Diversity of Organisms · Paper 1 — Free Biology HL Practice Questions

A rooted phylogenetic tree of the three domains is constructed using substitutions in the SSU rRNA gene. The root is placed on the branch leading to Bacteria, which has the greatest number of substitutions from the root. Archaea and Eukarya diverge from a node that is closer to the root than the node separating any other pair of domains. Which statement is supported by this tree?

A sediment sample from a deep-sea hydrothermal vent contains DNA from three previously unknown prokaryotic species. Sequencing of their small subunit ribosomal RNA (SSU rRNA) genes shows that Species X shares 95% nucleotide similarity with Archaeoglobus fulgidus (domain Archaea), Species Y shares 88% similarity with Thermotoga maritima (domain Bacteria), and Species Z shares 78% similarity with both reference sequences. Using the three-domain system and the principle that SSU rRNA similarity reflects evolutionary relatedness, which conclusion is most strongly supported?