Problem 4 - Campbell Biology

Question

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

Step-by-Step Solution

Answer

C: A paraphyletic group.

Step 1: Understanding Key Terms

Review the definitions of key terms: - Monophyletic group: Includes a common ancestor and all its descendants. - Paraphyletic group: Includes a common ancestor and some, but not all, of its descendants. - Polyphyletic group: Does not include the common ancestor of all members. - Ingroup: The set of organisms being studied. - Outgroup: A species or group outside the ingroup.

Step 2: Identify Relationships in the Problem

Note that species X, Y, and Z, along with T, share a common ancestor, and U and V are also descendants of that ancestor.

Step 3: Determine Characteristics of the Group

Given that the group includes T, X, Y, and Z, but not U or V, evaluate if this group includes all descendants of the common ancestor T.

Step 4: Compare to Definitions

Since the group T, X, Y, and Z does not include all descendants (specifically, U and V are excluded), it does not meet the definition of a monophyletic group. Identify it using other definitions.

Step 5: Choose the Correct Classification

Since a paraphyletic group includes a common ancestor and some but not all of its descendants, and this fits the given group, the grouping is best described as a paraphyletic group.

Key Concepts

monophyletic group

A monophyletic group is an important concept in phylogenetic classification. It includes a common ancestor and all of its descendants. Imagine a family tree where you include a parent and every single child, grandchild, and so on. That's a monophyletic group.
For example, suppose we consider all mammals. They all share a common ancestor, which means they form a monophyletic group. This type of grouping is useful because it reflects true evolutionary relationships.
In the original exercise, the group containing species T, X, Y, and Z is not monophyletic. It excludes species U and V, which are also descendants of the common ancestor T.

paraphyletic group

A paraphyletic group includes a common ancestor and some, but not all, of its descendants. This is like having a family reunion and inviting some, but not all, of your cousins.
In evolutionary terms, a classic example could be the group of reptiles. If you include lizards, snakes, and crocodiles but exclude birds (which also descended from the same ancestor), you're looking at a paraphyletic group.
In the exercise scenario, the group consisting of T, X, Y, and Z fits this description perfectly. It is paraphyletic because it includes the common ancestor (T) and some of its descendants (X, Y, and Z) but leaves out others (U and V). So, the correct classification for this set is a paraphyletic group.

polyphyletic group

A polyphyletic group is a bit different from the previous two. It includes organisms from different ancestors and does not include their most recent common ancestor.
This is akin to grouping animals that can fly, such as bats, birds, and insects, despite their flight ability arising independently and not from a common flying ancestor.
Understanding this concept helps avoid these incorrect groupings, ensuring more accurate evolutionary relationships. The group T, X, Y, and Z from the exercise is not polyphyletic because they all share a common ancestor, making this term inapplicable to the given grouping.

ingroup and outgroup analysis

Ingroup and outgroup analysis is crucial for understanding evolutionary relationships. The 'ingroup' consists of the organisms actively being studied, while the 'outgroup' is a species or group that is closely related but not part of the ingroup.
Using an outgroup helps root the phylogenetic tree correctly. It serves as a reference point and provides context for the evolutionary history of the ingroup.
In the exercise, if T, X, Y, and Z form the ingroup, species U or V could act as the outgroup. This context helps in rooting the tree and understanding the evolutionary pathway accurately.
By analyzing ingroup and outgroup relationships, scientists can better confirm the evolutionary linkages and divergences within a tree.