Q.50 In the phylogeny depicted above,
Introduction
Understanding monophyletic, paraphyletic, and polyphyletic groups is very important in
phylogenetic tree analysis. In the given tree, taxa A & B form one clade and C & D form another clade.
Both groups join at a deeper common ancestor.
Tree Structure
X
/ \
Y Y
/ \ / \
A B C D
Observations
- A and B are sister taxa
- C and D are sister taxa
- AB and CD form separate clades
- All four share common ancestor X
Option-wise Explanation
Option (1): ‘AB’ and ‘CD’ are monophyletic at the level of Y
✔ True
A monophyletic group includes a common ancestor and all its descendants.
AB and CD each include all descendants from node Y. Therefore both are monophyletic.
Option (2): ‘AB’ and ‘CD’ are paraphyletic at the level of Y
✖ False
Paraphyletic groups exclude some descendants. Here nothing is excluded, so this statement is incorrect.
Option (3): ‘ABCD’ are paraphyletic at the level of X
✖ False
ABCD includes all descendants of X, so it is monophyletic, not paraphyletic.
Option (4): ‘ABCD’ are polyphyletic at the level of X
✖ False
Polyphyletic groups do not share a recent common ancestor. ABCD share ancestor X, so this is wrong.
Concept Summary Table
| Group Type | Definition | Example in Tree |
|---|---|---|
| Monophyletic | Ancestor + all descendants | AB, CD, ABCD |
| Paraphyletic | Ancestor + some descendants | None |
| Polyphyletic | Unrelated branches grouped together | None |
Exam Trick Tips
- Complete branch → Monophyletic
- Missing members → Paraphyletic
- Mixed unrelated branches → Polyphyletic
Final Conclusion
Only statement (1) is correct.
Final Answer: (1)
