Which node represents the most recent common ancestor of organisms E and F?

Question

BlackTom AI · Accepted Answer

Question context: We are asked to identify which node represents the most recent common ancestor of organisms E and F. In a typical phylogenetic tree, the most recent common ancestor (MRCA) of two taxa is the lowest node that is ancestral to both taxa.

Option 6: This node would be a shared ancestor of E and F only if both lineages connect at or above node 6. If E and F diverged after node 6, then 6 cannot be their MRCA; it would be too ancestral. Without the tree showing a direct connection to both E and F at 6, this option is unlikely to be the MRCA.

Option 1: Similar reasoning applies. If the lineages leading to E and F diverge after 1, then 1 would be a more distant ancestor. The MRCA must be the lowest such common ancestor, so unless E and F only share a lineage back to 1, this would not be the MRCA.

Option 3: This is the correct choice per the provided answer key. Node 3 is positioned as the shared ancestor from which both E and F descend, and it is the lowest node common to both lineages. This satisfies the definition of MRCA, being the most recent node from which both organisms are descended.

Option 7: If E and F diverge after node 7, then 7 is either a more distant ancestor or not common to both lineages at all. The MRCA should be the lowest node that both lineages share; if 7 sits above the divergence point of E and F, it would not be the MRCA.

Summary: The MRCA is the lowest node that is ancestral to both E and F. Based on the standard interpretation of MRCA in a tree and the given answer, node 3 is that most recent common ancestor, while the other nodes either lie above the divergence point or are not common to both lineages.

Which node represents the most recent common ancestor of organisms E and F?单项选择题

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