A taxon (pl. taxa) is any group of organisms that is given a formal taxonomic name. Loosely, a monophyletic taxon is one that includes a group of organisms descended from a single ancestor , whereas a polyphyletic taxon is composed of unrelated organisms descended from more than one ancestor.
definitions fail to recognize
the fact that all organisms are related,
group is logically "monophyletic". In modern
usage, a monophyletic
taxon is defined as one that includes the most recent common
of a group of organisms, and all of its descendents [as in (a)]. Such
groups are sometimes called holophyletic.
It is also possible to recognize a paraphyletic
taxon as one that includes the most
recent common ancestor, but not all of its
[as in (c)]. A
taxon is defined as one that does
not include the common ancestor of all members
of the taxon [as in (b)].
Well-known monophyletic taxa include Mammalia and Aves (modern birds), recognizable as all furry and feathered vertebrates, respectively. Paraphyletic taxa include Pisces and Reptilia, the former comprising all ray-finned fish but excluding terrestrial descendants of fleshy-finned fish, and the latter comprising all scaly tetrapods but excluding mammals and birds with their modified scales. Polyphyletic taxa once in common usage include Agnatha for jawless lampreys and hagfish, and Insectivora for various toothless, insect-eating mammals such as anteaters and armadillos. Note that these latter groups are defined by 'absence' characters, and that although redwood trees are jawless and toothless, they are not included in those taxa.
Taxonomists tend to fall into two schools, "Evolutionary" or "traditional" systematics versus "Phylogenetic" or "cladistic" systematics. Since the 1970s, "phylogenetic systematics" has been replacing "traditional systematics" Because the older literature and textbooks often use "evolutionary" classifications, the student must understand both systems.
An unfortunate circumstance for the student is that the two schools use the same terms, but in different ways, and often refuse to recognize the alternative usage. Evolutionary taxonomists claim to recognize only "monophyletic" taxa, but use the term to include both holophyletic and paraphyletic taxa. Phylogenetic taxonomists also claim to recognize only "monophyletic" taxa, but limit the term to what is defined above as "holophyletic," although most reject that particular term. Both schools reject the use of polyphyletic taxa, although most phylogenetic taxonomists would use that term to included paraphyletic taxa.
Homework: The diagram above recognizes Taxa 1, 2, & 3 as examples of "monophyletic", "polyphyletic", and "paraphyletic" groups, respectively. (1) In the diagram, identify two additional examples each of groups that would be considered "holophyletic," "polyphyletic," and "paraphyletic". (2) Identify all possible "holophyletic" taxa.