Q.24
Identify the taxa that constitute a paraphyletic group in the given phylogenetic tree ?
(A) A, B, C
(B) D, E, F
(C) B, C, D
(D) C, D, E
The correct answer is option (A) – taxa A, B, C form a paraphyletic group in the given phylogenetic tree.
Understanding paraphyletic groups
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A paraphyletic group includes a common ancestor and some but not all of its descendant lineages.
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In contrast, a monophyletic group (clade) contains a common ancestor and all its descendants, while a polyphyletic group combines taxa that do not share an immediate common ancestor.
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Why A, B, C is the correct option (A)
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In the given rooted tree, A, B and C share a more recent common ancestor with each other than with D, E, F or G, so they clearly arise from a single internal node.
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However, that internal node also has descendant lineages leading to D, E, F and G; excluding these makes the set {A, B, C} a paraphyletic group because not all descendants of the common ancestor are included.
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Why option (B) D, E, F is incorrect
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D, E and F also share an internal node, but that node has only two branches: one leading to D and another that splits into E and F, so all descendants of this node are exactly D, E and F.
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Because D, E and F together include the common ancestor and all its descendants, {D, E, F} is a monophyletic clade, not a paraphyletic group, so option (B) is wrong.
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Why option (C) B, C, D is incorrect
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The set {B, C, D} cannot be obtained by taking a single internal node and including all or some of its descendants in one contiguous block on the tree.
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B and C share a closer common ancestor with each other than either does with D, and D is on a different branch; grouping B, C and D together therefore mixes parts of two adjacent clades and forms a polyphyletic or non-natural grouping, not paraphyletic, so option (C) is incorrect.
