Homology is a term used in comparative anatomy and evolutionary biology in reference to traits of organisms that have a common phylogenetic ancestry, but are now dissimilar in their structure, function, or behavior. Homology is based on the observation that there are basic, repeating patterns in the attributes of organisms, and that these are often due to a shared ancestry. However, depending on the environmental circumstances affecting the evolution of particular species within related groups (i.e., differences in natural selection), the attributes may eventually adopt differing forms. Nevertheless, their essential homology is retained, and this can often be discerned during earlier stages of development.
The study of homologies has been an important aspect of biology since the nineteenth century, and numerous examples have been identified in various groups of both closely and distantly related organisms. For example, the three tiny bones of the middle ear of humans (these are known as the malleus, incus, and stapes) are ultimately derived from certain jaw-bones of early ancestors of our vertebrate phylogeny. Similarly, the specific facial muscles which humans use to frown and smile are derived from muscles used by ancient fishes in our evolutionary lineage to pump water through their gills.
Another case of homology is the wings of birds, which are derived from the forelimbs of their ancient, reptilian ancestors. A botanical example involves certain elements of the floral structures of higher plants, such as petals and bracts, which are derived from the modified leaves of their ancestors. In all of these examples of homologies, structures occurring in ancient ancestors became evolutionarily modified through natural selection into dissimilar, but homologous structures.
As such, homology is different from analogy. The latter term refers to a similarity of the attributes of organisms (i.e., of their structure, function, or behavior), occurring because of convergent evolution, rather than through a common ancestry. Examples of analogous structures are the wings of flying insects, birds, and bats, all of which are rather similar in their shape and function, but are derived from different anatomical elements. Another case involves the drought-adapted morphology of certain non-related groups of plants that live in desert ecosystems, such as species in the cactus and spurge families (Cactaceae and Euphorbiaceae, respectively). These may have a similarly tubular or barrel-shaped stem, be armed with sharp spines, lack leaves, have a thick cuticle to conserve water, and have green stem tissues capable of photosynthesis. However, these plants of different families look similar because of convergent evolution, and not because they are closely related.
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