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The term monophyly, or monophyletic, derives from the two Ancient Greek words μόνος (mónos), meaning "alone, only, unique", and φῦλον (phûlon), meaning "genus, species", [4] [5] and refers to the fact that a monophyletic group includes organisms (e.g., genera, species) consisting of all the descendants of a unique common ancestor.
The term paraphyly, or paraphyletic, derives from the two Ancient Greek words παρά (pará), meaning "beside, near", and φῦλον (phûlon), meaning "genus, species", [2] [3] and refers to the situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) are left apart from all other descendants of a unique common ancestor.
It is usually implicitly assumed that species are monophyletic (or at least paraphyletic). However, hybrid speciation arguably leads to polyphyletic species. [12] Hybrid species are a common phenomenon in nature, particularly in plants where polyploidy allows for rapid speciation. [13]
[61] [62] Groups that have descendant groups removed from them are termed paraphyletic, [61] while groups representing more than one branch from the tree of life are called polyphyletic. [61] [62] Monophyletic groups are recognized and diagnosed on the basis of synapomorphies, shared derived character states. [63]
Cladogram (family tree) of a biological group. The green box (central) may represent an evolutionary grade (paraphyletic), a group united by conservative anatomical and physiological traits rather than phylogeny. The flanking red and blue boxes are clades (i.e., complete monophyletic subtrees).
A polyphyletic assemblage is one which is neither monophyletic nor paraphyletic. A polyphyletic assemblage is characterized by one or more homoplasies: character states which have converged or reverted so as to be the same but which have not been inherited from a common ancestor. No systematist recognizes polyphyletic assemblages as ...
Formal classifications based on cladistic reasoning are said to emphasize ancestry at the expense of descriptive characteristics. Nonetheless, most taxonomists presently avoid paraphyletic groups whenever they think it is possible within Linnaean taxonomy; polyphyletic taxa have long been unfashionable.
Under phylogenetic taxonomy, amphibians do not constitute a monophyletic group in that the amniotes (reptiles, birds and mammals) have evolved from an amphibian ancestor and yet are not considered amphibians. Such paraphyletic groups are rejected in phylogenetic nomenclature, but are considered a signal of serial descent by evolutionary ...