Complex sentence: Listing all these organisms (i.e. providing a full circumscription) requires the full phylogenetic tree to be known; in practice, there are only one or more hypotheses as to the correct tree.
Split into simpler sentences: Listing all these organisms (i.e. providing a full circumscription) requires the full phylogenetic tree to be known. In practice, there are only one or more hypotheses as to the correct tree.