Problem 4 - Campbell Biology

Question

Three living species \(\mathrm{X}, \mathrm{Y},\) and \(Z\) share a common ancestor \(\mathrm{T},\) as do extinct species \(\mathrm{U}\) and \(\mathrm{V}\) . 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

The group is a monophyletic taxon.

Step 1: Understand the Definitions

First, understand the definitions of the terms given in the options: - Monophyletic: A group of organisms that consists of a common ancestor and all its descendants.- Ingroup: The group of organisms that we are interested in; all other groups are outgroups.- Paraphyletic: A group that includes a common ancestor and some, but not all, of its descendants.- Polyphyletic: A group of organisms that does not include the most recent common ancestor.

Step 2: Analyze the Given Grouping

Examine the grouping species \(\text{T}, \text{X}, \text{Y}, \text{Z}\). Note that U and V, which are extinct, are not included. This means the group includes their common ancestor T and all its living descendants X, Y, and Z.

Step 3: Check for Monophyly

If a group is monophyletic, it includes a common ancestor and all of its descendants. Since T is the common ancestor and X, Y, Z are all T's descendants, this grouping is monophyletic.

Step 4: Verify Other Possibilities

Check the other options:- Ingroup: U as the outgroup is not representative here because U is not included.- Paraphyletic: This would require the exclusion of one or more living descendants.- Polyphyletic: This requires a group without a common ancestor included.

Step 5: Conclude the Correct Answer

Since species T, X, Y, and Z fit the definition of a monophyletic group, none of the other options appropriately describe the given grouping.

Key Concepts

Monophyletic Group

In phylogenetics, a monophyletic group, also known as a clade, is a group of organisms that consists of a common ancestor and all of its descendants. This includes both living and extinct species. For example, the group consisting of species X, Y, Z, and their common ancestor T, fits perfectly into this definition. This is because T is the common ancestor, and X, Y, and Z are all directly descended from it.
Monophyletic groups are important in evolutionary biology because they accurately represent the historical relationships between species. They help scientists understand how different species are related and how traits have evolved over time.

• Example: Birds are a monophyletic group if we include their common ancestor with reptiles.
• Significance: Helps in understanding evolutionary pathways and creating accurate phylogenetic trees.

Paraphyletic Group

A paraphyletic group includes a common ancestor and some, but not all, of its descendants. This means that while the group shares a common ancestor, it leaves out one or more descendant lineages. Using our species X, Y, Z, T, U, and V as an example, if we included T, X, Y, and Z, but excluded U and V, we would have a paraphyletic group.
This is significant because paraphyletic groups can give an incomplete or misleading picture of evolutionary relationships. They sometimes result from classifications based on shared traits rather than true evolutionary history.

• Example: Traditional classification of reptiles (excluding birds).
• Significance: May lead to outdated or inaccurate understanding of evolutionary relationships.

Polyphyletic Group

A polyphyletic group consists of organisms that do not share an immediate common ancestor. Instead, this group is characterized by members that have similar traits due to convergent evolution, not because they inherited them from a common ancestor. If we grouped together species X, Y, and Z, but excluded their common ancestor T, we would form a polyphyletic group.
Polyphyletic groups can be problematic in phylogenetics because they do not accurately represent evolutionary relationships. Scientists aim to avoid these groups in favor of monophyletic groups that provide a clearer picture of evolutionary history.

• Example: Grouping bats and birds based on the ability to fly, despite not sharing a common flying ancestor.
• Significance: Leads to misunderstandings in evolutionary biology and taxonomy.