15. Which of the following can be reasonably inferred based on this phylogeny of
four species?
a. The ancestral species at the root of the tree most closely resembles A
b. Species B arose after species A
c. A is equally related to both C and D
d. Since A is an outgroup, it is unrelated to B, C, and D

The correct answer is Option C: A is equally related to both C and D. This is because in the given phylogenetic tree, A branches off earlier as an outgroup, while C and D are sister taxa within the ingroup; the common ancestor that A shares with C is the same ancestor it shares with D, so its relatedness to both is equal.​

Introduction

Interpreting a phylogenetic tree correctly is essential for questions on evolutionary relationships, especially when an outgroup such as species A is compared with ingroup species B, C, and D. In this four-species phylogeny, understanding how branching order reflects common ancestry allows accurate inference that A is equally related to both C and D and clarifies why the other options are incorrect.​

Understanding the given phylogeny

In a rooted phylogenetic tree, the pattern of branches (topology) shows relatedness: taxa that share a more recent common ancestor are more closely related than those whose common ancestor is deeper in the tree. An outgroup is a taxon that diverged earlier than the ingroup; it shares an older common ancestor with all ingroup members and is used to root the tree and polarize character changes.​

In the diagram, species A branches off first and functions as the outgroup, whereas B, C, and D form the ingroup with C and D as sister taxa sharing the most recent common ancestor among them.​

Option A: ancestral species most closely resembles A

Statement: “The ancestral species at the root of the tree most closely resembles A.”

• The root represents the most recent common ancestor of all four species, not any particular modern species at the tips.​

• Outgroup A is simply the lineage that diverged earliest; it is not evidence that the root ancestor phenotypically most resembled A, because all lineages have continued to evolve from that ancestor.​

Therefore, Option A is incorrect because tree topology does not indicate that the ancestor looks more like any one extant species, including the outgroup.​

Option B: B arose after A

Statement: “Species B arose after species A.”

• Phylogenetic trees depict relative branching order and shared ancestry, but the tips are usually drawn in a straight vertical line and do not show which extant species is “older” in absolute time.​

• Saying “B arose after A” misinterprets the diagram, because both A and B represent present-day species whose lineages have been evolving since their divergence from a common ancestor; the tree does not show one modern species originating later than another.​

Thus, Option B is incorrect because phylogenies do not rank contemporary species by age of origin in that manner.​

Option C: A is equally related to C and D (correct)

Statement: “A is equally related to both C and D.”

• Relatedness in a phylogeny is determined by how recently taxa share a common ancestor, not by the physical distance of the tips on the page.​

• In this tree, A joins the B–C–D clade at a single, older node; the same ancestral node connects A with C and with D, so the depth of the most recent common ancestor between A–C and A–D is identical.​

Because A shares the same ancestral node with both C and D, it is equally related to C and D, making Option C the correct inference.​

Option D: A is unrelated to B, C, and D

Statement: “Since A is an outgroup, it is unrelated to B, C, and D.”

• An outgroup is less closely related to the ingroup than ingroup members are to each other, but it still shares a common ancestor with them; otherwise it could not be placed on the same tree.​

• The purpose of the outgroup is to provide a reference for rooting and character polarity, not to indicate absence of evolutionary relationship.​

Therefore, Option D is incorrect because an outgroup is related to the ingroup, just via an older divergence.​

Key takeaways for exam preparation

• Outgroups (like species A) branch off earlier and are used to root the phylogeny but remain evolutionarily related to all ingroup taxa.​

• When a single taxon (A) connects to a clade with sister taxa (C and D) via the same ancestral node, that taxon is equally related to each member of that clade, which is the principle tested in this phylogeny question.​