Homology
The term "homology" was defined in 1843 by Richard Owen, a noted British paleontologist, as the "same organ under every variety of form and function." Thus homologous structures can be defined in an evolutionary context as elements whose similarity in various taxa derives from their common origin in a shared ancestor. Homology may be based on:
- similarities in structure, or how an organ is shaped;
- topography, or the location of an organ;
- associations with other structures, an example of which would be bone-muscle relationships;
- development, including shared expression patterns of homologous genes.
The concept of homology is fundamental to comparative biology and phylogenetics systematics. Homology has historically been defined in terms of inheritance of a structure, with more or less modification, from a common ancestor. In this sense, attributes of two organisms are homologous when they are derived from an equivalent characteristic of the common ancestor. For example, whale flippers, bat wings, and human hands are homologous with respect to one another despite obvious differences in size, structure, and function. Whales, bats, and humans are descendants of a common mammalian ancestor, and their specialized appendages are simply modifications of the ancestral forelimb.
If two or more species have a similar trait that was not inherited from their common ancestor, the traits are said to be homoplastic. For example, insect wings, bird wings, and bat wings are considered to be homoplastic with respect to one another despite that they are all specialized appendages used for flight. The common ancestor of insects and amniotes (terrestrial vertebrates, including mammals and birds) did not have wings. Specialized appendages used in flight have evolved several times independently in the history of metazoans (multicellular animals). The developmental origins and underlying structural patterns are very different in these two groups of organisms. Insects and amniotes acquire wings in different ways during development and, despite similarities in the early development of bats and birds, the common ancestor of birds and bats did not have wings but rather some other type of forelimb. In addition, insect wings are foils made up primarily of chitin, a type of tissue, while bird and bat wings are highly complex with various tissue types organized into different structures such as feathers, skin, bone, blood, muscle, and nerves.
The question of whether an identical trait shared by two or more taxa is the result of homology or homoplasy usually cannot be decided based on a single character alone. In the above examples, one might be misled by grouping organisms based solely on the presence or absence of "wings." Rather, multiple characters are needed to provide an accurate hypothesis of relationships. Whales, bats, and humans have many more traits in common than are shared by bats, birds, and insects despite the fact that the latter three have wings. Therefore, mammals are considered to be a natural group and the forelimbs of mammals are considered homologous, whereas the wings of birds, bats, and insects (an unnatural group whose members do notshare a common, winged ancestor) are considered homoplastic. It is the pattern of relationships among taxa with the trait in question that determines the nature of the similarity.
Phylogenetic Relationships of Major Groups; Phylogenetics Systematics.
Bibliography
Futuyma, Douglas J. Evolutionary Biology. Sunderland, MA: Sinauer Associates, Inc.,1986.
Gould, Stephen J. Ontogeny and Phylogeny. Cambridge, MA: Harvard University Press, Belknap Press, 1977.
McKinney, Michael L. Evolution of Life. Englewood Cliffs, NJ: Prentice Hall, 1993.
Raff, Rudolf A. The Shape of Life. Chicago: University of Chicago Press, 1996.
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