Table of Contents
The criterion of position is one of the classical tools used to identify whether two structures in different organisms are homologous (same evolutionary origin) or analogous (similar function but independent origin). It focuses strictly on where a structure is located and how it is integrated into the overall body plan.
What “Position” Means in Comparative Anatomy
When biologists talk about the position of a structure, they mean:
- Its location relative to the main body axes (front–back, top–bottom, left–right).
- Its relationship to neighboring organs, bones, muscles, blood vessels, and nerves.
- Its attachment points and sequence within a series of similar elements (e.g., vertebrae, digits, ribs).
The key idea:
If structures in different species occupy the same relative position within a comparable basic body plan, this strongly suggests they are homologous, even if they look quite different or perform different functions.
Using the Criterion of Position
1. Relative Position in the Skeleton
Skeletal elements are especially well-suited for this criterion because they tend to have consistent spatial relationships.
Example: Forelimb bones of vertebrates
In mammals, birds, and reptiles, the main bones of the forelimb follow the same sequence from the body outward:
- One proximal bone: humerus
- Two more distal bones: radius and ulna
- Wrist bones: carpals
- Hand/paw/wing bones: metacarpals
- Fingers/toes: phalanges
A bat’s wing, a human arm, and a whale’s flipper all contain a humerus, radius, ulna, carpals, metacarpals, and phalanges in the same order and attached to the same shoulder girdle elements (scapula, often clavicle). This consistent pattern of position supports their homology as vertebrate forelimbs, despite very different shapes and functions.
By contrast, the wings of insects are located on the thoracic body segments and attach to the exoskeleton, not to an internal bony shoulder girdle. Their position within the body plan is fundamentally different, supporting the conclusion that insect wings and vertebrate wings are not homologous but analogous.
2. Position in Relation to Neighboring Organs
Soft tissues can also be compared by looking at the organs or structures around them.
Example: Gills and related structures in vertebrates
- In fish, gills lie in a series of pouches (gill arches) along the sides of the pharynx, just behind the head, between the mouth and the heart.
- In terrestrial vertebrate embryos, structures in the same region (pharyngeal arches and pouches) give rise to parts of the jaw, middle ear bones, and certain glands.
The shared basic position—lateral to the pharynx, between mouth and heart, arranged in series—indicates homology of the underlying arches, even though adult structures differ in form and function.
3. Position Within Repeated Series (Segmented or Serial Structures)
Many animals have repeated units (segments, vertebrae, ribs, or appendages). In such cases, which unit a structure belongs to also matters.
Example: Vertebrae in mammals
- Cervical vertebrae: situated in the neck, between skull and thorax.
- Thoracic vertebrae: bear ribs, located between cervical and lumbar vertebrae.
- Lumbar vertebrae: behind the ribcage, before the sacrum.
A long-necked giraffe and a human both have seven cervical vertebrae in the neck, in the same relative sequence between the skull and the first rib-bearing vertebra. The identity “cervical vertebra 1, 2, 3…” is defined largely by position, helping identify homology even if vertebrae differ greatly in shape and size.
Example: Arthropod limbs
In crustaceans, insects, and other arthropods, legs and other appendages are attached to specific body segments. Comparing which segment carries which appendage helps determine homology:
- Mouthparts of insects can be traced back to legs attached to the head segments in ancestral forms.
- Even if they are now modified into mandibles or maxillae, their positions on particular head segments, and their relation to other segmental features, reveal their homology with legs of other arthropods.
4. Positional Relationships in Embryos
Embryonic development often displays body plans more clearly and regularly than adult forms. The position of a structure in the embryo can help when adult positions are obscured by growth or specialization.
Example: Limb buds in vertebrate embryos
- In vertebrate embryos, forelimb buds always arise at a particular position along the front–back axis relative to developing organs such as the heart and liver.
- Hindlimb buds appear more posteriorly, near future pelvic structures.
If a limb-like structure appears at the same embryonic level and in the same topological relation to internal organs in different species, this is positional evidence of homology—even if the adult structure is highly altered, such as wings in birds or loss of visible limbs in some reptiles (e.g., some lizards and snakes still show limb-bud–like structures at predictable positions during development).
Distinguishing Homology from Analogy with Position
When Position Supports Homology
A structure is likely homologous to another if:
- It connects to the same skeletal elements in different animals.
- It arises from the same embryonic region.
- It has the same neighbors (e.g., nerves, blood vessels, adjacent organs).
- It occupies the same segment or level along the body axes (for example, always present between the same two vertebrae, or attached to the same body segment).
In such cases, even large differences in shape or function (digging claw vs. flying wing vs. swimming flipper) do not contradict homology.
When Position Suggests Analogy
Two structures are likely analogous (independent origin) when:
- They occupy clearly different regions of the body plan (e.g., external fin folds vs. internal limbs attached to a girdle).
- They are associated with different supporting elements (e.g., different bones, or no bones).
- They arise from different embryonic tissues or from different parts of the embryo’s axes.
Examples:
- The dorsal fin of a fish vs. the arm of a human. Both may help with movement, but the dorsal fin sits on the midline of the back and is supported by fin rays, whereas the arm is attached laterally to a pectoral girdle and built on a limb bone pattern. Their different position and skeletal context indicate analogy, not homology.
- Bird wings vs. insect wings. Both are used for flight, but bird wings are modified forelimbs with bones homologous to other vertebrate limbs, while insect wings are cuticular outgrowths on specific thoracic segments; their positions within the entire body plan differ fundamentally.
Limits and Caveats of the Criterion of Position
The criterion of position is powerful but not infallible. Several complications can arise:
- Shifts in position (translocation): During evolution, structures can move slightly along the body or change their orientation, making comparison less obvious. Detailed study of development and intermediate forms is often needed.
- Fusion and reduction: Neighboring structures may fuse, or parts may be lost. For example, skull bones in vertebrates can fuse into fewer elements; their original positional relationships must be reconstructed to identify homologies.
- Highly derived body plans: In some organisms (e.g., certain parasitic worms), the original segmentation or organ layout is greatly modified. Positional comparisons may be difficult, and other homology criteria (such as specific quality) become more important.
For these reasons, biologists rarely use the criterion of position alone. It is typically combined with other criteria for homology, such as detailed structural similarity and intermediate forms, to draw robust conclusions.
Summary
- The criterion of position evaluates homology based on where a structure is located in the organism’s body plan and its relationship to surrounding structures.
- Consistent relative position across different organisms (same neighbors, same attachment, same segment or axis level) is strong evidence for homology, even when form and function diverge.
- Differences in fundamental position often point to analogy, indicating that similar functions evolved independently.
- While powerful, the criterion of position must be used together with other homology criteria to account for evolutionary changes in body organization.