asexually. However, a study on viruses was performed that extended the application of the biological species concept to viruses as well as bacteriophages. Reproductive processes occur differently in viruses and the definition of interbreeding may need to be altered slightly depending on the species or organism of interest. For instance, in this study, viral gene flow was analyzed and “despite differences in cellularity and reproductive processes, the overwhelming majority of lineages in every group can be classified into species based on the BSC (Bobay and Ochman 2018).”
Though the biological species concept is not without flaws and cannot be applied to all organisms, it is the best definition to date of how to define a species. With the subspecies concept, it is rather difficult to determine when a subspecies has just emerged or when it becomes a distinct species from which it emerged from. The biological species concept draws a clear and definitive line between different species by using interbreeding as the deciding factor. Admittedly, the subspecies concept may have its uses in nature and may be even more suitable to use than the biological species concept sometimes. However, it has not been put into consistent practice and application enough to replace the biological species concept.