From computational biologists to forensic scientists, researchers from a range of disciplines are increasingly relying on phylogenetics--the classification of organisms and DNA sequences based on evolutionary relationships (see Hot Papers | Modern Phylogeneticists Branch Out). But the terminology used to describe this relatedness can be confusing even for evolutionists. Here are five primary definitions.
Clade: A monophyletic group of two or more taxa or DNA sequences. All members of any given clade are descended from a single common ancestor, and all descendents of that ancestor belong to the clade. If some descendents are not included, the group is termed paraphyletic. If the group includes some descendents but not the common ancestor, it is polyphyletic. Many biologists don't recognize paraphylies or polyphylies as valid groups and consider monophyletic groups the only true clades, as defined here. Others use the word clade more loosely and refer to paraphyletic clades and polyphyletic clades, ...
