Biological Classification and Systematics

The Linnaean classification

Scientific classification or biological classification is how species both living and extinct are grouped and categorized. Man’s desire to classify the natural world seems to be very deep rooted and the fact that many traditional societies have highly sophisticated taxonomies suggests the practice goes back to prehistoric times. However the earliest system of which we have knowledge was that of Aristotle, who divided living organisms into two groups – animals and plants. Animals were further divided into three categories – those living on land, those living in the water and those living in the air, and were in addition categorised by whether or not they had blood (those “without blood” would now be classed as invertebrates). Plants were categorised by differences in their stems.

Aristotle’s system remained in use for hundreds of years but by the 16th Century, man’s knowledge of the natural world had reached a point where it was becoming inadequate. Many attempts were made to devise a better system, but the science of biological classification remained in a confused state until the time of Linnaeus, who published the first edition of his Systema Naturae in 1735. In this work, he re-introduced Gaspard Bauhin’s binomial nomenclature and grouped species according to shared physical characteristics for ease of identification. The scheme of ranks, as used today, differs very little from that originally proposed by Linnaeus. A taxon (plural taxa), or taxonomic unit, is a grouping of organisms. A taxon will usually have a rank and can be placed at a particular level in the hierarchy.

The ranks in general use, in hierarchical order, are as follows:

Domain
Kingdom
Phylum (animals or plants) or Division (plants only)
Class
Order
Cohort
Family
Tribe
Genus
Species

The prefix super- indicates a rank above; the prefix sub- indicates a rank below. The prefix infra- indicates a rank below sub-. For instance:

Superclass
Class
Subclass
Infraclass

Even higher resolution is sometimes required and divisions below infra- are sometimes encountered, e.g. parvorder. Domains are a relatively new grouping. The three-domain system (Archaea, Bacteria and Eukaryota) was first proposed in 1990 (Woese), but not generally accepted until later. Many biologists to this day still use the older five-kingdom system (Whittaker). One main characteristic of the three-domain system is the separation of Archaea and Bacteria, previously grouped into the single prokaryote kingdom Bacteria (sometimes Monera). As a compromise, some authorities add Archaea as a sixth kingdom.

It should be noted that taxonomic rank is relative, and restricted to the particular scheme used. The idea is to group living organisms by degrees of relatedness, but it should be bourn in mind that rankings above species level are a bookkeeping idea and not a fundamental truth. Groupings such as Reptilia are a convenience but are not proper taxonomic terms. One can become too obsessed with whether a thing belongs in one artificial category or another – e.g. is Pluto a planet or (closer to home) does habilis belong in Homo or Australopithecus; does it really matter if we lump the robust australopithecines into Australopithecus or split them out into Paranthropus?

Systematics

Systematics is the study of the evolutionary relationships between organisms and grouping of organisms. There are three principle schools of systematics – evolutionary taxonomy (Linnaean or “traditional” taxonomy), phenetics and cladistics. Although there are considerable differences between the three in terms of methodologies used, all seek to determine taxonomic relationships or phylogenies between different species or between different higher order groupings and should, in principle, all come to the same conclusions for the species or groups under consideration.

Some Terminology and concepts

One of the most important concepts in systematics is that of monophyly. A monophyletic group is a group of species comprising an ancestral species and all of its descendants, and so forming one (and only one) evolutionary group. Such a group is said to be a natural group. A paraphyletic group also contains a common ancestor, but excludes some of the descendants that have undergone significant changes. For instance, the traditional class Reptilia excludes birds even though they evolved from an ancestral reptile. A polyphyletic group is one in which the defining trait evolved separately in different places on the phylogenetic tree and hence does not contain all the common ancestors, e.g. warm-blooded vertebrates (birds and mammals, whose common ancestor was cold-blooded). Such groups are usually defined as a result of incomplete knowledge. Organisms forming a natural group are said to form a clade, e.g. the amniotes. If however the defining feature has not arisen within a natural group, it is said to be a grade, e.g. flightless birds (flight has been given up by many unrelated groups of birds).

Characters are attributes or features of organisms or groups of organisms (taxa) that biologists use to indicate relatedness or lack of relatedness to other organisms or groups of organisms. A character can be just about anything that can be measured from a morphological feature to a part of its genetic makeup. Characters in organisms that are similar due to descent from a common ancestor are known as homologues and it is crucial to systematics to determine if characters under consideration are indeed homologous, e.g. wings are homologous if we are comparing two birds, but if a bird is compared with, say, a bat, they are not, having arisen through convergent evolution, a process where structures similar in appearance and function appear in unrelated groups of organisms. Such characters are known as homoplasies. Convergences are not the same as parallelisms which are similar structures that have arisen more than once in species or groups within a single extended lineage, and have followed a similar evolutionary trajectory over time.

Character states can be either primitive or derived. A primitive character state is one that has been retained from a remote ancestor; derived character states are those that originated more recently. For example the backbone is a defining feature of the vertebrates and is a primitive state when considering mammals; but the mammalian ear is a derived state, not shared with other vertebrates. However these things are relative. If one considers Phylum Chordata as a whole, the backbone is a derived state of the vertebrates, not shared with the acrania or the tunicates. If a character state is primitive at the point of reference, it is known as a pleisiomorphy; if it is derived it is known as an apomorphy (note that “primitive” trait in this context does not mean it is less well adapted than one that is not primitive).

Current schools of thought in classification methodology

Biologists devote much effort to identifying and unambiguously defining monophyletic taxa. Relationships are generally presented in tree-diagrams or dendrograms known as phenograms, cladograms or evolutionary trees depending on the methodology used. In all cases they represent evolutionary hypotheses i.e. hypotheses of ancestor-descendant relationships.

Phenetics, also known as numerical taxonomy, was developed in the late 1950s. Pheneticists avoid all considerations of the evolution of taxa and seek instead to construct relationships based on overall phenetic similarity (which can be based on morphological features, or protein chemistry, or indeed anything that can be measured), which they take to be a reflection of genetic similarity. By considering a large number of randomly-chosen phenotypic characters and giving each equal weight, then the sums of differences and similarities between taxa should serve as the best possible measure of genetic distance and hence degree of relatedness. The main problem with the approach is that it tends to group taxa by degrees of difference rather than by shared similarities. Phenetics won many converts in the 1960s and 1970s, as more and more “number crunching” computer techniques became available. Though it has since declined in popularity, some believe it may make a comeback (Dawkins, 1986).

By contrast, cladistics is based on the goal of producing testable hypotheses of genealogical relationships among monophyletic groups of organisms. Cladistics originated with Willi Hennig in 1950 and has grown in popularity since the mid-1960s. Cladists rely heavily on the concept of primitive versus derived character states, identifying homologies as pleisiomorphies and apomorphies. Apomorphies restricted to a single species are referred to as autapomorphies, where as those shared between two or more species or groups are known as synapomorphies.

A major task for cladists is identifying which is the pleisiomorphic and which is the apomorphic form of two character states. A number of techniques are used; a common approach is outgroup analysis where clues are sought to ancestral character states in groups known to be more primitive than the group under consideration.

In constructing a cladogram, only genealogical (ancestor-descendent) relationships are considered; thus cladograms may be thought of as depicting synapomorphy patterns or the pattern of shared similarities hypothesised to the evolutionary novelties among taxa. In drawing up a cladogram based on significant numbers of traits and significant numbers of taxa, the consideration of every possibility is beyond even a computer; computer programs are therefore designed to reject unnecessarily complex hypotheses using the method of maximum parsimony, which is really an application of Occam’s Razor.

The result will be a family tree – an evolutionary pattern of monophyletic lineages; one that can be tested and revised as necessary when new homologues and species are identified. Trees that consistently resist refutation in the face of such testing are said to be highly corroborated.

A cladogram will often be used to construct a classification scheme. Here cladistics differs from traditional Linnaean systematics. Phylogeny is treated as a genealogical branching pattern, with each split producing a pair of newly-derived taxa known as sister groups (or sister species). The classification is based solely on the cladogram, with no consideration to the degree of difference between taxa, or to rates of evolutionary change.

For example, consider these two classification schemes of the Phylum Chordata.

Classification Scheme A (Linnaean):

Phylum Chordata
Subphylum Vertebrata (vertebrates)
Superclass Pisces (fish)
Class Amphibia (amphibians)
Class Reptilia (turtles, crocodiles, snakes and lizards)
Class Mammalia (mammals)
Class Aves (birds)

Classification Scheme B (Cladistic):

Phylum Chordata
Subphylum Vertebrata
Superclass Tetrapoda
Subclass Lissamphibia (recent amphibians)
Superclass Amniota
Class Mammalia (mammals)
Class Reptilomorpha
Subclass Anapsida (turtles)
Subclass Diapsida
Infraclass Lepidosaura (snakes, lizards, etc)
Infraclass Archosauria
Order Crocodilia (crocodiles, etc)
Class Aves (birds)

In Scheme A, crocodiles are grouped with turtles, snakes and lizards as “reptiles” (Class Reptilia) and birds get their own separate grouping (Class Aves). This scheme considers physical similarities as well as genealogy; but the result is the scheme contains paraphyletic taxa. Scheme B strictly reflects cladistic branching patterns; the reptiles are broken up, with birds and crocodiles as a sister group Archosauria (which also included the dinosaurs). All the groupings in this scheme are monophyletic. It will be noted that attempts to append traditional Linnaean rankings to each group runs into difficulties – birds should have equal ranking with the Crocodilia and should therefore be also categorised as an order within the Archosauria; not their own class, as is traditional.

Traditional Linnaean systematics, now referred to as evolutionary taxonomy, seeks to construct relationships on basis of both genealogy and overall similarity/dissimilarity; rates of evolution are an important consideration (in the above example, birds have clearly evolved faster than crocodiles); classification reflects both branching pattern and degree of difference of taxa. The approach lacks a clearly-defined methodology; tends to be based on intuition; and for this reason does not produce results amenable to testing and falsification.

© Christopher Seddon 2008

Linnaeus – Princeps Botanicorum

There are very few examples of scientific terminology that have become sufficiently well-known to have become a part of popular culture. The chemical formula for water – H2O – is certainly one; it is so familiar it has even featured in advertisements. Another is the equation E = mc squared – while not everybody knows that it defines a relationship between mass and energy, most will have heard of it and will be aware it was formulated by Albert Einstein.

But the most familiar scientific term of all has to be Homo sapiens – Mankind’s scientific name for himself.

The term was originated by the 18th Century Swedish scientist Carl von Linné (1707-78), better known as Linnaeus, who first formally “described” the human species in 1758. It means (some would say ironically!) “wise man” or “man the thinker”. It is an example of what biologists call the binomial nomenclature, a system whereby all living things are assigned a double-barrelled name based on their genus and species. These latter terms are in turn part of a bigger scheme of classification known as the Linnaean taxonomy, which – as the name implies – was introduced by Linnaeus himself.

Man has been studying and classifying that natural world throughout recorded history and probably much longer. A key concept in classification of living organisms is that they all belong to various species, and this is a very old idea indeed, almost certainly prehistoric in origin. For example, it would have been obvious that sheep all look very much alike, but that they don’t look in the least bit like pigs, and that therefore all sheep belong to one species and all pigs belong to another. Today we refer to organisms so grouped as morphological species.

In addition, the early Neolithic farmers must soon have realised that while a ewe and a ram can reproduce, and likewise a sow and a boar; a ewe and a boar, or a sow and a ram cannot. Sheep and pigs are different biological species, though this definition of a species was not formalised until much later, by John Ray (1628-1705), an English naturalist who proclaimed that “one species could never spring from the seed of another”.

The first attempt at arranging the various species of living organisms into a systematic classification was made by the Greek philosopher Aristotle (384-322 BC), who divided them into two groups – animals and plants. Animals were further divided into three categories – those living on land, those living in the water and those living in the air, and were in addition categorised by whether or not they had blood (broadly speaking, those “without blood” would now be classed as invertebrates, or animals without a backbone). Plants were categorised by differences in their stems.

Aristotle’s system remained in use for hundreds of years but by the 16th Century, Man’s knowledge of the natural world had reached a point where it was becoming inadequate. Many attempts were made to devise a better system, with some notable works being published by Conrad Gessner (1516-65), Andrea Cesalpino (1524-1603) and John Ray (1628-1705).

In addition Gaspard Bauhin (1560-1624) introduced the binomial nomenclature that Linnaeus would later adopt. Under this system, a species is assigned a generic name and a specific name. The generic name refers to the genus, a group of species more closely related to one another than any other group of species. The specific name represents the species itself. For example lions and tigers are different species, but they are similar enough to both be assigned to the genus Panthera. The lion is Panthera leo and the tiger Panthera tigris.

Despite these advances, the science of biological classification at the beginning of the 18th Century remained in a confused state. There was little or no consensus in the scientific community on how things should be done and with new species being discovered all the time, the problem was getting steadily worse.

Step forward Carl Linné, who was born at Rashult, Sweden, in 1707, the son of a Lutherian curate. He is usually known by the Latinised version of his name, Carolus Linnaeus. It was expected that young Carl would follow his father into the Church, but he showed little enthusiasm for this proposed choice of career and it is said his despairing father apprenticed him to a local shoemaker before he was eventually sent to study medicine at the University of Lund in 1727. A year later, he transferred to Uppsala. However his real interest lay in Botany (the study of plants) and during the course of his studies he became convinced that flowering plants could be classified on the basis of their sexual organs – the male stamens (pollinating) and female pistils (pollen receptor).

In 1732 he led an expedition to Lapland, where he discovered around a hundred new plant species, before completing his medical studies in the Netherlands and Belgium. It was during this time that he published the first edition of Systema Naturae, the work for he is largely remembered, in which he adopted Gaspard Bauhin’s binomial nomenclature, which to date had not gained popularity. Unwieldy names such as physalis amno ramosissime ramis angulosis glabris foliis dentoserratis were still the norm, but under Bauhin’s system this became the rather less wordy Physalis angulata.

This work also put forward Linnaeus’ taxonomic scheme for the natural world. The word taxonomy means “hierarchical classification” and it can be used as either a noun or an adjective. A taxonomy (noun) is a tree structure of classifications for any given set of objects with a single classification at the top, known as the root node, which applies to all objects. A taxon (plural taxa) is any item within such a scheme and all objects within a particular taxon will be united by one or more defining features.

For example, a taxonomic scheme for cars has “car” as the root node (all objects in the scheme are cars), followed by manufacturer, model, type, engine size and colour. Each of these sub-categories is known as a division. An example of a car classified in the scheme is Car>Ford>Mondeo>Estate>2.3 Litre>Metallic silver. An example of a taxon is “Ford”; all cars within it sharing the defining feature of having been manufactured by the Ford Motor Company.

The taxonomy devised by Linnaeus, which he refined and expanded over ten editions of Systema Naturae, had six divisions. At the top, as in the car example is the root note, which Linnaeus designated Imperium (Empire), of which all the natural world is a part. The divisions below this were Regnum (Kingdom), Classis (Class), Ordo (Order), Genus and Species.

The use of Latin in this and other learned texts is worth a brief digression. At the time few scientists spoke any contemporary language beyond their own native tongue, but most had studied the classics and so nearly all scientific works were published in Latin, including Sir Isaac Newton’s landmark Philosophiae Naturalis Principia Mathematica (The Philosophy of Natural Mathematical Principles) and Linnaeus’ own Systema Naturae. One notable exception was Galileo’s The Dialogue Concerning the two Chief World Systems, aimed at a wider audience and thus an early example of “popular science” (though it certainly wasn’t very popular with the Inquisition!).

Linnaeus recognised three kingdoms in his system, the Animal kingdom, the Plant Kingdom and the Mineral Kingdom. Each kingdom was subdivided by Class, of which the animal kingdom had six: Mammalia (mammals), Aves (birds), Amphibia (amphibians), Pisces (fish), Insecta (insects) and Vermes (worms). The Mammalia (mammals) are those animals that suckle their young. It is said that Linnaeus adopted this aspect as the defining feature of the group because of his strongly-held view that all mothers should breast feed their on babies. He was strongly opposed to the then-common practice of “wet nursing” and in this respect he was very much in tune with current thinking.

Each class was further subdivided by Order, with the mammals comprising eight such orders, including the Primates. Orders were subdivided into Genera, with each Genus containing one or more Species. The Primates comprised the Simia (monkeys, apes, etc) and Homo (man), the latter containing a single species, sapiens (though Linnaeus initially also included chimpanzees and gibbons).

The Linnaean system did not accord equal status to apparently equal divisions; thus the Mineral Kingdom was ranked below the Plant Kingdom; which in turn sat below the Animal Kingdom. Similarly the classes were assigned ranks with the mammals ranking the highest and the worms the lowest. Within the mammals the Primates received top billing, with Homo sapiens assigned to pole position therein.

This hierarchy within a hierarchy reflected Linnaeus’ belief that the system reflected a Divine Order of Creation, with Mankind standing at the top of the pile and indeed the term “primate” survives to this day as a legacy of that view. It should be remembered that the prevalent belief at the time of Linnaeus was that the Earth and all living things had been produced by God in their present forms in a single act. This view, now known as Creationism, wasn’t seriously challenged until the 19th Century.

Linnaeus’ system was an example of natural theology, which is the study of nature with a view to achieving a better understanding of the works of God. It was heavily relied on by the deists of that time. Deists believe that knowledge of God can be deduced from nature rather than having to be revealed directly by supernatural means. Deism was very popular in the 18th Century and its adherents included Voltaire, Thomas Jefferson and Benjamin Franklin.

Though some were already beginning to question Creationism, Linnaeus was not among them and he proclaimed that “God creates, Linnaeus arranges”. It has to be said that modesty wasn’t Linnaeus’ strongest point and he proposed that Princeps Botanicorum (Prince of Botany) be engraved on his tombstone. He was no doubt delighted with his elevation to the nobility in 1761, when he took the name Carl von Linné.

Linnaeus did have his critics and some objected to the bizarre sexual imagery he used when categorising plants. For example, “The flowers’ leaves…serve as bridal beds which the Creator has so gloriously arranged, adorned with such noble bed curtains, and perfumed with so many soft scents that the bridegroom with his bride might there celebrate their nuptials with so much the greater solemnity…”. The botanist Johann Siegesbeck denounced this “loathsome harlotry” but Linnaeus had his revenge and named a small and completely useless weed Siegesbeckia! In the event Linnaeus’ preoccupation with the sexual characteristics of plants gave poor results and was soon abandoned.

Nevertheless, Linnaeus’ classification system, as set out in the 10th edition of Systema Naturae, published in 1758, is still is considered the foundation of modern taxonomy and it has been modified only slightly.

Linnaeus continued his work until the early 1770s, when his health began to decline. He was afflicted by strokes, memory loss and general ill-health until his death in 1778. In his publications, Linnaeus provided a concise, usable survey of all the world’s then-known plants and animals, comprising about 7,700 species of plants and 4,400 species of animals. These works helped to establish and standardize the consistent binomial nomenclature for species, including our own.

We have long ago discarded the “loathsome harlotry” and the rank of Empire. Two new ranks have been added; Phylum lies between Kingdom and Class; and Family lies between Order and Genus, giving seven hierarchical ranks in all. In addition, prefixes such as sub-, super-, etc. are sometimes used to expand the system. (The optional divisions of Cohort (between Order and Class) and Tribe (between Genus and Family) are also sometimes encountered, but will not be used here). The Mineral Kingdom was soon abandoned but other kingdoms were added later, such as Fungi, Monera (bacteria) and Protista (single-celled organisms including the well-known (but actually quite rare) Amoeba) and most systems today employ at least six kingdoms.

On this revised picture, Mankind is classified as follows:

Kingdom: Animalia (animals)
Phylum: Chordata (possessing a stiffening rod or notochord)
Sub-phylum: Vertebrata (more specifically possessing a backbone)
Class: Mammalia (suckling their young)
Order: Primates (tarsiers, lemurs, monkeys, apes and humans)
Family: Hominidae (the Hominids, i.e. modern and extinct humans, the extinct australopithecines and, in some recent schemes, the great apes)
Genus: Homo
Species: sapiens

It should be noted that we while we now regard all equivalent-level taxa as being equal, the updated scheme would work perfectly well if we had continued with Linnaeus’ view that some taxa were rather more equal in the eyes of God than others, and it is in no way at odds with the tenets of Creationism. The Linnean Taxonomy shows us where Man fits into the grand scheme of things, but it has nothing to tell us about how we got there. It was left for Charles Darwin to point the way.

© Christopher Seddon 2008

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