Historical Development of the Concept of Homol-
ogy.
It is in the light of these goals and difficulties
that one should
consider the prevalent evolutionary
definitions of homology and analogy.
According to
G. R. De Beer, “The sole condition which organs
must
fulfill to be homologous is to be descended from one
and the same
representative in a common ancestor.”
G. G. Simpson also defines
homology in terms of com-
mon inheritance:
“Homology is resemblance due to
inheritance from a common
ancestry.” Usually, this
evolutionary sense of homology is
contrasted with
analogy, resemblance due to common function. A fre-
quent example of homology is the relation
between
a bird's wing and a mammalian forearm; of analogy,
the
relation between the wing of a bird and of an
insect.
It has recently been claimed by several authors that
the evolutionary
definition of homology is viciously
circular, for “in order to
show that a part of one
organism, x, is homologous
with a part y of another
organism, it must be shown
that they are derived from
a part z in a common
ancestor. But homology itself
is invoked in identifying parts x and y with part z”
(Jardine, 1967). It will be argued
below that the circu-
larity of De Beer and
Simpsons' definition is not at
all vicious, but whether it is or not, the
circularity
points up the priority of a nonevolutionary, phenotypic
sense of homology, defined solely in terms of pheno-
typic similarity, and logically independent of criteria
of
descent.
A nonevolutionary sense of phenotypic homology
was widely utilized prior to
Darwin. In 1818, Geoffrey
St. Hilaire, in his
théorie des
analogues (where
analogue is roughly
equivalent to phenotypic homology), argued
that animals conform to a common
type; the analogy
of parts was to be established by showing that they
occupied corresponding relative positions in different
animals—the
principe des connexions.
Owen, in his
Lectures... (1843), defined homologue as “The
same
organ in different animals under every variety of form
and
function.” In 1847 he distinguished special
homology
“the correspondence of a part or organ,
determined by its
relative position and connections,
with a part or organ in a different
animal,” from gen-
eral homology,
“a relation in which a part or a series
of parts stands to the
fundamental or general type.”
Several authors in addition to
Owen, most notably the
poet Goethe, advocated this Platonic idealism of
fun-
damental types, or common bauplans
(construction
plans) of which diverse organisms were seen as imper-
fect realizations.
Darwin himself utilized Geoffroy St. Hilaire's con-
cept of homology in the Origin of Species...
(1859):
“... If we suppose that the ancient progenitor, the
archetype as it may be called, of all mammals, had
its limbs constructed on
the existing general pattern,
for whatever purposes they served, we can at
once
perceive the plain signification of the homologous
construction
of limbs throughout the whole class.”
Darwin, of course, thought
homology evidence in favor
of evolution, but he
utilized a nonevolutionary, pheno-
typic
concept of homology.
Both the Platonic idealism of fundamental types, and
the nonevolutionary
sense of homology soon came
under attack. In 1870, E. R. Lankester argued
that St.
Hilaire's and Owen's use of homology “belongs to
the
Platonic school.... Professor Owen... would under-
stand by homologue 'the same organ in different ani-
mals under every variety of form and
function'....
But how can the sameness (if we may use the word)
of an
organ under every variety of form and function
be established or
investigated?... to settle this ques-
tion of
sameness, an ideal 'type' or a group... had
to be evolved from the human
mind,... then it could
be asserted that organs might be said to be the
'same'
in two animals” (“On the Use...,”
pp. 34-43). In the
place of this idealist, purely phenotypic concept
of
homology, Lankester proposed a definition resting in
part on common
ancestry. “Structures which are ge-
netically related, in so far as they have a single rep-
resentative in a common ancestor, may
be called
homogenous” (idem). The term
“homogeny” was not
accepted, but its definition was
used to define
“homology” in the evolutionary sense
later advocated
by Simpson, De Beer, and others.
Are Lankester's points well taken? It is true that
Owen's and Goethe's
fundamental type or common
bauplan has the features of a Platonic Ideal of
which
all organisms of the type were imperfect realizations.
Their
major motivation for the supposition of a funda-
mental type was the belief that in order to class a group
of
organisms together, the organisms must jointly share
some common features,
the fundamental type.
This understanding of the nature of universals has
been criticized by the
philosophers Ludwig Wittgen-
stein and
(more recently) Morton Beckner. Wittgen-
stein points out that we apply the concept “game”
to
diverse objects which share different attributes in par-
tially overlapping ways but have no single
attribute
in common. Classification of objects together requires
only
that we have repeatable criteria for their classifi-
cation. Thus phenotypic homology need not lead
to
idealism.
Furthermore Lankester's definition of homogeny
(homology) failed to meet his
own criticisms of Owen,
for to establish that two structures have a single
repre-
sentative in a common ancestor
requires showing that
the two structures are homologous to that single
an-
cestral representative; and the only
criterion of homol-
ogy to which Owen can
allude is phenotypic homology.
If phenotypic homology inevitably led to
idealism, then
Owen's redefinition of homology would fail to escape
the disaster.
There were, however, more valid grounds for the
introduction of Lankester's
sense of homology. It was
argued that evolutionary theory provided a
criterion
by which to distinguish “true” from
accidental homol-
ogy. Because the traits of
organisms overlap in complex
ways, phenotypic clusters can be made discrete
only
somewhat arbitrarily. Hence, phenotypic homology is
somewhat
arbitrary. However, evolution of organs in
one species took place,
presumably, in only one way.
By utilizing the criterion of descent of two
structures
from the same representative in a common ancestor,
evolutionists like Simpson and De Beer felt they were
providing a less
arbitrary and more biologically mean-
ingful
sense of homology. With the wide adoption of
this evolutionary sense of
homology, the meaning of
the term had changed. In its new sense,
homology
required the theory of evolution for its definition. This
is
one of many cases in science in which a theory built
in part with data
described by one concept, later is
used to change the meaning of that
concept itself.
But the clarity of aim of the evolutionists to intro-
duce a concept of homology which overcame the ar-
bitrariness of mere phenotypic homology
does not
completely attain its goal. To establish that two struc-
tures are homologous, in Simpson's
sense, requires
showing that a phylogenetic hypothesis is true. But part
of the
evidence that the hypothesis is true stems from
phenotypic resemblance of
structures in fossil and
living organisms. And that phenotypic
homology
suffers from the arbitrariness which the evolutionist
hoped
to avoid. Nevertheless what the evolutionist does
do is to construct and
attempt to verify phylogenetic
hypotheses on the basis of
all the available information,
including not only phenotypic
homology but also tem-
poral relations among
structures in the fossil record,
as well as considerations about possible
sequences of
change in structures, which minimize the total number
of
changes which must be supposed to have occurred.
For the evolutionist, the
statement that two structures
are homologous is the result of a great deal
of theory
building. Because the evolutionist utilizes other criteria
in addition to phenotypic similarity in asserting that
two structures are
homologous, he is able to say that
two phenotypically dissimilar structures
are homol-
ogous; for example, the bones of
the mammalian mid-
dle ear and their homologues
in fish.
The evolutionary sense of homology has held sway
for nearly a century. It is
currently facing vigorous
attack by systematists who wish to substitute a
purely
phenotypic sense of homology. The argument is not
merely
semantic, for it expresses very different con-
victions about how best to do systematics.
The pheneticists (who wish to classify organisms on
the basis of clusters of
phenotypic traits) raise three
major arguments against the traditional
evolutionary
taxonomist. 1. The evolutionary sense of homology is
claimed to be viciously circular, for homology must
be utilized to show
that two structures are descended
from the same
representative in an ancestor. 2. The
pheneticists argue that the
traditional evolutionist has
poorly defined criteria for asserting the
phenotypic
similarity of organisms. 3. Worse, it is argued, the
traditional taxonomist interjects speculative phylo-
genetic hypotheses into his very phenotypic classifica-
tion schemes, thus rendering
them biased and unscien-
tific, and
rendering suspect any purported evolutionary
homologies derived from the data.
In place of these putatively objectionable practices,
the pheneticist, and
in particular, the numerical tax-
onomist,
wishes to substitute less biased, more reliable
methods (Sokal and Sneath,
1963); therefore, they have
introduced “operational
homology” (Sokal and Camin,
1965). Unit characters, such as
eyes: red, blue, green,
etc., are chosen (Colless, 1967); and, on the basis
of
a set of such characters and their “states” for
each
organism in a sample, similarity among groups of orga-
nisms is computed. Many techniques of calculation of
similarity have been generated (Sokal and Sneath),
depending upon whether
each unit character is con
sidered of equal or differing weight. Diverse functions
on these
characters are computed as measures of simi-
larity or “distance” between organisms, and
diverse
types of cluster and factor analysis are utilized to
generate
clusters.
The claims for scientific respectability of these tech-
niques are: 1. The criteria by which organisms are
judged similar are explicit and repeatable, in contrast,
the pheneticists
allege, to the unclear criteria by which
traditional taxonomists talk of
similarity among orga-
nisms. 2. The
phenotypic clusters found by these
techniques are supposed to be free of
phylogenetic
speculations, or, more strongly, free of theory, and
unvarnished data on which to build theory.
Can these arguments be maintained by the pheneti-
cist against the traditional evolutionary taxonomist?
First, the
circularity ascribed to the evolutionary sense
of homology also applies to
the purely phenotypic
sense of homology. The evolutionist's homology
is
circular because of his reference to the “same”
an-
cestral structure; but that
“same” is just the circle the
phenotypic sense of
homology requires. If it is a vicious
circle, both evolutionist and
pheneticist would be
trapped, but the circle is benign. Many concepts
can
only be defined by reference to a self-defining set of
terms. For
example, “homologus,” “similar,”
“resem-
bling,”
“almost the same,”... etc. Benign circles are
not
limited to biological concepts, a “rule” cannot
be
understood without the ideas of “correct” and
“wrong,” nor they without it.
The pheneticist's hope for a “theory-free” opera-
tional homology is illusory. No
operation is completely
free of theory; measuring the same ear length
again
requires a theory about measuring rods not changing
length in
these circumstances. Nor are the pheneticists'
similarity measures free of
phylogenetic bias. Different
choices of unit characters or different
computation
schemes will yield different phenetic clusters consistent
with diverse phylogenetic sequences. Suppose the
pheneticist deduced from
his clusters a presumptive
phylogenetic sequence which happened to be incon-
sistent with the temporal relations in
the fossil record.
Faced with disconfirming evidence, he might doubt
the
deductions, doubt the fossil record, or doubt the ade-
quacy of his clustering techniques or choice of unit
characters. If he were willing to consider altering his
unit characters or
clustering techniques, then he would
be doing what he accuses the
traditional evolutionist
taxonomist of doing, namely, redefining his
phenotypic
classification scheme to fit with other phylogenetic data
or hypotheses. If he would never change his clustering
technique, he might
be asked to justify his position,
and would find it hard to do so.
Finally, the pheneticist is wrong in asserting that
the traditional evolutionist taxonomist must be unsci-
entific since he cannot make explicit
and simple the
criteria by which he judges organisms similar. An art
critic can correctly recognize a Picasso, but would
probably be unable to
make all his criteria explicit and
simply measurable. While the numerical
taxonomist's
measures are probably more repeatable, and perhaps
more
easily learned than those of the traditional tax-
onomist, numerical methods nevertheless are not nec-
essarily more meaningful measures of
phenotypic simi-
larity than the traditional
evolutionist methods.
In brief, the differences between those who support
a phenotypic and those
who favor the evolutionary
sense of homology is not as great as the furor
of current
debate makes it appear. Both utilize a phenotypic sense
of
homology. The pheneticist must admit that his oper-
ational homology is neither theory-free nor unbiased
with regard
to phylogenetic hypotheses, and that the
evolutionary taxonomist is not
necessarily unscientific
for his lack of simple measuring operations.
