A
clade (from
ancient GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
,
klados, "branch") is a term used in modern
alpha taxonomyAlpha taxonomy is the science of finding, describing and categorising organisms, thus leading to the recognition of proposed taxonomic groups, or taxa , which may then be named....
, the scientific classification of living and fossil
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s, to describe a monophyletic group, defined as
a group consisting of a single common ancestor and all its descendants.
[The term "monophyletic group" is used in this article in the conventional sense of "an ancestor and all its descendants." A case has been made that semantically, such groups should simply be referred to as "holophyletic," but this term has not yet acquired widespread use.
]
A
clade (from
ancient GreekAncient Greek is the historical stage in the development of the Greek language spanning across the Archaic , Classical , and Hellenistic periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek...
,
klados, "branch") is a term used in modern
alpha taxonomyAlpha taxonomy is the science of finding, describing and categorising organisms, thus leading to the recognition of proposed taxonomic groups, or taxa , which may then be named....
, the scientific classification of living and fossil
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s, to describe a monophyletic group, defined as
a group consisting of a single common ancestor and all its descendants.
[The term "monophyletic group" is used in this article in the conventional sense of "an ancestor and all its descendants." A case has been made that semantically, such groups should simply be referred to as "holophyletic," but this term has not yet acquired widespread use. For more information, see Holophyletic]
The common ancestor of any group of reasonable size and most of that ancestor's descendants will usually be long extinct.
This may not be true in very small and recent clades, where the "last common ancestor" and all of the ancestor's descendants are still living. It is not necessary for a clade to contain any living representatives.
The context
Ever since
DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
showed that all organisms share common ancestry, taxonomy has consistently attempted to represent and reflect the
evolutionary history of organismsThe evolutionary history of life on Earth traces the processes by which living and fossil organisms evolved. It stretches back over , possibly as far as , and evolution continues, even in humans. All present-day organisms use the same large set of complex chemical reactions, which indicates that...
. The
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...
and
RNARibonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...
analysis used in modern
molecular biologyMolecular biology is the study of biology at a molecular level. The field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...
has greatly helped in illuminating this history by providing large amounts of new phylogenetic information which was previously unavailable to taxonomists. These techniques of study are known as molecular phylogenetics, and they have given rise to the modern disciplines of
cladisticsCladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
and
phylogenetic systematicsA phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the...
.
This new information and new insights have made very clear what the limitations of the old Linnaean system of taxonomy were and are. As a result, many taxonomists are gradually revising in radical ways the taxonomy of the groups that they study. For an example of a taxonomy (in this case a taxonomy of the gastropods) which has been
partially revised in order to incorporate insights from molecular work, see
Taxonomy of the Gastropoda (Bouchet & Rocroi, 2005)Currently the leading system of classifying the gastropods is the taxonomy of the Gastropoda as revised by Bouchet & Rocroi. This system of taxonomy was set forth in a paper entitled "Classification and Nomenclator of Gastropod Families", which was published in the journal Malacologia in 2005 ...
.
Updating taxonomy
The term "clade" did not exist in the older
Linnaean taxonomyLinnaean taxonomy is either# the particular classification of Linnaeus, as set forth in his Systema Naturae and subsequent works...
, which was by necessity based only on
morphologicalIn biology morphology is the form, structure and configuration of an organism.This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs...
similarities between organisms. The concept embodied by the word "clade" does not fit well into the rigid
hierarchyA hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another and with only one "neighbor" above and below each level. These classifications are made with regard to rank, importance, seniority, power status or authority...
that the Linnaean system of taxonomy uses; indeed,
cladisticsCladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
and
Linnaean taxonomyLinnaean taxonomy is either# the particular classification of Linnaeus, as set forth in his Systema Naturae and subsequent works...
are not really compatible.
Linnaean taxonomy demands that all organisms be placed neatly into a rigid, ranked, hierarchy of taxa, such that one individual kind of organism must belong in one of each of the categories:
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
, genus, family, order, class, phylum and kingdom. Because of this necessity to "file things away neatly", the Linnaean system is often very convenient indeed in organizing such things as large museum reference collections; however it does not represent well the process of change that actually happens over evolutionary time.
Because clades can be nested at
any level, they do not have to be neatly slotted into a
rankA ranking is a relationship between a set of items such that, for any two items, the first is either 'ranked higher than', 'ranked lower than' or 'ranked equal to' the second....
in an overall hierarchy. In contrast, the Linnaean taxa of "order," "class" etc. must all be used when naming a new taxon. They cannot be avoided, and each one implies a certain (admittedly very poorly defined) level of diversity which is supposed to be equivalent throughout the system.
Species arise by gradual modification, not sudden complete changes or jumps (although, see also "
punctuated equilibriumPunctuated equilibrium is a theory in evolutionary biology which proposes that most sexually reproducing species will experience little evolutionary change for most of their geological history . When evolution occurs, it is localized in rare, rapid events of branching speciation...
"), thus there is no sound biological basis to make a distinction between a species and its "descendant" species. This is another important area where cladistics is more valuable to biologists than Linnaean taxonomy; intermediate taxa can be named according to their relationship to named taxa using the stem group terminology.
For example, the famous fossil organism
ArchaeopteryxArchaeopteryx, sometimes referred to by its German name Urvogel , is the earliest and most primitive bird known...
has a lot of bird-like characteristics, but is not a true bird in the modern sense. It is, in effect, a 'great-aunt' of the group that contains all modern birds and their shared ancestors. The Linnaean system would define the taxon 'Aves' to include all modern birds, and base this taxon on a number of shared characteristics. Since modern birds are not descended from
Archaeopteryx, and
Archaeopteryx has characters which no living birds possess, it could not be included within the existing taxon 'Aves'. The scientists who described the organism had to make a decision: did they erect a new taxon, which would carry no inherent relationship to Aves; or did they modify the characteristics of the existing group to include the unusual fossil? In the case of
Archaeopteryx, the describers opted for the latter option. However, this approach cannot be followed
ad infinitum. Each time a fossil is found to be basal to the birds, this approach would require relaxing still further the criteria for inclusion in 'Aves'. Ultimately, as more transitional forms are found, the definition of 'Aves' would become so broad as to include all dinosaurs; as basal dinosaurs are described, the definition of 'Aves' would have to be extended still further to incorporate all reptiles... and so on - the logical conclusion, in the presence of a complete fossil record, would be that the definition of 'Aves' would be relaxed so far that it would include all life. In cladistic terms, however,
Archaeopteryx can be considered a stem group to the bird clade - it branched off from the bird lineage before the first member of that lineage resembled a true bird.
Defining clade names
Since taxonomy intends to reflect evolutionary relationships, in order to be valid in evolutionary terms a taxon must be monophyletic—that is, it must be a clade. The definition of clades differ somewhat between Linnaean and cladistic nomenclature. Linnaean units are defined by a small number of key traits. Three methods of defining clades have been proposed in cladistics: node-, stem-, and apomorphy-based:
- In node-based naming, taxon name A refers to the least inclusive clade containing X and Y.
- In stem-based naming, A would refer to the most inclusive clade containing X and Y, but not Z.
- In apomorphy (derived feature)-based naming, A would refer to the clade identified by a feature synapomorphic (sharing a derivation) with a feature in specimen (taxon) X. This definition is basically similar to the Linnaean system.
Differences between a Linnaean or an apomorphy-based clade and a node-based one become obvious when the phylogenetic hypothesis changes.
Here is an example comparing the traditional Linnaean approach to a node-based naming definition:
- Suppose that all we want to do is to name a clade ("A"), containing X and Y. In the Linnaean system we would assign all taxa to the relevant categories species, genus, and family, and then designate type species. No explicit reference to the actual phylogeny is made when these categories are used. The clade A would then be accompanied by a short definition of the "defining trait" (apomorphy). The node-based alternative is started with an explicit reference to evolutionary history, and nothing but the clade containing X and Y needs to be named. When the hypothesis of relationship changes, the phylogenetic alternative is cleaner and more explicit about what it refers to.
See also
- Cladistics
Cladistics is a form of biological systematics which classifies living organisms on the basis of shared ancestry...
- Phylogeny
- Paraphyly
In phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all the descendants of that ancestor.- Relation to monophyletic groups :...
- Polyphyly
A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
- Phylogenetic nomenclature
Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
- Binomial nomenclature
The formal system of naming species is called binominal nomenclature , binary nomenclature , or the binomial classification system...
- Crown group
A crown group is the smallest monophyletic group, or "clade", to contain the last common ancestor of all members, and all of that ancestor's descendants...
External links