Monophyly
Encyclopedia
In common cladistic usage, a monophyletic group is a taxon
(group of organisms) which forms a clade
, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly
. Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies
).
Monophyly is contrasted with the terms paraphyly
and polyphyly
, which are most easily understood from the second diagram in this article. In current usage, a paraphyletic group consists of all of the descendants of a possibly hypothetical closest common ancestor minus one or more monophyletic groups (most usually one). A paraphyletic group is thus 'nearly' monophyletic (consistent with the meaning of the prefix 'para', namely 'near' or 'alongside'.) A polyphyletic group is any group other than a monophyletic group or a paraphyletic group, which like a paraphyletic group contains only some of the descendants of their closest common ancestor, but unlike a paraphyletic group is not characterized by the missing descendants forming one (or more) monophyletic groups.
These definitions have taken some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists.
and evolutionary taxonomy
models led to an initial rift, not entirely healed, between the cladistic and more traditional schools of thought. Extreme cladists challenged the validity of Linnean taxa such as the Reptilia. Because birds, although descended from reptiles, are not themselves considered to be reptiles, cladists demanded that the taxon Reptilia be dismantled: a request that taxonomists were unwilling to heed. This stand-off was eventually resolved to a degree by the construction of the term 'paraphyletic' to describe closely related groups which included most but not all of the descendants of a common ancestor.
However, the coining of this term led to yet more confusion. Some scientists considered paraphyletic groups to be monophyletic (as they shared a common ancestor), where others insisted that monophyletic should continue to refer only to holophyletic groups. Another term, polyphyletic, fell outside of the definition of monophyly. A strict explanation of a paraphyletic group has not been published, but the consensus appears to be that paraphyletic groups consist of a monophyletic group, minus one or more smaller constituent clades – for instance the paraphyletic class Reptilia being "Amniote
s minus birds and mammals". Polyphyletic groups can be thought of as a number of unrelated clades, for instance "warm blooded animals" = "birds plus mammals". Non-holophyletic groups are of little use for analysis of evolutionary processes, hence the calls for their "unnaming" - even though they are useful to scientists who are less concerned with the evolutionary past of groups. Naming is also a problem for monophyletic groups: because the number of ancestors from which to root monophyletic groups is almost infinite, giving each clade a unique name is impossible - as illustrated by the failed attempts to instigate a system called the Phylocode
. Names obfuscate the really interesting part, which is the branching order, and are therefore of little utility to the cladist - at odds with the taxonomist, who since the time of Linnaeus
has been naming species.
Intermediate, and particularly fossil, taxa can be considered to fall 'just outside' a widely accepted taxon. Examples are the very tetrapod
-like fossil elpistostegalian
fishes Panderichthys
and Tiktaalik
. The skull and upper finn structures are very similar to those of the first tetrapods, yet the limbs terminated in fins with fin-rays rather than feet with digits. The two are close to, but not members of the group consisting of four footed vertebrates. To reflect this phylogenetic proximity, they are termed termed 'stem group tetrapods' or simply 'stem tetrapods' - i.e. it lies on a branch more closely related to the lineage that led to tetrapods as recognised by a taxonomist than to their nearest living relatives, the lungfish
. This concept closes the gap between taxonomy and cladistics at a broader scale, but is difficult to apply at a species-level resolution.
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
(group of organisms) which forms a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly
Holophyly
Holophyletic is a term posited as a semantically correct replacement for the term monophyletic as used by cladists...
. Monophyletic groups are typically characterized by shared derived characteristics (synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
).
Monophyly is contrasted with the terms paraphyly
Paraphyly
A group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
and polyphyly
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...
, which are most easily understood from the second diagram in this article. In current usage, a paraphyletic group consists of all of the descendants of a possibly hypothetical closest common ancestor minus one or more monophyletic groups (most usually one). A paraphyletic group is thus 'nearly' monophyletic (consistent with the meaning of the prefix 'para', namely 'near' or 'alongside'.) A polyphyletic group is any group other than a monophyletic group or a paraphyletic group, which like a paraphyletic group contains only some of the descendants of their closest common ancestor, but unlike a paraphyletic group is not characterized by the missing descendants forming one (or more) monophyletic groups.
These definitions have taken some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists.
Definitions
On the broadest scale, definitions fall into two groups.- The widest, and arguably the semantically correct meaning of the word, is any two or more groups sharing a common ancestor. This very broad definition strips the term of scientific utility. Therefore, scientists today restrict the term to holophyletic groups only – that is, groups consisting of all the descendants of one (usually hypothetical) common ancestor. However, when considering taxonomic groups such as genera and species, the most appropriate nature of their common ancestor is unclear. Assuming that it would be one individual or mating pair is unrealistic for sexually reproducing species, which are by definition interbreeding populations.
- However, using a broader definition, such as a species and all its descendants, does not really work to define a genus. A satisfactory and comprehensive cladistic definition of a species or genus is in fact impossible, and reflects the impossibility of seamlessly impressing a gradualistic model of continual change over the 'quantum' Linnean model, where species have defined boundaries, and intermediaries between species cannot be accommodated.
Controversy
This incompatibility with the LinnaeanLinnaean taxonomy
Linnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...
and evolutionary taxonomy
Evolutionary taxonomy
Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship and overall similarity. This type of taxonomy considers taxa rather than single species, so that...
models led to an initial rift, not entirely healed, between the cladistic and more traditional schools of thought. Extreme cladists challenged the validity of Linnean taxa such as the Reptilia. Because birds, although descended from reptiles, are not themselves considered to be reptiles, cladists demanded that the taxon Reptilia be dismantled: a request that taxonomists were unwilling to heed. This stand-off was eventually resolved to a degree by the construction of the term 'paraphyletic' to describe closely related groups which included most but not all of the descendants of a common ancestor.
However, the coining of this term led to yet more confusion. Some scientists considered paraphyletic groups to be monophyletic (as they shared a common ancestor), where others insisted that monophyletic should continue to refer only to holophyletic groups. Another term, polyphyletic, fell outside of the definition of monophyly. A strict explanation of a paraphyletic group has not been published, but the consensus appears to be that paraphyletic groups consist of a monophyletic group, minus one or more smaller constituent clades – for instance the paraphyletic class Reptilia being "Amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s minus birds and mammals". Polyphyletic groups can be thought of as a number of unrelated clades, for instance "warm blooded animals" = "birds plus mammals". Non-holophyletic groups are of little use for analysis of evolutionary processes, hence the calls for their "unnaming" - even though they are useful to scientists who are less concerned with the evolutionary past of groups. Naming is also a problem for monophyletic groups: because the number of ancestors from which to root monophyletic groups is almost infinite, giving each clade a unique name is impossible - as illustrated by the failed attempts to instigate a system called the Phylocode
PhyloCode
The International Code of Phylogenetic Nomenclature, known as the PhyloCode for short, is a developing draft for a formal set of rules governing phylogenetic nomenclature...
. Names obfuscate the really interesting part, which is the branching order, and are therefore of little utility to the cladist - at odds with the taxonomist, who since the time of Linnaeus
Carolus Linnaeus
Carl Linnaeus , also known after his ennoblement as , was a Swedish botanist, physician, and zoologist, who laid the foundations for the modern scheme of binomial nomenclature. He is known as the father of modern taxonomy, and is also considered one of the fathers of modern ecology...
has been naming species.
Intermediate, and particularly fossil, taxa can be considered to fall 'just outside' a widely accepted taxon. Examples are the very tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
-like fossil elpistostegalian
Elpistostegalia
Elpistostegalia or Panderichthyida is an order of prehistoric lobe-finned fishes which lived during the Late Devonian period . They represent the advanced tetrapodomorph stock, the fishes more closely related to tetrapods than the osteolepiform fishes...
fishes Panderichthys
Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...
and Tiktaalik
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
. The skull and upper finn structures are very similar to those of the first tetrapods, yet the limbs terminated in fins with fin-rays rather than feet with digits. The two are close to, but not members of the group consisting of four footed vertebrates. To reflect this phylogenetic proximity, they are termed termed 'stem group tetrapods' or simply 'stem tetrapods' - i.e. it lies on a branch more closely related to the lineage that led to tetrapods as recognised by a taxonomist than to their nearest living relatives, the lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
. This concept closes the gap between taxonomy and cladistics at a broader scale, but is difficult to apply at a species-level resolution.