Q.75 Carl Woese established that short subunit rRNA sequences can be used to reveal
evolutionary relationships between various organisms. Based on this, which one
of the following options is the established phylogenetic arrangement of the three
domains of life?
(A) I
(B) IV
(C) II
(D) III

The correct option is (D) III. In Woese’s three‑domain system, Eukarya and Archaea are sister groups, sharing a more recent common ancestor with each other than either does with Bacteria.​​

Introduction

Carl Woese revolutionized biological classification by comparing small subunit rRNA sequences and proposing the three‑domain system of life: Bacteria, Archaea and Eukarya. This molecular phylogeny showed that Archaea are more closely related to Eukarya than to Bacteria, overturning the traditional single “prokaryote” group.​

What Woese’s three‑domain tree shows

• All cellular life is grouped into three domains: Bacteria, Archaea and Eukarya.​

• Phylogenetically, Bacteria branch off first, while Archaea and Eukarya form sister clades on a separate branch.​

• This means the last common ancestor of Archaea and Eukarya is more recent than their common ancestor with Bacteria.​

Analysis of the four options

Option I

• In option I, the tree shows Archaea and Bacteria as closer to each other, with Eukarya branching later from them.

• This incorrectly groups Archaea with Bacteria, contradicting Woese’s evidence that Archaea are distinct from Bacteria and more closely related to Eukarya.​

• Therefore, option I is incorrect.

Option II

• Option II places Bacteria and Archaea together as sister groups, with Eukarya branching separately.

• This again suggests that Archaea and Bacteria share a more recent common ancestor with each other than either does with Eukarya, which disagrees with rRNA‑based phylogenies.​

• Hence, option II is also incorrect.

Option III

• In option III, Eukarya and Archaea emerge from a common branch, while Bacteria diverge earlier on a separate branch.

• This topology matches the classic Woese three‑domain tree, where Archaea and Eukarya are sister domains and Bacteria form an earlier, distinct lineage.​

• Therefore, option III correctly represents the established phylogenetic arrangement and corresponds to answer choice (D) III.

Option IV

• Option IV shows Archaea branching off first, with Bacteria and Eukarya as sister groups.

• This implies Bacteria are closer to Eukarya than Archaea are, which conflicts with Woese’s three‑domain hypothesis and standard textbooks.​

• Thus, option IV is incorrect.

Key takeaway about Woese’s phylogeny

• The defining feature of Woese’s three‑domain system is that Archaea and Eukarya are sister domains, distinct from Bacteria, as revealed by small subunit rRNA sequence comparisons.​

• Any tree that does not place Eukarya and Archaea together, with Bacteria on a separate, earlier branch, does not depict Woese’s established phylogenetic arrangement.​