Francisco Aboitiz
Neuroscience Office
73-346 Center for the Health Sciences
University of California
Los Angeles
California 90024
Ideas in Theoretical Biology
"Homology: A Comparative or a Historial Concept?"
<Acta Biotheoretica> 37: 27-29, 1988
ABSTRACT
The meaning of the word 'homology' has changed. From being a
comparative concept in pre-Darwinian times, it became a historical
concept, strictly signifying a common evolutionary origin for either
anatomical structures or genes. <This historical understanding of
homology is not useful in classification; therefore I propose a return
to its pre-Darwinian meaning>.
* * *
The distinction between homology and analogy was used by pre-Darwinian
biologists as a criterion for animal classification. There were at
least two ways of establishing homology (Russell, 1916; Aboitiz,
1987). One definition stated that, in different animals, homologous
structures were those who shared a set of topographic relations to
other organs. According to a second definition, homology consisted of
having a common embryological origin. Although there was some
disagreement about what homology was (still other definitions were
seldom proposed, and also different kinds of homologies were sometimes
suggested), all the proposed criteria were empirically testable, and
the concept was very useful for the elucidation of taxonomies
(Russell, 1916). With the rise of evolutionary ideas, homologies
became 'suggestive' of a common ancestry in different groups (Darwin,
1872).
Today, however, the word 'homology' is currently understood as
strictly signifying a common evolutionary origin for either anatomical
traits or genes or proteins (Gould, 1987, 1988; Reeck et al., 1987).
In different species, two structures or genes are considered
homologous only if they are derived from the same structure or gene in
a (hypothetical) common ancestor. Under this definition, homology is
not anymore a criterion for classification; rather, it is established
after taxonomies are elucidated. There is no expression for organs
sharing a relative position in body (or having the same embryological
origin) in different species. If these organs happen to be originated
separately in evolution, they will not be homologous anymore.
Furthermore, in molecular biology there is no place for a comparative
definition of homology. As expressed above, this definition bears
relation to the concept of a body plan that determines the relative
position of each organ. It makes no sense today to speak either of a
'genetic plan' determining the 'relative positions' of genes, or of a
common embryological origin for two genes.
A science of morphogenesis as the realization of a body plan is
beginning its renaissance (Goodwin, 1982; Thom, 1972). In this
approach, evolutionary considerations are considered as secondary.
<What matters are the processes that realize a body plan, and not
whether a shared plan implies the same ancestry. Under this
perspective, perhaps a return to the pre-Darwinian concept of homology
will be more useful to biologists, since the historical interpretation
is not very practical in phylogenetic classification>. If this turns
out to be the case, the term 'homology' would have to be restricted to
the discipline of morphology, and should not be used in molecular
biology.
REFERENCES
Aboitiz, F. (1987). 'Homology' in anatomy and molecular biology.
<Cell> 51: 515-516.
Darwin, C. (1872). <The Origin of Species>., 6th ed. New York:
Collier Eds., 1962.
Goodwin, B.C. (1982). Development and evolution. <Jl. Theor. Biol.>
97: 43-55.
Gould, S.J. (1987). <Time's Arrow, Time's Cycle>. Cambridge: Harvard
University Press.
Gould, S.J. (1988). Conference on molecular data and systematics.
Symposium on "The Impact of Molecular Analyses on Our Understanding of
Evolution". University of Southern California, March 11.
Reeck, G.R. et al. (1987). A terminology muddle and a way out of it.
<Cell> 50: 667.
Russell, E.S. (1916). <Form and Function>. Chicago: Univ. of Chicago
Press, 1982.
Thom, R. (1972). <Structural Stability and Morphogenesis>. Mass.: W.A.
Benjamin.
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