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.

[ALL EMPHASES ADDED]


Index - Evolution or Creation

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