There is little agreement among scientists about the definition of the word "species." However, most biologists would agree that a species is a detectable, naturally occurring group of individuals or populations that is on an evolutionary path independent from other such groups. Several more detailed definitions have been articulated over the years; two that have gained prominence are the biological species concept (BSC) and the phylogenetic species concept (PSC).
In 1942 biologist Ernst Mayr defined the biological species concept as follows: "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." This definition places emphasis on restriction of gene flow among groups. Reproductive isolation means that individuals from two groups are unable to interbreed successfully, that is, produce healthy, fertile offspring. So according to this definition, an individual is a member of a particular species if it can breed successfully with members of that species but not with members of other species.
Interbreeding between two different groups is called hybridization and is viewed differently by different scientists. In animals, hybrid offspring of two different species are thought to be unhealthy or infertile as adults, but in plants hybrid offspring are often thought to be more vigorous than their parents. As a result, plant biologists and animal biologists differ regarding the significance of interbreeding in answering species questions, and most plant biologists are not proponents of the BSC.
Objections to the BSC include the fact that the extent of hybridization can range from very little to extensive, making its interpretation subjective. Also, it requires guesswork regarding the species status of groups that do not occur in the same place and thus have no opportunity to interbreed, and it cannot easily be applied to organisms in the fossil record or to those that lack sexual reproduction. Furthermore, it is now known that hybridization can occur between two independent groups that are not each other's nearest relatives. Thus, putting two hybridizing groups into one species could misrepresent evolutionary history by excluding other more closely related (and often reproductively isolated) groups.
A phylogenetic species concept was articulated by Joel Cracraft in 1987 as follows: "a species can be defined as an irreducible cluster of organisms, within which there is a parental pattern of ancestry and descent, and which is diagnosably distinct from other such clusters." This definition views a species as being the smallest possible grouping of organisms in time and space that can be differentiated from other groupings, with the basis for the differentiation being inherited. So an individual is a member of a species if it shares the inherited characteristics of the species, irrespective of whether it can hybridize with a member of another species. The primary objection to this definition is that it is too vague.
These are but two examples of the numerous definitions from a century of ongoing debate about the definition and meaning of species. Scientists often approach the species question differently depending on what organisms they are studying and the way in which they are studying them. Traditionally, organisms have been grouped into species based on aspects of their appearance or particular behaviors. More recently, analysis of deoxyribonucleic acid (DNA) has joined the list of techniques for differentiating or grouping organisms. Additionally, there are specific criteria used for different groups. In plants, for instance, plant chemistry, insect associations, and number of chromosomes may be important indicators of species status. As another example, scientists studying bacteria may use such characteristics as shape, biochemistry, and conditions favoring growth to help them answer species questions. Thus, there is no simple, universally agreed-upon definition of species.
Ann E. Kessen and Robert M. Zink
Keller, Evelyn Fox, and Elisabeth A. Lloyd, eds. Keywords on Evolutionary Biology. Cambridge, MA: Harvard University Press, 1992.
Otte, Daniel, and John A. Endler. Speciation and Its Consequences. Sunderland, MA: Sinauer Associates, 1989.
MAYR, ERNST (1904–)
German-born U.S. evolutionary biologist who helped found the "modern synthesis," the melding of evolutionary theory with genetics. Mayr's greatest contribution was to explain how new species can arise. When a population is isolated, on an island, for example, it can evolve separately from the rest of the species. Mayr's views have defined evolutionary biology for nearly three-quarters of a century, and he has won two prestigious prizes, the Balzan Prize and the Japan Prize.