Serial homology is the development of special features from a structure that occurs in each section of an organism's body. A common example of serial homology is the specialized limb structure of arthropods. Each section of an arthropod body has a pair of appendages. These appendages can develop in the embryo as legs, wings or feelers.
The ability to adapt whole sets of limbs to new challenges is a major part of the success arthropods have enjoyed. From an ancestral form similar to a millipede, insects have developed six legs, two or four wings, sensitive antennae and complex mouth parts.
Vertebrates also demonstrate serial homology, though vertebrate segmentation is less pronounced than it is among arthropods. The serial repetition of vertebral sections, each with its bone, nerve complex and blood vessels, is a good example of this segmentation. Air-breathing vertebrates, known as tetrapods, all have four limbs, two to a section, that have sometimes been modified in ways unique to the species.
Serial homology works to the advantage of organisms in several ways. It makes adding or deleting sections as easy as a single-point mutation to a controller gene. It permits the elaboration of specialized structures in the body, and it even ensures bilateral symmetry, as a single gene shift affects both sides of the section simultaneously.