In a phylogenetic tree with four tips — A, B, C, D — where A and B share a node, then that node and C share a node, then that node and D share a node, which two organisms are most closely related?
AA and D, because they are at opposite ends of the tree
BA and B, because they share the most recent common ancestor
CC and D, because they appear closest together in reading order
DB and C, because they are both in the middle of the tree
Relatedness on a phylogenetic tree is determined by the most recent common ancestor, not by position in reading order. A and B share a node directly — their common ancestor is more recent than the common ancestor of any other pair. The visual left-to-right arrangement carries no evolutionary significance.
Question 2 True / False
A species of deep-sea fish and a dolphin both have streamlined, torpedo-shaped bodies. This similarity is strong evidence that they share a recent common ancestor.
TTrue
FFalse
Answer: False
Similar traits can arise independently in distantly related lineages through convergent evolution, driven by similar selective pressures (in this case, aquatic locomotion). Body shape in this example is an analogous character — similar in form and function but not inherited from a shared ancestor. Phylogenies must be built from shared derived characters (synapomorphies) that trace back to a common ancestor, not from any similarity in appearance.
Question 3 Short Answer
What is a synapomorphy, and why do phylogeneticists prefer synapomorphies over general similarities when building trees?
Think about your answer, then reveal below.
Model answer: A synapomorphy is a derived character state shared by two or more taxa that was inherited from their most recent common ancestor. Phylogeneticists prefer them because they specifically mark clades — groups of organisms sharing a single ancestral lineage — whereas general similarity can arise from convergent evolution in unrelated lineages and would produce a misleading tree.
The distinction between ancestral (plesiomorphic) and derived (apomorphic) character states is the foundation of cladistic analysis. A synapomorphy is a shared derived character — meaning it evolved once in a common ancestor and was passed to descendants. Using these markers ensures the tree reflects true genealogical history rather than superficial resemblance caused by adaptation to similar environments.