Dictionary of the History of Ideas Studies of Selected Pivotal Ideas |
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9 | I. | BIOLOGICAL HOMOLOGIESAND ANALOGIES |
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Dictionary of the History of Ideas | ||
BIOLOGICAL HOMOLOGIES
AND ANALOGIES
Introduction.
In his study of evolution, or arrange-
ment of organisms into taxa, the systematic biologist
makes
continual use of the concepts of homology and
analogy. The historical
development and current
meanings of these concepts are of interest both
because
they help show how the science of systematics is done,
and
because of the intriguing frequency with which
philosophically mistaken
arguments have intruded into
the science.
The fundamental aim of taxonomy is to classify
organisms into groups in a
biologically meaningful way.
The Linnean hierarchy, built along lines
formally pro-
posed by Aristotle, is the
familiar result. A major pur-
pose of
evolutionary biology is to discover the actual
diverging sequences of
organisms, commonly described
by phylogenetic trees, and the causes of
those diver-
gences.
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
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
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
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.
Serial, Sexual, and Genetic Homology.
While the
evolutionary and phenotypic senses of homology are
the
most fundamental, several derivative senses of the
term are commonly
utilized. Serial, sexual, and genetic
homology apply to entire organs;
amino acid sequences
in proteins and base sequences in DNA allow the
application of homology at the molecular level.
Serial homology refers to more or less identical,
repeating structures in an
organism, for example, the
vertebrae or teeth. The criteria for serial
homology
include phenotypic homology of different structures
in the
same adult, phenotypic homology of structures
in the embryo, similarity of
connections of the repeat-
ing parts, and
phylogenetic arguments. The phenotypic
similarity of teeth, vertebrae,
ribs, etc. of an adult
mammal are obvious. Even if vertebrae in the
adult
were grossly different, however, their claim to serial
homology
might still be made on the basis of the serial
homology of the embryonic
somites from which they
arose. Thus, ontogenetic data can be utilized to
dem-
onstrate serial homology.
Similarity of connections of
the humerus and femur help establish them as
serially
homologous structures. Even if structures were very
different, if they could be shown to have evolved from
structures which
were themselves serially homologous,
then the derived structures might also
be said to exhibit
serial homology.
The occurrence of serially homologous structures
provides an important clue
about both evolution and
ontogeny. It appears that it is relatively easy to
evolve
by changing the number of repeating units which occur
in an
organism. Unfortunately, very little is known
about how repeating
structures are generated, or how
their number is controlled.
Sexual homology refers to structures which differ
between the two sexes of
the same species, but which
derive from a common embryonic rudiment. For
ex-
ample, the mammalian penis and clitoris
are homolo-
gous structures deriving from the
same region of the
genital ridge. The criteria for sexual homology are
therefore the phenotypic homology of embryonic parts
and careful
comparative anatomy of the stages of em-
bryonic development in the two sexes.
In its original sense, genetic homology meant that
if structures in two
organisms were the consequence
of the action of the
“same” gene in both organisms,
then those structures
were homologous. The homology
of the structures was a consequence of the
homology
of the gene(s). Unfortunately, there is no simple corre-
spondence between the genotype and
phenotype of an
organism. Alteration of a single gene may have effects
on many phenotypic traits, and alteration of each of
many genes may have
the same effect on a given
phenotypic character. A consequence of this
com-
plexity is the great difficulty in
trying to prove that
a given phenotypic character in two organisms is
due
to the action of the “same” gene(s) in both. It
is the
common experience of geneticists that if two initial
populations undergo an identical selection regimen
which successfully
maximizes some trait, then the gene
modifications which underlie that
change in the two
populations can differ, and usually do differ
strikingly.
Application of the notion of genetic homology in such
instances is useless. Because of these difficulties, genetic
homology is
not widely utilized.
Ontogenetic criteria of homology had wide appli-
cation when the Recapitulation Theory was accepted.
If ontogeny
recapitulates phylogeny, then to establish
the evolutionary sense of
homology between two or-
gans—that
they were derived from the same part in
a common ancestor—it was
sufficient to show they had
the same origin in ontogeny. In 1870, Karl
Gegenbaur
stated that special homology is the relationship be-
tween two organs which have had a common evolu-
tionary origin, and which, as a
corollary, have arisen
from the same embryonic Anlage. However, E . B.
Wilson (1895, p. 101) and De Beer (1958) have shown
cases of homologous
adult structures derived from
different embryological origins. Such a
straightforward
application of ontogenetic criteria no longer suffices
to prove that two adult structures are homologous in
the evolutionary
sense.
Molecular Homology.
In the 1950's and 1960's the
notion of homology has begun to be
extended to the
molecular level, to the comparison of nucleotide se-
quences in the DNA's of different organisms,
and to
the comparison of amino acid sequences in proteins
from
different organisms. If the work to establish mo-
rewards. One of the most difficult handicaps of classical
genetics and evolutionary studies is, as we have noted,
the lack of a simple relation between the genotype
and phenotype of an organism. A consequence of this
complexity is that if two classes of organisms exhibit
a certain degree of phenotypic difference, one cannot
usually determine the extent of genotypic difference
between the two classes. Thus, from classical genetics
we can usually know neither the extent of genotypic
change underlying the observed phenotypic alterations
in evolution, nor, therefore, the rate at which the
genotype changes in evolution.
These difficulties are partially overcome by consid-
ering nucleotide sequences in DNA and amino acid
sequences in
proteins. Since the nucleotide sequence
of the DNA is the genotype,
comparison of nucleotide
sequences in different organisms is the most
direct
means of assessing the extent of genetic change in
evolution,
and genetic homology between species.
However, such an assessment is not as
straightforward
as one might have hoped. In the first place, direct
analyses of truly long sequences of bases in DNA are
not currently
available. Estimations of similarity of
nucleotide sequences between DNA's
utilize indirect
techniques which only establish approximate ho-
mology, not identity of sequence. These
techniques
will be described later.
Even were it possible to obtain quite detailed nu-
cleotide sequences for the DNA of two organisms, say
bacteria,
estimation of the extent of genotypic differ-
ence between the two would remain difficult. The
concept of the
extent of genotypic difference is ambig-
uous.
Ambiguity resides in the dual reference of
“genotype”
to the actual physical structure of the
DNA, the sequence of bases, and
also to the DNA as
the carrier of genetic information. If one is
referring
to the physical genotype, then the extent of difference
between two genotypes is simply the number of ho-
mologous loci at which the nucleotides differ. Alter-
ation in the informational genotype is related in
rather
complex ways to alteration in the physical genotype.
Amino
acids are coded for by triplets of nucleotides;
most amino acids are coded
for by two or more codons.
Thus, some nucleotide substitutions change a
codon to
a second codon for the same amino acid. Such a substi-
tution alters the physical genome,
but leaves the infor-
mational genotype
unaltered. Physical genotypes
different at many loci can be the same
informational
genotype. Conversely, nearly identical physical geno-
types can be radically different
informational geno-
types. This possibility is
a consequence of the fact that
codons are triplets, and an amino acid
sequence is
specified by a sequence of triplets in which the nucleo
tides are “read” from a specific starting
point, three
at a time. A deletion of a single nucleotide can cause
a
“reading frame shift” in which all codons downstream
from the deletion are misread and a large number of
incorrect amino acids
are incorporated into the pro-
tein. A small
change in the physical genotype yields
a large change in the informational
genotype. If one
is concerned with the extent and rate of alteration
of
the informational genotype in evolution, one must view
with caution
data derived from estimates of physical
homology of the DNA's of various
organisms.
Indirect physical techniques to study the extent of
base sequence homology
of the physical genotype de-
pend upon the DNA's
duplex structure whose comple-
mentary
strands may be separated and caused to re-
combine. Since single-stranded DNA components from
different origins
may also be induced to form “hybrid”
structures, a
means is afforded by which to assess ge-
netic
relationships among organisms. It can be shown
that duplex formation
between single strands derived
from DNA of the same or nearly identical
species
occurs readily, but fails to occur if the strands are
derived
from very different organisms.
Results of such studies (Bolton, p. 77) indicate that
phenotypically similar
animals have very similar DNA
base sequences. Furthermore, “...
the similarities and
differences in polynucleotide sequences
quantitatively
indicate the extent of the taxonomic category to which
the systematist refers. Thus, among the primates, a
superfamily distinction
means that about one-quarter
of the polynucleotide sequences are different,
half are
different for subordinal separation, and about three-
quarters for ordinal
distinction.” Bolton also notes that
“the
quantitative similarities in polynucleotide se-
quences among vertebrates can be related to the time
at which
the lines of organisms in the present diverged
from one another in the
geologic past according to
the paleontologist's judgment.”
Bolton's figure shows
a linear decrease in the logarithm of DNA
similarity
with time.
While Bolton's data gives a good indication of the
rate of alteration of a
physical genome in evolution,
it remains difficult to relate the results to
the extent
and rate of change of the informational genotype in
evolution.
A conclusion reached by Britten and co-workers
(1968, p. 529), is that many
nucleotide sequences occur
repeatedly in the DNA of higher organisms, there
being
many DNA families, each with many nearly identical
copies of one
sequence. The existence of these homolo-
gous
DNA sequences renders the relation between the
physical and informational
genotype even more com-
plex, for the functional
significance of the redundant
DNA is not known. Britten's data also casts
doubt on
cies, for Bolton probably measured only highly re-
dundant DNA sequences.
In contrast to changes in nucleotide sequences which
may occur without
alteration of the informational
genotype, changes in amino acid sequence
are evi-
dence, by definition, of alteration of
the informational
genotype. With the exception of substitutions of nu-
cleotides which do not change the amino
acid specified,
substitution of a single nucleotide results in the substi-
tution of a single amino acid at a
locus in the poly-
peptide. Since the
assignment of codons to amino acids
is now fairly well established, it is
now possible to say
which amino acid substitutions can occur by substi-
tution of a single nucleotide in a
codon. Some amino
acid substitutions cannot be made by altering a
single
nucleotide, but would require the simultaneous alter-
ation of two or three nucleotides; or
else, since nucleo-
tide substitutions must
usually occur one at a time,
intermediate proteins with an amino acid
different
from both the first and final form, must have existed.
Partial or complete sequences of amino acids have
now been worked out for
several sets of homologous
proteins in different organisms, for example,
hemo-
globin and cytochrome C (Fitch and
Margoliash, 1967).
By utilizing arguments about minimal possible
changes
causing sequences of amino acid substitutions, coupled
with
assumptions about nonreversal of changes, it is
possible to arrange
contemporary proteins into pre-
sumptive
branching phylogenetic sequences (ibid.).
Evidence supporting the deduced branching phylo-
genetic relations can be sought in the
fossil record. The
form of argument utilized is closely similar to
that
noted by E . O. Wilson in 1965 for deducing consistent
possible
phylogenies based on gross phenotypes of
contemporary organisms. Utilizing
such techniques on
cytochrome C, Fitch and Margoliash (1967) have pro-
duced a phylogenetic tree linking fungi,
yeasts, nema-
todes, fish, birds, and mammals,
which is very similar
to phylogenetic trees proposed by classical
zoologists.
The number of amino acid substitutions, coupled with
time
estimates derived from the paleontological record,
can give an estimate of
the rate of mutation of the
informational genotype. It will be of
particular interest
to compare the rates for proteins performing
diverse
functions, for the rate must depend in part upon the
strictness of selective constraints on workable amino
acid sequences.
The extent of homology in amino acid sequence for
some proteins is enormous;
neurohypophysial peptide
hormones hardly differ from man to shark
(Acher,
1969). Other proteins exhibit far less homology, differ-
ing in many loci in many different ways.
The occur-
rence of such proteins, all
performing the same func
tion in different animals, has led some biologists to
suppose
that some amino acid substitutions do not
affect protein function and are
therefore not subject
to selection. By random drift, large numbers of
such
substitutions are claimed to accumulate, so that these
homologous
proteins differ at many loci but continue
to function.
Amino acid sequence homology is also utilized to
help establish possible
common evolutionary ancestry
for different proteins. For example, the
alpha, beta,
delta, and gamma chains of hemoglobin have long
identical
sequences (Fitch and Margoliash, 1967). This
argues strongly that the four
protein chains were de-
rived from some single
gene, perhaps by its endorepli-
cation
to form the sort of redundant DNA of which
Britten has spoken, and then the
further evolution of
the four genes.
Clearly, the extension of the concept of homology
to the molecular level
promises to be exceptionally
rewarding.
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.
BIBLIOGRAPHY
R. Acher, “Évolution des Structures des hormones Neuro-
hypophysaires,”
La Spécificité zoologique des
Hormones
hypophysaires et leurs activités,
Éditions du Centre National
de la Recherche Scientifique, No.
177, (Paris, 1969).
M. Beckner, The Biological Way of
Thought (Berkeley, 1968).
E . T. Bolton, “The
Evolution of Polynucleotide Sequences
in DNA,” Mendel Centenary: Genetics, Development, and
Evolution, ed. R. M. Nardone (Washington, D.C., 1968). R. J.
Britten and D. E . Kohne, “Repeated Sequences in DNA,”
Science,
161 (1968), 529. D. H. Colless, “An
Examination
of Certain Concepts in Phenetic Taxonomy,” Systematic
Zoology,
16 (1967), 7. C. Darwin, On the
Origin of Species
by Natural Selection (London, 1859; many
reprints). G. R.
De Beer, Vertebrate Zoology
(London, 1928); idem, Embryos
and Ancestors, 3rd
ed. (Oxford and New York, 1958). W. M.
Fitch and F. Margoliash,
“Construction of Phylogenetic
Trees,” Science,
155 (1967), 279. K. Gegenbaur, Grundzuge
der Vergleichenden Anatomie, 2nd ed. (Leipzig,
1870).
R. Goldschmidt, Theoretical Genetics
(Berkeley, 1955).
N. Jardine, “The Concept of Homology in
Biology,” British
Journal for the
Philosophy of Science,
18 (1967), 125. E . R.
Lankester, “On
the Use of the Term Homology in Modern
Zoology,” Annals and Magazine of Natural History,
6, series
4 (1870), 34-43. R. Owen, Lectures on the Comparative
Anatomy and Physiology of
the Invertebrate Animals (Lon-
don,
1843); idem, “Report on the Archtype and Homologies
of the
Vertebrate Skeleton,” Report of the British
Association
for the Advancement of Science,
16 (1847), 169. J. M. Rendel,
Canalization and Gene Control (London, 1967). E .
G.
St. Hilaire, Philosophie Anatomique (Paris,
1818). G. G.
Simpson, Principles of Animal
Taxonomy (New York and
London, 1961). R. R. Sokal and P. H. A.
Sneath, Principles
of Numerical Taxonomy (San
Francisco, 1963). R. R. Sokal
and J. H. Camin, “The Two
Taxonomies: Areas of Agree-
ment and
Conflict,” Systematic Zoology,
14 (1965), 176.
E . B. Wilson, “The
Embryological Criterion of Homology,”
Biological Lectures, 1894, Marine Biological
Laboratory
(Woods Hole and Boston, 1895). E . O. Wilson,
“A Consist-
ency Test for
Phylogenies Based on Contemporaneous Spe-
cies,” Systematic Zoology,
14 (1965), 214. L. Wittgenstein,
Philosophical Investigations (Oxford, 1958).
STUART A. KAUFFMAN
[See also Analogy in Early Greek Thought; Biological Con-ceptions in Antiquity; Evolutionism; Game Theory; Genetic
Continuity; Recapitulation.]
Dictionary of the History of Ideas | ||