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Dictionary of the History of Ideas

Studies of Selected Pivotal Ideas
  
  
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Analogy. Analogy is commonly defined as similarity
of function, and is opposed to the evolutionary defini-
tion of homology in terms of common ancestry. It is
often unclear whether analogy is meant to be restricted
to nonhomologous structures. The source of this un-
clarity rests, in part, upon uncertainty whether the
evolutionary, or a phenotypic notion of homology
should be utilized. Granted the evolutionary sense of
homology, it becomes possible to distinguish simi-
larities between organisms which are not due to ho-
mology; thus, a bird's wing and a butterfly's wing are
not homologous despite phenotypic similarity. If we
utilize only a phenotypic concept of homology, it is
unclear how we are to distinguish similarities between
organisms which are homologous from similarities
which are not homologous but analogous. A virtue of
the evolutionary concept of homology, therefore, is
that it allows us to discuss ways in which distinct
(nonhomologous) phylogenetic lines have become
phenotypically similar. The notion of analogy facili-
tates descriptions of phylogenetic convergence.

The notion of analogy can be extended to the mo-
lecular level. One can consider different molecular
structures performing the same function, for example,
different oxygen carrying pigments, or structurally
different enzymes capable of catalyzing the same reac-
tion. The matter is of great importance, for it would
be helpful to have some estimate of the number of
diverse ways in which any chemical (catalytic) job
might be accomplished in order to gain insight into
the difficulty which evolution faced in finding at least
one workable mechanism, or in evolving new ones.
Despite its importance, little work has been done in
this potentially interesting area.


242

In summary, homology and analogy are working
tools with which the biologist attempts to classify
organisms into hierarchically nested taxa, formulate
phylogenetic hypotheses, discuss evolutionary forces,
describe ontogenetic similarities, and, in short, carry
on his science.