In biological classification, a phylum is best described as

• A. A single species
• B. A group defined by occupying the same habitat
• C. A grouping based on shared genetic sequences only
• D. A major division defined by a basic body plan or major organizational features

Answer: (D)

A phylum is a major taxonomic division defined by a fundamental body plan or organizational features that distinguish large groups of organisms. It groups creatures that share a basic structural blueprint—such as overall body symmetry, tissue organization, type of body cavity, and key anatomical plan—rather than by size, habitat, or simply by genetic similarity. For example, echinoderms and chordates are placed in different phyla because their core body plans are fundamentally different, even though they may share some evolutionary relationships. This level sits above class and below kingdom, capturing broad, structural differences that reflect long-term evolutionary divergence. While genetic data are useful for refining classifications, a phylum is not defined solely by genetics; it is rooted in the prominent, shared body-plan characteristics that define a major line of organisms.