A: Morphological phylogeny. B: Molecular phylogeny.
In the traditional morphological phylogeny (A), the phylum Platyhelminthes is depicted as a sister taxon to the rest of the protostome phyla and as having diverged earlier from the lineage that led to the rest of the protostomes. In the molecular phylogeny (B), Platyhelminthes is depicted as a lophotrochozoan phylum. What probably led to this change?
A) Platyhelminthes ceased to be recognized as true protostomes.
B) Similarity of genetic information has led to realization that some morphological features are evolutionarily flexible (e.g., coelom formation) and so cannot be used as a diagnostic character.
C) All Platyhelminthes must have a well-developed lophophore as their feeding apparatus.
D) Platyhelminthes' close genetic ties to the arthropods became clear as their Hox gene sequences were studied.

Question

A: Morphological phylogeny.  B: Molecular phylogeny. 
In the traditional morphological phylogeny (A), the phylum Platyhelminthes is depicted as a sister taxon to the rest of the protostome phyla and as having diverged earlier from the lineage that led to the rest of the protostomes. In the molecular phylogeny (B), Platyhelminthes is depicted as a lophotrochozoan phylum. What probably led to this change?
A) Platyhelminthes ceased to be recognized as true protostomes. 
B) Similarity of genetic information has led to realization that some morphological features are evolutionarily flexible (e.g., coelom formation) and so cannot be used as a diagnostic character. 
C) All Platyhelminthes must have a well-developed lophophore as their feeding apparatus. 
D) Platyhelminthes' close genetic ties to the arthropods became clear as their Hox gene sequences were studied.

Quizplus · Accepted Answer

Molecular data showing similarity between Platyhelminthes and other lophotrochozoan phyla led to a reassessment of their phylogenetic position. Morphological traits such as coelom formation are not always reliable as diagnostic characters because they can evolve independently and are not always shared by closely related taxa.

A: Morphological Phylogeny. B: Molecular Phylogeny