Why are taxonomic ranks not always assigned in the same way by all scientists?

Taxonomic ranks are not always assigned in the same way by scientists because there are no explicit, universal biological characters that define specific ranks like phylum, class, or order. The determination of these ranks often relies on the subjective judgment of taxonomists, leading to varying opinions on how to categorize organisms within the hierarchy.1

The methodology used to classify organisms also significantly influences these assignments, as different schools of thought prioritize different criteria:

• Traditional Evolutionary Taxonomy: This approach aims to represent evolutionary information but allows for subjective decisions, such as grouping organisms based on physical and ecological similarity even when their evolutionary history differs.2
• Phenetics (Numerical Taxonomy): This method classifies organisms based on the total number of shared phenotypic characteristics, regardless of ancestry. It is considered less credible today because defining what constitutes a "character" often requires subjective decisions.32
• Cladistics (Phylogenetic Taxonomy): Currently the dominant approach, this method classifies groups solely based on their most recent common ancestor using shared derived characters, or synapomorphies.32

Because classification is subject to revision as new information is discovered, and because different authorities may interpret evolutionary relationships or morphological data differently, an order in one scientist's classification may be a superorder or class in another's.1