Polyphyly
Encyclopedia
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. 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 therefore it is frequently a goal to eliminate groups that are found to be polyphyletic. This is often the stimulus for major revisions of the classification schemes. A polyphyletic group can be "fixed" either by excluding clade
s or by adding the common 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. However, many other taxonomists would argue that there is a valid place for groups that are paraphyletic, i.e. contains its most recent common ancestor but does not contain all the descendants of that ancestor.
-based schools of taxonomy
, the existence of polyphyletic groups (as well as paraphyletic groups) in a classification is discouraged. Monophyletic groups (that is, clades) are considered by these schools of thought to be the most important grouping of organism
s, for the following reasons:
For example, the group consisting of warm-blooded
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded. 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
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
together such that every group is descended from a single common ancestor and therefore it is frequently a goal to eliminate groups that are found to be polyphyletic. This is often the stimulus for major revisions of the classification schemes. A polyphyletic group can be "fixed" either by excluding 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...
s or by adding the common 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
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
it should be the aim of classification to ensure that all groups have this property. However, many other taxonomists would argue that there is a valid place for groups that are paraphyletic, i.e. contains its most recent common ancestor but does not contain all the descendants of that ancestor.
Cladistics generally discourages polyphyletic groups
In most cladisticsCladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
-based schools of taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
, the existence of polyphyletic groups (as well as paraphyletic groups) in a classification is discouraged. Monophyletic groups (that is, clades) are considered by these schools of thought to be the most important grouping of organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s, 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". 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". Or "Warm-blooded animals are the Aves clade plus the Mammals clade". Because polyphyletic and paraphyletic groups are defined in terms of clades plus or minus other clades, they are considered less fundamental than monophyletic (single, whole) clades.
- For a given evolutionary tree of, say, N nodes, there are exactly N clades, one per node. However, the number of paraphyletic groups of one clade minus a descendant clade is N2, one for each child of each node. The total number of polyphyletic groups is exponentially larger than that, a power set on the order of 2N. 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 evolutionConvergent evolutionConvergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
) to be polyphyletic, rather than monophyletic, then such a group loses its original significance.
External links
- Funk, D. J., and Omland, K. E. (2003). Species-level paraphyly and polyphyly: Frequncy, cause and consequences, with insights from animal mitochondrial DNA. Annu. Rev. Ecol. Evol. Syst. 34: 397–423.