Q.36 Determine the correctness or otherwise of the following Assertion (a) and Reason (r).
Assertion:UPGMA method produces ultrametric tree.
Reason:Sequence alignment is converted into evolutionary distances in UPGMA method.
(A) Both (a) and (r) are true and (r) is the correct reason for (a)
(B) Both (a) and (r) are true and (r) is not the correct reason for (a)
(C) (a) is true but (r) is false
(D) (a) is false but (r) is true
Correct Answer: (B) Both (a) and (r) are true but (r) is not the correct reason for (a). UPGMA generates ultrametric trees assuming constant evolutionary rates, while distance conversion from alignments serves multiple phylogenetic methods beyond ultrametricity.
Assertion-Reason Breakdown
UPGMA (Unweighted Pair Group Method with Arithmetic Mean) clusters taxa hierarchically from distance matrices, assuming a molecular clock where all tips equidistant from root. This produces ultrametric trees where maximum distance between siblings exceeds root-to-tip distances.
Statement Validity
Assertion (a) true: UPGMA outputs rooted ultrametric dendrograms by averaging distances between clusters iteratively, forcing equal root-to-leaf paths reflecting equal divergence times.
Reason (r) true: Multiple sequence alignments convert to pairwise evolutionary distances (e.g., Jukes-Cantor, Poisson correction) via substitution models, creating input matrices for tree-building algorithms including UPGMA.
Why (r) not causal: Distance conversion represents universal preprocessing, not UPGMA-specific; ultrametricity arises from clustering algorithm and molecular clock assumption, not distance calculation itself.
Option Explanations
Option (A): Incorrect linkage—distance conversion precedes tree construction but ultrametric property emerges from UPGMA’s equal-rate clustering, not matrix generation.
Option (C): Incorrect—reason holds; alignments routinely transform into distances for neighbor-joining, maximum parsimony preprocessing.
Option (D): Incorrect—assertion correct; UPGMA dendrograms satisfy ultrametric inequality: for taxa i,j,k, d(i,j) ≤ max{d(i,k), d(j,k)}.
Phylogenetic Methods Context
UPGMA suits closely related taxa with clocklike evolution but distorts trees under rate variation; neighbor-joining relaxes ultrametricity for additive distances, better reconstructing true topologies from heterogeneous alignments.
