Structures with a common evolutionary origin, even if they now appear quite different in different organisms, are called homologous structures (or homologies). The homology of superficially different structures can be demonstrated by one or more of the following criteria:

(1) Similarity of anatomical construction
(2) Similarity of location relative to other structures
(3) Similarity of embryonic development.

The wing of a bird, forelimb of a cat, and flipper of a whale fulfill all three criteria, because they have similar arrangements of bone and muscle, are in similar regions of the body, and develop from similar embryonic primordia. All 3 structures are modifications of the same basic structure. The pectoral fin of a perch lacks some of the structural similarity, but is in a similar place and develops in much the same way, and so is probably homologous, but at a different level, with the other 3 structures. You have seen many examples of homology this semester, from the homology of a petal and a stamen to the homology of arthropod mouthparts and legs. Some structures, however, are similar in many respects (form, function, location, etc.) but are not homologous. For example, on the basis of embryological evidence, mandibles of insects and crustaceans were derived independently, perhaps from the appendages of different primitive segments. If so, the mandibles of the two groups are analogous. That is, they are similar in function and thus perhaps in general form, but not in evolution or development - they do not derive from a common structure.

(A) The Uses of Homologous Structures to Biology

Analysis of homologies is fundamental in evolutionary biology. One application of homology is in systematics, where homologous structures are used as evidence of common descent. The existence of homologous forelimbs in land vertebrates indicates their phylogenetic similarity, as does the existence of strobili in higher plants, and ctenidia in molluscs. Lack of homology between the mandibles of insects and crustaceans argues against their being a monophyletic group.

Even more significant, analysis of homology is instrumental in learning how structures evolve. A "new" structure (like a flower, a pair of wings, mandibles, or legs) may be quite adaptive for the organism bearing that structure, but very often the structure itself is not totally new. Instead, the function may be new, but the structure is simply a modification of a pre-existing structure that had served a different function. Flowers are strobili whose function has changed primarily from protection of sporangia to include advertisement for animal pollinators. Similarly, insect wings may be modified thermoregulatory structures, the filter-feeding structures of bivalve molluscs are modified gills, and head appendages of arthropods are modified legs. Such changes in function may involve considerable luck. Sometimes existence of a structure in the right place may predispose it to a change in function.

The Chordata (which is, by the way, our own phylum) exhibits an interesting, sometimes amazing, series of homologies. In this laboratory, you will examine the basic characteristics of chordates, and then study the manner in which several of these characteristics have become modified in the evolutionary radiation of the phylum.