Homology, Homoplasy, Analogy and Serial Homology

In evolutionary biology the term homology means similarity in origin (implying a common ancestor), homoplasy means similarity in appearance but not in origin, and analogy means similarity in function but not in origin. Thus the wings of different species of birds are homologous as wings, but there is no homology between the wings of a bird and a bat. Wings of bird and bat, considered as wings are merely analogous, but both evolved from parts of the arm, forearm, and hand of four-limbed, five-fingered vertebrates, and as such are homologous as forelimbs (along with seal's fingers and the human upper extremity). The wings of the hummingbird and the huminingmoth, however, are not homologous by any standard (except perhaps as animal appendages); they are analogous as wings.

Evaluation of structures in terms of homology, analogy, and homoplasy can be valuable in determining whether apparently similar organisms have evolved in a convergent or parallel manner, and prior knowledge of the evolutionary history of particular species, aids in identifying homologous and analogous structures. Independent information is also important in making such assessments, but it should be obvious that parallel evolution following divergence of lines tends to result in homologies, while analogies and homoplasy are likely results of convergent evolution.

Serial homology is the similarity of structures between one part of an animal with another, part of the same animal. In this case, two or more structures of a single individual are compared. The arm and leg of man illustrate an instance of serial homology. The humerus bone of the upper arm corresponds to the femur of the leg (thigh). The radius and ulna of the fore arms correspond to the tibia and fibula of the legs (shin). The wrist bones (carpals) may be comparable with ankle bones (tarsal). The bones of the palm of the hand (metacarpals) correspond to the bones of the foot (metatarsals). Finally, the bones of the fingers as well as toes correspond to each other (phalanges). However, the arms of man are suited for grasping and handling, while the legs are used in locomotion.

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