What is a Species?

The meaning of species

The concept of a species is of pivotal importance in biological science and in simple terms means “types of organisms”. Man has undoubtedly been familiar with the notion that there are “kinds” of animals since prehistoric times, and this view, known as the morphological species, dovetailed neatly with Platonic Essentialism, which states that everything existing in our world is derived from an “ideal” form or “essence”: “essence of cat”, “essence of dog”, etc, which exist in a higher plane of reality to our imperfect world. For centuries it was a central dogma that species were God-given and immutable, a positive hindrance to understanding evolution, or even accepting that it takes place. An advance on the morphological species concept was the biological species concept, which was formalised by John Ray (1628-1705), an English naturalist who proclaimed that “one species could never spring from the seed of another”. Again, though, it would certainly have been understood in prehistoric times when agriculture was adopted in many parts of the world at the end of the last ice age, if not tens of millennia before.

The morphological and biological species concepts are today still the species concepts with which the general public are most familiar, but despite its central role, there is no consensus among biologists as to how “species” should be defined.

Species concepts

The following definitions are only some of those in current use, but they are probably the most widely-used by biologists:

Biological or Isolation species. Actually or potentially interbreeding populations which are reproductively isolated from other such populations (Mayr). Stated in another way, species are reproductively isolated groups of populations. The means by which they are isolated is known as a reproductive isolation mechanism or RIM. There are two types of RIM – pre-mating (which prevents the animals from mating) and post-mating (where the offspring are either not viable or infertile). Currently the most commonly-used concept, but it is only useful for sexually-reproducing organisms, and not useful when considering extinct organisms.

Specific mate recognition species. Members sharing a specific mate recognition system to ensure effective syngamy within populations (Patterson); focuses on pre-mating RIMs.

Phylogenetic species. The smallest diagnosable cluster of individual organism (that is, the cluster of organisms are identifiably distinct from other clusters) within which there is a parental pattern of ancestry and descent (Cracraft).

Evolutionary species. A lineage evolving separately from others and with its own unitary evolutionary role, tendencies and historical fate (Simpson).

All members [of a species] have the same number of chromosomes, and every location along the length of a chromosome has its exact opposite number in the same position along the length of the corresponding chromosome in all other members of the species (Dawkins, 1986). Only useful if genetic material is available and necessitates sequencing entire genomes to apply in practice!

Unfortunately none of the above concepts are wholly satisfactory. Dawkins’ is the most precise but has the problems noted above. Even the biological species definition has its envelope pushed when considering, for example, fertile hybrid big cats, bears and dolphins, all of which are very occasionally encountered in the wild. Possibly the definition of a RIM should be extended split the post-mating RIM into a “strong” version (non-viable or infertile offspring) and a “weak” one (in which the offspring are at a selective disadvantage due to hybrid behaviour, etc, albeit fertile).

(The so-called “ring” species concept is now dubious with the classic circumpolar herring gull complex having recently been shown not to be a ring species (Liebers, de Knijff & Helbig, 2004)).

Nomenclature

In scientific classification, a species is assigned a binomial or two-part name in Latin. The genus is listed first (with its leading letter capitalized), followed by a specific name (which should always be in lower case, even if the root is a proper noun, e.g. neanderthalensis). The binomial should be italicised. For example, humans belong to the genus Homo, and are in the species Homo sapiens. Genus is used to group closely-related animals in genera – e.g. (common) chimpanzees (Pan troglodytes) and pygmy chimpanzees or bonobos (Pan paniscus). The genus may be abbreviated to the initial letter, e.g. E. coli for Escherichia coli.

When an unknown species of known genera is being referred to, the abbreviation "sp." in the singular or "spp." in the plural may be used in the place of the second part of the scientific name.

This binomial nomenclature, and most other purely formal aspects of the biological codes of nomenclature, were formalized by the Swedish naturalist Karl von Linné, usually known as Carolus Linnaeus, or simply Linnaeus (1707-1778) in the 18th Century and as a result are called the "Linnaean system" (although binomial nomenclature was actually introduced much earlier, by Gaspard Bauhin (1560-1624), but it failed to gain popularity).

Subspecies

Subspecies are segments of a species that differ morphologically to some degree from other such segments, but still meet other criteria of being a single species. The 75% rule may be used: 75% of individuals classified in one subspecies are distinguishable from all the members of other subspecies within the same species. Subspecies are geographic by nature and cannot by definition ever be sympatric, i.e. occupying the same geographical range.

In the past, numerous subspecies were recognised. Many have since been found to merely represent samples taken at different points on a cline (gradual change across a geographical range), and have largely been discarded. However some species can genuinely be divided into subspecies, such as the lion (Panthera leo), which shows considerable variation in mane appearance, size and distribution. Groves (1988) mentions four subspecies – the North African lion; the Asian lion; the Common African lion and the Cape lion.

If subspecies of a species are recognized, it is said to be polytypic; if there are none it is said to be monotypic. The scientific name of a subspecies is a trinomen, which is the binomen followed immediately by a subspecific name, e.g. Homo sapiens idaltu. If there is a need for subspecific taxa in animal nomenclature, a trinomen may be described for each subspecies. Note that if subspecies are recognised, there must be at least two. A species cannot have a single subspecies.

© Christopher Seddon 2008