In common cladistic usage, a monophyletic group is a taxon (group of organisms) which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly. It is contrasted with the terms paraphyly, which is a taxon consisting of an ancestor and some of its descendants, and polyphyly, which is a taxon that does not share a common ancestor. Monophyletic groups are characterized by shared derived characters (synapomorphies).
However, this definition of the term took some time to be accepted. When the cladistic school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, for some time taxonomists would use the term without even defining it, leading to great confusion in the early literature.[1]
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On the broadest scale, definitions fall into two groups.
This incompatibility with the Linnean model led to an initial rift, not entirely healed, between the cladistic and Linnean 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.[4]
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.[4] 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 smaller constituent clade – for instance "Reptiles minus birds". 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.[4] 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[4] - 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. For instance, Archaeopteryx appears more reptilian than bird – it has teeth and a number of other reptilian characteristics. But it also has feathers, which have traditionally been considered as an avian trait. It lacks a number of other traits shared by all birds, so can't fall within the bird clade. To reflect this phylogenetic proximity, it is termed a 'stem group bird' - i.e. it lies on a branch close to the lineage that led to true birds, as recognised by a taxonomist. This concept closes the gap between taxonomy and cladistics at a broader scale,[6] but is difficult to apply at a species-level resolution.
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Monophyletic means common descent from a single ancestor. Biologists have introduced a taxonomy. If there is a group, that is made of a common ancestor (or parent), and all its descendants (children), they call that group monophyletic (Greek: "of one race").
A group which contains organisms that have not descended from a common ancestor is called polyphyletic. If a goup contains the common ancestor, but leaves out some of its descendants, it is called paraphyletic. All living things on this planet are thought, on the basis of DNA analysis, to be monoplyletic, that is, to have one origin, not many.
Example: all organisms in the genus Homo are inferred to have come from the same ancestral form in the family Hominidae, and no other descendants are known. Thus the genus Homo is monophyletic. If, on the other hand, it were discovered that Homo habilis had developed from a different ancestor than Homo sapiens, and this ancestor were not included in the genus, then the genus would be polyphyletic. Since biologists by and large tend to prefer monophyletic groups, in this case, they would likely either split the genus or broaden it to include the additional forms.
Example of polyphyletic group: Sea slugs.
Sometimes taxonomists are frustrated fitting plant species into a monophyletic group because of polyploidy. There is clear evidence that some polyploid plant species are of multiple origins (the species has arisen more than once). For example Salsify, the hybrid goat's-beard, (Tragopogon miscellus) has formed as many as 20 times in eastern Washington.[1]
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