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.

Verified step by step guidance

1. Identify the characteristics of the group consisting of species T, X, Y, and Z. Since they all share a common ancestor T and include all descendants of T in the group, we need to determine the type of taxonomic group they form.
2. Understand the definitions of different taxonomic groups: Monophyletic groups consist of an ancestor and all of its descendants. Paraphyletic groups consist of an ancestor and some, but not all, of its descendants. Polyphyletic groups consist of various organisms that do not share an immediate common ancestor.
3. Analyze the group T, X, Y, and Z in relation to U and V. Since U and V are also descendants of T but are not included in the group, this suggests that the group might be missing some descendants of the common ancestor T.
4. Compare the characteristics of the group with the definitions: The group T, X, Y, and Z does not include all descendants of T (missing U and V), which rules out it being a monophyletic group.
5. Conclude that the group consisting of T, X, Y, and Z, which excludes U and V, forms a paraphyletic group, as it includes the common ancestor and some, but not all, of its descendants.

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.