A recent paper published in PLoS Biology estimated that the total number of eukaryotic species on the planet was 8.7 million (+/- 1.3 million). The sheer diversity of life on Earth is certainly breathtaking, if this figure is accurate it means that 86% of all land species and 91% of all marine species remain to be discovered. However, although we can now estimate how many species there are, there is still no definition of “species” that works in all situations. In fact there may be as many as 22 (or more) species concepts in use today (Mayden 1997).
The question of how best to define “species” is something that biologists have struggled with for well over a century, this has become known as “the species problem”. Charles Darwin himself mentioned it in On the origin of species saying:
No one definition has satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species. Generally the term includes the unknown element of a distinct act of creation.
He later revised his view, writing in The descent of man:
But it is a hopeless endeavour to decide this point on sound grounds, until some definition of the term “species” is generally accepted; and the definition must not include an element that cannot possibly be ascertained, such as an act of creation.
In an ideal situation all of life could be neatly organised into discrete groups, completely isolated and distinct from one another. But the world is not ideal and nature is a lot messier than that. The boundaries that determine where one species ends and another begins are often blurred and overlapping.
One of the most widely used species definitions was proposed by the renowned evolutionary biologist Ernst Mayr and is known as the biological species concept or BSC. Mayr defined a species as:
groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups
Mayr’s definition works well in most cases, particularly for multicellular sexually reproducing organisms. However, for the small number of multicellular organisms that reproduce by parthenogenesis and for single-celled organisms such as bacteria and archaea this definition is useless. There are other situations in which the BSC is less than helpful. For example, some species may occasionally hybridise and produce fertile offspring. It is not clear whether reproductive isolation between species has to be absolute or if some small degree of hybridization is permissable under this definition.
The BSC is also difficult to apply to populations that change gradually over a continuous gradient such that neighboring populations can interbreed but populations at the far ends of the gradient cannot. This situation in known as a cline or, when the cline is circular, a ring species.
Take for example a hypothetical situation in which there are three neighboring populations which we’ll call A, B and C. Population A can successfully interbreed with population B and population B can successfully interbreed with population C. However, when A and C come together they are completely incompatible with each other and cannot interbreed. In this case it is not at all clear how many species should be recognised. We could classify A, B and C as three separate species but that doesn’t seem right since populations A and B or B and C do fulfill Mayr’s criteria by interbreeding. We could classify them all as one species but that doesn’t seem right either since A and C cannot interbreed and so do not fill Mayr’s criteria.
Although unusual, situations just like the example above have been found in nature. One such case is that of the Greenish warbler (Phylloscopus trochiloides). This small and unremarkable bird from northern and central Asia occurs in Siberia in two distinct forms which have different colouration, different songs and which do not interbreed. In most cases this would be enough to classify these two groups as completely separate species. However, the two forms are linked by a continuous chain of gradually changing and interbreeding populations which encircle the Tibetan plateau to form a ring. Thus there is gene flow through the ring connecting the two Siberian populations yet there are no clear points at which the ring can be divided in separate species. At present Phylloscopus trochiloides is divided into five subspecies.
An alternative to Mayr’s BSC that is growing in popularity is the phylogenetic species concept (PSC) this has been defined as:
a monophyletic group composed of the smallest diagnosable cluster of individual organisms within which there is a parental pattern of ancestry and descent
Put simply a species, as defined under the PSC, is a group containing all the descendents of one common ancestor that are united by traits that evolved uniquely in that population. These are known as derived characters and may be morphological, behavioural or chemical so long as they have a genetic basis and are heritable.
The PSC differs from the BSC in that it focuses on the phylogenetic history of organisms rather than their ability to interbreed. It therefore solves many of the problems with the BSC for example, hybridizing, parthenogenetic and clinal populations can be easily divided into species using the PSC.
However, although the PSC has many benefits it has also been criticised for not defining clearly how much variation is permissible within a species or how many derived characters are required to elevate a population to a species status. For example it has been suggested that derived character traits as small as one extra hooklet on the barbule of the primary feathers in a population of swans could, theoretically, distinguish that population as a distinct evolutionary unit and therefore as a separate species (McKitrick and Zink 1988). In this way the PSC has the potential to spilt populations up into very small groups which would be of no practical use to science.
The BSC and PSC are the two most widely used species concepts however there are many more.
- As used by Linaeus and many other naturalists of his time the typological species concept was one of the first to be proposed. Under this concept species are groups of individuals united by shared morphology such that they can be distinguished from other groups. In other words they were groups of similar looking individuals. Typological species were defined by reference to a type specimen, usually kept in a museum, to which individuals could be compared. We now know that phenotypes can vary substantially within a species (e.g. amongst eusocial insects such as ants) and so the concept is no longer used.
- Ecological species are defined as populations occupying the same ecological niche.
- Genetic species are defined based on the similarity of DNA sequences. This may be determined by DNA fingerprinting or genome sequencing.
- The mate recognition species concept defines a species as a group of organisms that recognize each other as potential mates. Like the BSC this only applies to sexually reproducing species.
“The species problem” is unlikely ever to be resolved. While species concepts have been developed that work well in a wide range of cases none is applicable to every situation. In the words of Theodosius Dobzhansky:
…the futility of attempts to find a universally valid criterion for distinguishing species has come to be fairly generally, if reluctantly, recognized.
Grant, P. R. & Grant, B. R. (1992) Hybridisation of bird species. Science 256, 193-197.
Irwin, D.E., Bensch, S. and Price, T.D. (2001) Speciation in a ring. Nature. 409, 333-337.
Mayden, R. L. (1997) A hierarchy of species concepts: the denouement in the saga of the species problem. Methodology. 54, 381-424.
Mayr, E. (1942) Systematics and the origin of species from the viewpoint of a zoologist. New York: Columbia University Press. p 334.
McKitrick, M. C. & Zink, R. M. (1988). Species concepts in ornithology. Condor. 90, 1-14.
Mora, C., Tittensor, D. P., Adl, S., Simpson, A. G. B. and Worm, B. (2011) PLoS Biology. 9, e1001127.