What is a species?

Enacted in 1972, the Endangered Species Act is widely regarded as having saved various species with low population numbers from extinction. The number of individuals necessary to preserve an endangered species is variable, depending on fecundity, genetic diversity, available habitat, and other factors. These are evaluated by the U.S. Fish and Wildlife Service on an individual case basis.

And, of course, implementation of the Endangered Species Act is crucially dependent upon what constitutes a species.

The traditional definition, often called the biological species concept, recognizes a species as a population of organisms that potentially interbreed, producing fertile offspring.  In practice, individuals are seldom held in captivity to check for interbreeding potential and the fertility of their offspring.  So in most cases, biological species are recognized by their physical traits, which often conform to interbreeding potential.  Hybrids between species are recognized, but these do not interfere with the definition as long as the hybrids are sterile.

The traditional view of species is not without its problems.  For instance, how do we classify two populations of individuals that could interbreed, but they are isolated by geography, so that they never come in contact?  Some biologists prefer a Phylogenetic Species Concept, in which at least one feature of the species in question (known as the diagnostic character) is found nowhere else in nature.

In recent years, with the advent of molecular biology, ecologists are using quantitative measures of genetic differences to recognize species.  Armed with standards that define how many gene differences would categorize a species, individuals can be gathered from disparate habitats and analyzed for differences in DNA.  The methods, increasingly simple, are quantitative, reproducible, and definitive—unlike the physical traits of color, size variation, and behavior that are used in the biological species definition.

This implementation of molecular biology should dramatically clarify the definition of species for the Endangered Species Act.  Some small groups of organisms that have been recognized as separate species may now be found to be of the same genetic composition and thus a single species, perhaps with enough individuals to avoid the designation of “endangered.”   In other cases, populations that have been recognized as a single species may be found to differ in genome, enough to be categorized as separate species.  One or both newly recognized species might qualify as endangered.  Salmon that return to different rivers to spawn may have enough genetic differences to warrant recognition of each riverine population as a species, deserving of special protection under the law.

The total number of species in an area constitutes its biodiversity. As the molecular species definition is applied to an increasing number of species in the field, we should not be dismayed by changes in what we recognize as a species. These changes are not the result of bad science, but better science.



Hunter, M.L., J.P. Gibbs and V.D.  Popescu.  2019.  Fundamentals of Conservation Biology. 4th ed. (Chapter 3, Species Diversity), John Wiley, New York