Which one of the following phylogenetic trees illustrates the relationship between Archaea, Bacteria and Eukarya? The figure shows four candidate cladograms, labelled P, Q, R and S, each grouping the three domains of life in a different branching pattern.
Show Hint
Molecular evidence groups Archaea and Eukarya as sister lineages, with Bacteria branching off earliest.
Step 1: Recall the three-domain system.
Carl Woese's three-domain classification, based on ribosomal RNA sequence comparisons, splits all cellular life into Bacteria, Archaea, and Eukarya. This system remains the standard framework for showing how these three fundamental lineages are related to one another.
Step 2: Recall which two domains are more closely related.
Molecular data (ribosomal RNA sequences, and later comparisons of the machinery for DNA replication, transcription, and translation, along with histone-like proteins) consistently show that Archaea share more derived, informational-processing features with Eukarya than either domain shares with Bacteria. This means Archaea and Eukarya split from a more recent common ancestor, while Bacteria branched off earlier from the base of the tree of life.
Step 3: Translate this into a tree shape.
A correct tree must show Archaea and Eukarya joining together first, at an inner node, before that combined branch joins with Bacteria further down at the root. Bacteria sits as the outgroup, the first lineage to branch off.
Step 4: Match each candidate tree to this rule.
In tree P, Archaea and Eukarya join at an inner node, and Bacteria branches off separately at the root. This matches the correct pattern.
In tree Q, Bacteria and Eukarya are drawn as the closer pair, with Archaea as the outgroup. This is wrong, since it groups Bacteria with Eukarya instead of Archaea with Eukarya.
In tree R, the same incorrect grouping appears, Bacteria and Eukarya joined first and Archaea separate, just drawn with a different branch style.
In tree S, Bacteria and Archaea are grouped together, with Eukarya as the outgroup. This is also wrong, since it fails to place Eukarya next to Archaea.
Final Answer:
Only tree P shows Archaea and Eukarya as the closer pair with Bacteria branching off earliest, which is the accepted relationship among the three domains.
\[ \boxed{P} \]
