I admit I don’t really know much about that. But Wikipedia says:
In biology, phylogenetics (/ˌfaɪloʊdʒəˈnɛtɪks, -lə-/)[1][2][3] is the study of the evolutionary history and relationships among or within groups of organisms. These relationships are determined by phylogenetic inference methods that focus on observed heritable traits, such as DNA sequences, protein amino acid sequences, or morphology.
Where morphology is:
Morphology is a branch of biology dealing with the study of the form and structure of organisms and their specific structural features.[1]
This includes aspects of the outward appearance (shape, structure, colour, pattern, size), i.e. external morphology (or eidonomy), as well as the form and structure of the internal parts like bones and organs
Yes, fish would be a morphology that includes gills, fins, streamlined body shape for swimming, etc. But there is no good phylogenetic definition because all the animals we call “fish” do not form a monophyletic group. The monophyletic group encompassing all fish would include also mammals, reptiles including birds, and amphibians because these all have fish ancestors
I admit I don’t really know much about that. But Wikipedia says:
Where morphology is:
It sure seems to me like that should cover gills.
Yes, fish would be a morphology that includes gills, fins, streamlined body shape for swimming, etc. But there is no good phylogenetic definition because all the animals we call “fish” do not form a monophyletic group. The monophyletic group encompassing all fish would include also mammals, reptiles including birds, and amphibians because these all have fish ancestors
The challenge was to come up with a group “which includes both trout and sharks but not humans”. It seems like “gills” satisfies that pretty well.
Animals with gills do not form a monophyletic group.