Question

11. Distinguish between a monophyletic, paraphyletic, and polyphyletic taxa. Give an example of each. How does the concept of a clade relate to these concepts.

Answer 1

A monophyletic taxon is defined as one that includes the most recent common ancestor of a group of organisms, and all of its descendents.

A polyphyletic taxon is defined as one that does not include the common ancestor of all members of the taxon.

A paraphyletic taxon as one that includes the most recent common ancestor, but not all of its descendents.

Well-known example of monophyletic taxa include Mammalia and Aves, 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.

A clade is a group of organisms that consists of a common ancestor and all its lineal descendants and points towards convergent evolution. Monophylectic taxa have the most common ancestors, while polyphylectic taxa include the least common acestors among their members.