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.
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.
