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"Homology and Homoplasy: a Philosophical Perspective"

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Homology and Introductory article Homoplasy: A Article Contents . The Concept of Homology up to Darwin . Adaptationist, Taxic and Developmental Concepts of Philosophical Perspective Homology . Developmental Homology Ron Amundson, University of Hawaii at Hilo, Hawaii, USA . Character Identity Networks Online posting date: 15th October 2012 Homology refers to the underlying sameness of distinct Homology is sameness, judged by various kinds of body parts or other organic features. The concept became similarity. The recognition of body parts that are shared by crucial to the understanding of relationships among different kinds of organisms dates at least back to Aristotle, organisms during the early nineteenth century. Its but acceptance of this fact as scientifically important to biology developed during the first half of the nineteenth importance has vacillated during the years between the century. In an influential definition published in 1843, publication of Darwin’s Origin of Species and recent times. Richard Owen defined homologues as ‘the same organ in For much of the twentieth century, homology was different animals under every variety of form and function’ regarded as nothing more than evidence of common (Owen, 1843). Owen had a very particular kind of struc- descent. In recent times, however, it has regained its for- tural sameness in mind. It was not to be confused with mer importance. It is regarded by advocates of evo- similarities that are due to shared function, which were lutionary developmental biology as a source of evidence termed ‘analogy.’ Bird wings are not homologous to insect for developmental constraint, and evidence for biases in wings, even though they look similar and perform the same phenotypic variation and the commonalities in organic function. Because the similarities are ascribed to functional form. The recent growth in understanding of develop- need, they are identified as analogues. However, the wings mental genetics has made possible a new theory about the of birds, the front fins of porpoises, the forelegs of horses and the arms of humans are all homologous. Even though mechanical causes of homology, which was impossible they look different and function in different ways, they are under the naı¨ve views regarding genetic causation that the same in the sense of homology (see Figure 1). were available during most of the twentieth century. This Some body parts bear very little resemblance to their new theory, proposed by Gu¨nter Wagner (2007), states homologues in other species. The incus and malleus, tiny that homologies of character are due to the development ear bones in mammals, are homologous with the bones that of a certain kind of genetic regulatory network called a form the jaw joint in reptiles. A distinct but related concept character identity network. that Owen termed serial homology identifies repeated elements within an individual organism. All of the ver- tebrae in an individual’s body are serial homologues, as are The Concept of Homology up to their right and left limbs. An interesting pre-Darwinian discovery of serial homology was the identification of the Darwin parts of flowers (petals, sepals, etc.) with the leaves of plants. The concept of serial homology has varied in its The organic world is amazingly diverse, but patterns of importance within evolutionary thought, in ways that will unity can be seen throughout. Evolutionary biology has the be discussed in ‘Character Identity Networks’. See also: double task of accounting for both the diversity and the Aristotle of Stagira; Homology in Character Evolution; unity of life. These two aspects are tied to the two most Owen, Richard central concepts in evolutionary theory: adaptation and A heated biological debate during the early nineteenth homology. Adaptation to diverse situations is believed to century concerned the relative importance of the principles account for the diversity of life forms. Relations of hom- of unity of type versus conditions of existence. Unity of ology express their unity. type expressed the consistency of generalised categories (types) that reflected the organisation of nested sets of eLS subject area: Bioethics & Philosophy homologies within comparative anatomy. Owen proposed the vertebrate archetype as the generalised form of ver- How to cite: tebral skeletons, and sometimes spoke of the archetype as Amundson, Ron (October 2012) Homology and Homoplasy: A an abstract platonic idea. Conditions of existence, in con- Philosophical Perspective. In: eLS. John Wiley & Sons, Ltd: Chichester. trast, expressed the diverse adaptations of organisms to DOI: 10.1002/9780470015902.a0003445.pub2 their environment and to internal physiological needs. eLS & 2012, John Wiley & Sons, Ltd. www.els.net 1 Homology and Homoplasy: A Philosophical Perspective Whale Frog Horse Lion Human Bat Bird Figure 1 Homologous characters can have different shapes and functions. Forelimbs of seven tetrapod species show that corresponding body parts have similar designs but can serve different functions, from swimming to flying. Hence, functional necessity cannot explain the similarity of homologous characters. Reproduced from Wagner (2007) p. 475, with permission of Nature Publishing Group. Now termed adaptationists, the advocates of conditions of homology from causal explanation to summary of existence included British natural theologians such as the evidence was reinforced by the Darwinian E. R. Lankester. followers of William Paley. Adaptationists claimed that Lankester disliked Owen’s terminology, which sounded types were scientifically disreputable. A true understanding mystical to many English-speaking theorists. In 1870, of biology required attention only to function. Lankester argued that Owen’s term homology should be Many twentieth century scholars have attributed the replaced by the term homogeny, which would be defined as antievolutionism of the early nineteenth century to the characters that ‘have a single representative in a common allegedly mystical ‘typological thinking’ of unity of type ancestor’ (Lankester, 1870). Most evolutionists have advocates. But nineteenth-century typologists were very retained the term homology, but used Lankester’s histor- diverse in their views on metaphysics and on evolution. ical definition in place of Owen’s, which had simply said They ranged from mystical to materialistic and even that homologues were ‘the same organ in different animals included T. H. Huxley, Darwin’s strongest supporter. _’ without offering any definition of sameness. In place of Typologists were united not by metaphysical or anti- analogy, Lankester offered homoplasy, a term that desig- evolutionary commitments, but by a belief in the import- nates any biological similarity that does not fit the histor- ance of homology over adaptation. Evolutionary debates ical definition of homology. Because analogy requires within the twentieth century reiterated the pre-evolution- evidence for an adaptive cause of the similarity, homoplasy ary debates regarding the relative importance of homology has come to seem the simpler concept. The term homology versus adaptation as the most basic unifying principles of is still widely held to involve Lankester’s historical concept: biological understanding. See also: History of Compara- ‘A feature in two or more taxa is homologous when it is tive Anatomy; Huxley, Thomas Henry derived from the same (or a corresponding) feature of their One of Darwin’s great successes was his use of the well- common ancestor’ according to Ernst Mayr (Mayr, 1982). established facts about homology as evidence for common See also: Lankester, Edwin Ray ancestry. Darwin rhetorically transformed Owen’s arche- The actual criteria used for recognising homologies have type into an ancestor, and showed how nested sets of changed little between pre-Darwinian days and modern homologies are just what we would expect from genea- times. One is the principle of connections originally logical relationships. Darwin considered ‘the homological articulated by Geoffroy Saint-Hilaire in the 1820s. Hom- argument’ as one of his best arguments for common ologous body parts might differ greatly in shape and even ancestry. Still, Darwin’s replacement of Owen’s archetype composition, but can be recognised by their topological with an ancestor did reduce the status of homology within connections to the body parts around them. A second cri- biological theory. For Darwin, homology was merely evi- terion is the embryological origin of a body part. Body dence for ‘the fact of’ evolution (meaning descent with parts that differ in adults are sometimes observed to modification). So homology was no longer involved in develop out of the same parts in early embryos. Embry- causal explanation. Instead, it counted only as post ology allowed the discovery of the ear bone/jaw bone hoc evidence for common descent. This downgrading of homology above. Here, the homological sameness is 2 eLS & 2012, John Wiley & Sons, Ltd. www.els.net Homology and Homoplasy: A Philosophical Perspective inferred from the similarity of topological connections, treated with great scepticism. One reason for this scepti- even though the bones themselves are very dissimilar. A cism was the difficulty in identifying such genes, but third traditional criterion is the existence of transitional another was the belief that interspecies sterility guaranteed forms of the character in adults of related species. that no possible mechanisms could maintain the kinds of Lankester’s historical definition of homology has
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