What is a species?
It’s not a question many think about, especially not those who don’t focus on biology. Even within biology,the lines between species are not usually questioned. When a definition is provided, it usually is something like “if two organisms can breed and produce equally fertile offspring, they are the same species.” This allows for a healthy degree of variation within a species, which can be seen in animals such as dogs, but clearly delineates between closely-related species. However, even this definition of a species, which is known as the “biological concept,” does not hold up to scrutiny. For example, the extinct Neanderthals are typically considered to be a distinct species from Homo sapiens, yet Neanderthals are believed to have interbred with Homo sapiens given that many modern-day humans possess Neanderthal DNA. Among species in which interspecies hybridization is commonplace, a “species continuum” can develop in which the line between two species is blurred. Also, the biological concept is of no use when classifying organisms which asexually reproduce, such as bacteria.
The problem of how best to define a species has puzzled biologists for a long time, especially since Darwin defined a mechanism for how species originate. Indeed, Darwin stated his surprise at how “entirely vague and arbitrary” the distinctions between species was. The biological concept became standard in part because of Ernst Mayr’s 1942 proposal to define species in such a way .
Since Mayr’s formalization, some newer methods of distinguishing species have been formulated which attempt to resolve some of the problems with the biological concept, but even these still have some crucial issues. One of these methods is the phylogenetic species, which merely defines a species as a lineage of organisms which all possess a certain trait or traits. The Barcode of Life project has used the cytochrome c oxidase gene to delimit species, but this method has little correspondence with other traditional methods.
In some cases, determining what counts as a species affects policy, as what is or isn’t a species can determine which animals are covered by legal protections. For example, the endangered red wolf of the American Southwest is the result of ancient hybridization between wolves and coyotes. Some biologists argue that the red wolf populations have remained distinct long enough that they qualify as their own species, but others disagree given its hybrid origin. However, the Endangered Species Act does not cover hybrid animals, so in order to qualify for protection it would have to be considered its own species.
