body plan
A body plan is the basic architectural blueprint shared by a group of organisms — the arrangement of axes, symmetry, segments and major organs. Just as many different houses share the same floor plan of rooms and hallways, a fly, a beetle and a butterfly all share an insect body plan even though their details differ enormously.
Body plans are laid down early in development as the embryo establishes its main axes (head-to-tail, back-to-belly) and divides into regions, each assigned an identity by patterning genes. This early framework constrains everything built afterward, which is why related animals look like variations on a common theme rather than wholly independent designs.
Because the genes that set up these plans are ancient and widely shared, comparing body plans across species reveals deep evolutionary relationships. The great diversity of animal forms arose largely by modifying a modest number of underlying plans rather than inventing entirely new ones from scratch.
Vertebrates from fish to humans share a body plan with a dorsal nerve cord, a segmented backbone and paired appendages, even as fins become limbs and limbs become wings.
One shared plan, endless anatomical variations.
“Body plan” describes a level of organization, not a single gene; it emerges from the combined action of axis-setting signals, segmentation genes and identity genes acting in sequence.