The group of "warm-blooded animals" is polyphyletic.

In phylogenetics, a taxon is polyphyletic (Greek for "of many races") if the trait its members have in common evolved separately in different places in the phylogenetic tree. A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Equivalently, a polyphyletic taxon does not contain the most recent common ancestor of all its members.

For example, the group 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. In Biology, a warm-blooded Animal species is one whose members maintain thermal Homeostasis; that is they keep their body temperature at a roughly constant Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Warm-bloodedness evolved separately in the ancestors of mammals and the ancestors of birds, so it is not a true phylogenetic grouping.

Scientific classification aims to group species together such that every group is descended from a single common ancestor, and the elimination of groups that are found to be polyphyletic is therefore a common goal, and is often the stimulus for major revisions of the classification schemes. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. A polyphyletic group can be "fixed" either by excluding clades or by adding the common ancestor. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor

Opinions differ as to whether valid groups need to contain all the descendants of a common ancestor. Groups that do so are called monophyletic, and according to cladistics it should be the aim of classification to ensure that all groups have this property. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor Cladistics is the hierarchical classification of Species based on evolutionary ancestry However, many other taxonomists would argue that there is a valid place for groups that are paraphyletic, i. 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 e. contain only the descendants of a common ancestor, but do not contain all its descendants.

Examples of Polyphyly

• The group of all warm-blooded animals is polyphyletic because it includes birds and mammals, yet descendants of their nearest ancestor include cold-blooded animals, such as reptiles. Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers

• The group of all organisms that use photosynthesis is polyphyletic, because it includes plants and some bacteria, and descendent of their nearest common ancestor include many non-photosynthetic organisms, such as animals. Photosynthesis is a Metabolic pathway that converts Light Energy into Chemical energy.

• The group of flying animals is polyphyletic, because it includes bats (mammals) and birds, and descendents of their nearest common ancestor include non-flying animals such as primates.

Cladistics Generally Discourages Polyphyletic Groups

In most cladistics-based schools of taxonomy, the existence of polyphyletic groups (as well as paraphyletic groups) in a classification is discouraged. Cladistics is the hierarchical classification of Species based on evolutionary ancestry Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos 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 Monophyletic groups (that is, clades) are considered by these schools of thought to be the most important grouping of organisims, for the following reasons:

• Clades are simple to define: a typical clade definition is "All descendants of the nearest common ancestor of species X and Y". A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor On the other hand, polyphyletic and paraphyletic groups are always defined in terms of clades, for example "reptiles are the Sauropsid clade, minus the Aves clade". Reptiles, or members of the class Reptilia are air-breathing Cold-blooded Vertebrates that have skin covered in scales as opposed to hair or feathers Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Or "Warm-blooded animals are the Aves clade plus the Mammals clade". Birds ( class Aves) are bipedal endothermic ( Warm-blooded) Vertebrate animals that lay eggs. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands Because polyphyletic and paraphyletic groups are defined in terms of clades, they are considered less important than clades.

• For a given evolutionary tree of, say, N nodes, there are exactly N clades (one per node). However, the number of paraphyletic groups and polyphyletic groups is exponentially larger than that, on the order of N². Yet only a small fraction of the paraphyletic groups are given names or discussed.

• Polyphyletic groups often have their origin in traditional taxonomy, based on similar morphological characteristics. The original perception may have been that the group was entirely descended from a single ancestor. If such a group is later discovered (for instance, due to convergent evolution) to be polyphyletic, rather than monophyletic, then such a group loses its original significance. Convergent evolution describes the acquisition of the same biological trait in unrelated lineages

References

• Colin Tudge (2000). Colin Tudge (born 22 April 1943) is a British science writer A biologist by training he is the author of numerous works on food agriculture genetics 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. The Variety of Life. Oxford University Press. ISBN 0198604262.