Paraphyly

Paraphyly is a term used in phylogenetics, the study of evolutionary relationships among species. It refers to a group of organisms that includes an ancestor but not all of its descendants. This is in contrast to a monophyletic group, which includes an ancestor and all of its descendants, and a polyphyletic group, which does not include the common ancestor.

Definition

The concept of paraphyly was developed in the context of cladistics, a method of classifying organisms based on their evolutionary relationships. A paraphyletic group is defined as a group that includes the most recent common ancestor of all its members, but excludes some of the descendants of that ancestor. This can occur when a subgroup of organisms evolves distinct characteristics and is classified as a separate group.

Examples

A classic example of a paraphyletic group is the reptiles. In traditional classifications, reptiles include lizards, snakes, turtles, and crocodiles, but exclude birds. However, birds are descended from a subgroup of dinosaurs, which are considered reptiles. Therefore, if birds are excluded, the group is paraphyletic.

Another example is the group of fish. This group is considered paraphyletic because it excludes the tetrapods (amphibians, reptiles, birds, and mammals), which are descended from a subgroup of fish.

Implications for Classification

The recognition of paraphyletic groups has important implications for biological classification. In the Linnaean system, which is based on morphological characteristics, many traditional groups are paraphyletic. However, in cladistic classifications, only monophyletic groups (also known as clades) are recognized.

This has led to some controversy in taxonomy. Some taxonomists argue that paraphyletic groups are unnatural and should be eliminated from classifications. Others argue that paraphyletic groups are useful for describing patterns of evolution and should be retained.

See also

• Monophyly
• Polyphyly
• Cladistics
• Phylogenetics
• Evolution