Three living species X, Y, and Z share a common ancestor T, as do extinct species U and V. A grouping that consists of species T, X, Y, and Z (but not U or V) makes up a. a monophyletic taxon. b. an ingroup, with species U as the outgroup. c. a paraphyletic group. d. a polyphyletic group.

Key Concepts

Monophyletic Taxon

A monophyletic taxon includes an ancestor and all of its descendants, forming a complete branch on the tree of life. This concept is crucial for understanding evolutionary relationships, as it reflects a true lineage. In the context of the question, the grouping of species T, X, Y, and Z represents a monophyletic group because it includes the common ancestor T and all its descendants.

Ingroup and Outgroup

In phylogenetics, the ingroup refers to the group of species being studied, while the outgroup is a related species or group that helps to establish the evolutionary relationships. In this question, species X, Y, and Z form the ingroup, and species U serves as the outgroup, providing a reference point for understanding the characteristics and evolutionary history of the ingroup.

Paraphyletic and Polyphyletic Groups

A paraphyletic group consists of an ancestor and some, but not all, of its descendants, while a polyphyletic group includes species that do not share a recent common ancestor. Understanding these definitions is essential for classifying species accurately. In the question, the grouping of T, X, Y, and Z is not paraphyletic or polyphyletic, as it includes all descendants of T.