AMER. ZOOL., 20:217-227 (1980)
The Definition and Recognition of
Biological Adaptation1
WALTER J. BOCK
Department of Biological Sciences, Columbia University,
New York, New York 10027
SYNOPSIS. Biological adaptation is a property of phenotypic features of organisms relative
to selection demands of the environment. Adaptive features are ones having properties
of form and function which permit the organism to maintain successfully the synerg
between a biological role of that feature and a stated selection force. The degree of
goodness of the adaptation can be measured by means of the amount of energy needed
to maintain the synerg with less energy indicating better adaptation. Adaptation does not
apply to a close fit between different features or between the form and function of a
feature or to the mutual interactions between features of a structural network. The state
of being adapted is independent of the process of becoming adapted as adaptations need
not evolve under the control of selection forces to which they are now adapted. Selection
is here considered as the demands placed on the organism by the environment with which
the organism must cope to continue surviving as an individual. Adapted features have
been judged using three methods—the comparative, the correlative and the synthetic—of
which only the last is valid. The synthetic method requires study of the forms and functions of the feature in the laboratory and of the biological roles of the feature and the
selection demands of the environment in the field. It is suggested that the best approach
for the study of adaptations is a team effort with constant feedback between the laboratory
and field phases of study. It is urged that attention be given to the development of an
ecological morphology to supplement functional morphology and provide the necessary
foundation for proper elucidation of biological adaptations.
INTRODUCTION
I am concerned in this paper only with
adaptation as a causal evolutionary mechanism—how the concept is defined both as
a state of being and as a process and how
particular adaptive features are recognized. This must be distinguished from
adaptive historical narratives, i.e., the explanation of the evolution of a particular
feature, such as the avian wing or the human brain, or of a particular taxon, such
as the snails or the flatfishes, by adaptive
evolution. Confusion of the elucidation of
the causal mechanism of adaptive evolutionary change with adaptive historical
narratives has been a prime factor in previous misunderstandings of adaptation as
an evolutionary phenomenon. I will not
consider adaptive historical narratives
herein.
Adaptation is a concept applicable only
to phenotypic features of individuals. It
does not apply to individuals, to populations or to species. One must be cognizant
of levels of organization in living organisms and realize that biological concepts
1
From the Symposium on Morphology and Analysis apply only to particular levels of organiof Adaptation presented at the Annual Meeting of the zation. Students of evolutionary theory
American Society of Zoologists, 27-30 December have paid scant attention to this facet of
The concept of biological adaptation is
one of the original ideas in evolutionary
studies as Darwin was primarily concerned
with providing a scientific explanation for
this phenomenon with his theory of evolution by heritable variation and natural
selection. It is a core concept in evolutionary studies but has remained relatively
poorly understood. A major reason for the
failure to elucidate the concept of adaptation compared to important advances in
our understanding of other evolutionary
mechanisms such as the origin and production of genetical variation and the process of speciation has been, in my opinion,
the minor role that morphologists have
had in the establishment and clarification
of evolutionary mechanisms. Yet with the
rise of functional morphology over the
past quarter-century and the increasing interest in ecological correlates of morphology, I believe that this situation will
change.
1978, at Richmond, Virginia.
217
218
WALTER J. BOCK
ORGANISM
ENVIRONMENT
Feature=the Adaptation
Umwelt
-^depends
upon v \
Synerg
Form
Biological
Role
Faculty-
Function
Ol
Selection
Environmental
Factor
Force ^
determines
Feedback Relationship
O
Organism = 2 Synergs = Niche
LABORATORY STUDIES
Descriptive and functional
morphology to ascertain:
a)the form (spectrum of possible
forms during life)
b)the function(s)
= solving the "black box"of the
organism at all levels of
organization
FIELD STUDIES
Behavior and ecology
to ascertain:
a)the biological role(s)
b)1he environmental factor(s)
acting on the organism and
the feedback relationship
c)the synerg(s)-the interactions
between the biological roles
and the selection forces
Both required to determine the adaptation
FIG. 1. Simplified scheme to illustrate the components of an adaptation and the range of laboratory and
field studies required for the synthetic determination of an adaptation. The adaptation is a feature of the
organism and depends on the properties of the synerg, i.e., the adaptation is relative to a selection force
arising from the environment. The selection force is a demand placed on the organism by the environment
but the exact properties of the selection force are determined by the feedback relationship between the
organism and the environmental factor via the biological role. After the adaptation has been established,
additional study is needed to ascertain (measure) its degree of goodness. (Figure taken from Bock, 1977a.)
theory development and application and Attribu tes offea tn res
have almost nilly-willy applied the same
An adaptation is a feature, i.e., phenoconcept to several levels of organization. typic aspect, of an organism. It is incorrect
Thus adaptation applies to features of or- to speak of morphological adaptations,
ganisms and fitness applies to individuals, physiological adaptations,- behavioral adnot to features or populations. The same aptations and so forth except as a shortterm such as survival may be applied with hand indication of the type of biological
different meanings to individuals and to research being done; it is only the feature
species. One hears terms such as adapted that is or is not the adaptation. Moreover
genes, adaptive individuals, adapted species it is essential to subdivide a feature into its
and adaptive evolution of new major taxa. component subunits and to consider the
Many of these combinations are meaning- adaptive significance of each part. A bone,
ful but must be defined carefully once the for example, must be decomposed into its
basic concept of adaptation is worked out. condyles, sites of muscle attachment, outer
BIOLOGICAL ADAPTATION
219
size and shape, internal arrangement of Functions associated with biological roles
trabeculae, etc. and the adaptive nature of are utilized ones. The biological role intereach part considered. If a feature, such as acts with one or more selection forces aristhe wing of Corvus corax or the brain of ing from environmental factors of the orHomo sapiens, can be shown to be adaptive ganism's environment. The link between
with respect to particular selection forces, the biological role and the selection force
it does not necessarily follow that all parts is the synerg; it determines the adaptation.
of the feature are adaptive to this or any The sum total of all synergs of the organism is the niche which may be considered
other selection forces.
To comprehend the concept of adapta- to be the organism because no living creation or to recognize a particular adapta- ture is independent of its environment.
tion, the feature must be apportioned into
its several attributes as done by Bock and Environment
von Wahlert (1965) and Bock. (1979), and
Adaptation is not an absolute property
summarized in Figure 1. The properties of of features, but one that is relative to, and
form and of function must be described hence can be judged only with respect to,
carefully at the onset of the study of any the external environment of the organism.
adaptation; the latter is the subject of func- Moreover it is essential to specify the pretional morphology. Definition and study of cise environmental factors of the organism
function continues to be a major source of when defining adaptation and when asconfusion; I restrict it firmly to all physical sessing particular adaptations. The outer
and chemical properties of a feature aris- shape of a fish is not adapted for a fluid
ing from its form including all properties environment but for the details of the fluid
from increased levels of organization. A environment of that particular species.
feature may have a spectrum of forms over Thus it is essential to know the species-spetime resulting from age changes, wear, cific environment or umwelt of the species
normal "physiological" alterations as mus- before attempting to judge an adaptation.
cle contraction and many other factors.
It is not sufficient to know the umwelt
Probably all features have a number of of the species, but the exact selection
functions of which only a part, usually a forces arising from the umwelt factors and
few, are utilized by the individual. The rest acting on the species because the adaptaare unutilized, but are nevertheless valid tion is judged with respect to the selection
functions and may become utilized (have force and not to the environmental factor
biological significance) with changes in the itself. The same environmental factor can
activities of the organism. Study of form give rise to different selection forces deand of function constitutes the laboratory pending how the species interacts with it.
part of morphological study and can be Flatfish interact with the ocean bottom difdone without reference to the normal en- ferently; hence different selection forces
vironment of the species. A combination arise and select for different adaptations
of a form and function constitutes a fac- in various groups of flatfish (von Wahlert,
ulty.
1961). Winter cold and food shortage will
generate
different selection force on small
The biological role is how the organism
uses the faculty and hence the feature in song birds that migrate to warmer climes
the course of its life history; the use may and on small rodents that store food and/
be active or passive on the part of the in- or hibernate over winter. Selection forces
dividual. A faculty may have one or more do not arise unilaterally from the environbiological roles. These roles can be ascer- ment, but depend upon a feedback relatained only by observation of the organism tionship between the organism and the enliving freely in its natural environment; vironment. Selection depends on the
studies by functional morphologists, no environment and how the organism intermatter how detailed or how carefully acts with the environment, e.g., how the
done, cannot elucidate biological roles so flatfish uses the ocean bottom. This feedlong as this work is done in the laboratory. back relationship includes the fact that the
220
WALTER J. BOCK
organism modifies noticeably many or
most aspects of its umwelt in its interaction
with it. Animals eat food, and thereby reduce the amount available to themselves.
Plants grow and thereby crowd one
another and shade the ground. But not all
environmental factors are modified by interaction with organisms. Terrestrial animals do not exert any noticeable effect on
their oxygen supply, nor do they lessen the
winter's cold.
Selection
Adaptation must be defined with respect
to external selection arising from the external environment of the organism. Individual adapted features are assessed with
respect to particular selection forces of the
umwelt of the species. The concept of adaptation and the concept of selection are
closely associated; hence selection must be
defined carefully.
Darwin in his On the origin of species de-
fines selection both as a mechanism and
as a result. Later evolutionists, following
the lead of population geneticists, have
generally used a "result" definition of selection, e.g., natural selection is the nonrandom differential reproduction of genotypes. The mechanism of selection is left
unspecified. I will be concerned only with
the causal mechanism of selection under
real conditions of living organisms which
are finite populations at or close to the carrying capacity of their environment. Natural selection, the causal mechanism, is the
action of the umwelt on the phenotype of
the organism such that demands are
placed on that organism with which it must
cope if it is to continue to survive as an
individual. This is only one of the several
types of interactions between the organism
and its environment (e.g., Waddington,
1960, p. 401). Continued survival as an individual is an essential prerequisite for reproduction. A selection force is the demand of a single environmental factor of
the umwelt on the phenotype of the individual (i.e., on a feature) through a particular biological role. The mechanism of selection may be density dependent or
density independent. It is associated with
a struggle for existence of the individual
because that individual must be able to
cope with the demands placed on it by selection. However, selection is not always a
struggle for existence between individuals.
Desert mammals coping with excessive
heat or Arctic mammals coping with extreme cold will be involved in a struggle
for existence but not between individuals.
Population geneticists point out that a
struggle for existence is not necessary to
achieve evolutionary change by natural selection. All that is needed is to have different fecundity of two genotypes in an
expanding population. But such conditions exist only for short temporary periods of time when the population is below,
and usually well below, the carrying capacity of its environment. Non-struggle mechanisms leading to nonrandom differential
reproduction of genotypes are the exception, not the rule.
(Here a problem in concepts and terms
should be mentioned. The result definition has been used so long for selection
that the term is best restricted to nonrandom differential reproduction of genotypes regardless of the causal mechanisms
responsible. Thus a different term must be
coined for what is here used for the mechanism of selection and individual selection
forces to avoid confusion. No suitable substitute terms are known to me in the evolutionary literature. In this paper I will use
selection and selection force in the mechanism sense of the environment placing
demands on the organism.)
Selection will always be used as demands
arising from the external environment. I
reject the notion of internal selection
which implies demands on the organism
arising from its internal environment.
Adaptation
The term adaptation has been used for
a number of concepts including "universal
adaptation" and "physiological ( = somatic)
adaptation" (see Bock and von VVahlert,
1965). I will be concerned only with "evolutionary adaptation" which is the phenomenon of an organism being well suited
to (i.e., able to cope with) the demands of
its umwelt. Evolutionary adaptation connotes both a state of being and a process,
BIOLOGICAL ADAPTATION
but the distinction between the two is usually clear. The concept of adaptation has
always been used to denote features of organisms which are well suited to the environment of the organism—a fitting of the
feature to the particular demands of the
environment. Because the same environmental factor may exert different selection
forces depending on how various species
interact with it (Bock and von Wahlert,
1965) it is clear that adaptation must be
defined and judged with respect to the
precise selection force, not to the environmental factor.
An adaptation is a feature of an organism, a part of its phenotype, that has at
least one biological role interacting with a
selection force—it forms a synerg with the
environment. Recall that selection forces
place demands on the organisms. An ad-
221
adaptation is needed that is independent
of other evolutionary measures and that
could be determined under laboratory or
field conditions. Based on the facts that all
organisms must utilize energy to maintain
each and every synerg and that all organisms have a certain, limited amount of energy available to them at any time, Bock
and von Wahlert (1965) suggested that the
degree of adaptation could be judged by
a measure of the energy required by the
organism to maintain the synerg—a sort of
efficiency measure. Hence the degree of adaptation, the state of being, is defined as the
amount of energy required by the organism to
maintain successfully the synerg of the stated
adaptation with a lower energy requirement indicating a better degree of adaptation. Thus
the degree of adaptation is inversely related to the energy required to maintain the
aptation, the state of being, can be defined as synerg. The amount of energy1 must be
a feature having properties of form and func- given in terms of calories-g wt" to comtion which permit the organism to maintain suc- pensate for different body sizes in comcessfully the synerg between a biological role of parative studies. The adaptiveness of difthat feature and a stated selection force. The ferent individuals, belonging to the same
selection force must be known before a or different species, can be compared but
feature can be considered to be an adap- must be judged against the same selection
tation. By successful, I mean that the in- force. Comparisons are best done between
dividual organism survives as an individ- closely related species and become less useual. Previously (Bock and von Wahlert, ful as the amount of evolutionary relation1965; Bock, 1979), I had included "and ship between species decreases because dereproduces to leave progeny in the next tails of the adaptations and/or the nature
generation" as part of the condition of suc- of the selection forces become too differcessful. This appears to be correct intu- ent.
itively because one of the essential criteria
Evolutionary change is adaptive if it
for continued evolutionary success is to modifies the adaptive nature of features;
leave progeny. Nevertheless upon reflec- hence it must always be judged relative to
tion, it seems more reasonable that the stated selection forces, regardless of
crux of adaptation is whether these fea- whether the selection forces have retures permit continued survival of the in- mained constant or have altered during
dividual (see Caplin, I979a,b). It is neces- the period of evolutionary change. In
sary to ascertain the relationship between either case adaptive evolution of a feature
adaptations of an individual and its ability may be judged in terms of energy utilizato reproduce, i.e., to determine what rela- tion with reduction in energy requirement
tionships may exist between adaptation indicating increased adaptation. Thus adand fitness.
aptation, the process, is defined as any evoluSuccess is a relative term and some mea- tionary change in the form-function complex of
sure of success or of the relative degree of a feature which reduces the amount of energy
goodness of the adaptation is needed. Ad- required by the organism to maintain successaptation is not an absolute all or none re- fully the synerg of the stated adaptation.
lationship nor is it necessary to postulate
The use of energy as the measure of the
that adaptations are always (or ever) max- degree of adaptation is possible, at least
imum, best or optimum. Some measure of theoretically, for any adaptation and could
222
WALTER J. BOCK
be applied now for most structures of the
skeletomuscular system, for example. Its
use would eliminate the need to consider
adaptation in terms as perfect or optimum
because a quantitative measure is available.
Some problems do exist with this measure
in that it is possible to suggest evolutionary
changes in features which everyone would
regard as adaptive but which would require more energy (Bock, 1977a; 1979). A
simple case is the length of muscle fibers
in a skeletal muscle—longer fibers are
needed if the muscle must shorten over a
greater distance. Hence if the selection
force demands that a structure, e.g., the
tongue of a woodpecker, be moved over a
longer distance, then a longer fibered
muscle would be better adapted. But a
longer fibered muscle would require more
energy as it contracts and shortens. For
those adaptations associated with feeding,
these difficulties could be overcome by defining the degree of adaptation in terms of
the energy acquired by the organism compared to the energy required. Application
of such a ratio would not work for all features and additional analysis is required to
develop a more comprehensive measure of
the degree of adaptation. The problem
may lie in the definition of adaptation and
in the vagueness of the term "successful"
which may have to be measured simultaneously in several different ways including
the energy measure.
A number of workers have applied optimization theory to ecology and evolution
(see reviews by Pyke et al., 1977; and by
Maynard Smith, 1978) with the clear implication that adaptations and adaptive
evolutionary changes can be judged under
this concept. Several workers (e.g., Curio,
1973, p. 1047; Gutmann, 1977 and earlier)
have advocated the evaluation of adaptations and of adaptive change under optimization theory (called the economy principle by these workers) with the claim that
this follows directly from the use of energy
utilization as a measure of the degree of
adaptation advanced by Bock and von
Wahlert (1965). I must differ with this extension of the concept of measuring the
degree of adaptation with optimization
theory (economy principle) because of the
lack of standard against which comparison
is made. In discussions of optimization the
environmental factors against which the
adaptation must be judged are not specified or only specified in a vague way as the
environment or selection. Frequently the
details of the feature being considered as
the adaptation are also left vague; this criticism does not apply to Gutmann's (1977
and elsewhere) analyses. Because the selection forces are constantly changing in
the evolution of a species, it is difficult to
appreciate how optimization theory can be
applied except in a vague abstract way
which would not permit useful estimates
of the degree of adaptation and its correlation with other evolutionary measures
such as fitness of the individual. (See also
Lewontin, 1978, 1979 who also objects to
the use of optimization theory).
The concept of adaptation has been misapplied to a number of phenomena. It
does not indicate a close fitting between
two or more features of an organism such
as muscles being adapted to the skeleton.
Nor does it indicate a fit between the form
and the function (or biological role) of a
feature. Features of an organism cannot
be adapted to its "internal environment"
because the internal environment would
be another attribute of the organism. Nor
can features be adapted to "internal selection" (Stebbins, 1974; Gutmann, 1977) as
this concept implies selection arising from
the internal environment. It is not valid to
speak of "adapted genes" because the concept of adaptation applies to phenotypic
features. Nor is it valid to speak of adapted
individuals, populations or species. If the
concept of adaptation is extended to
genes, individuals, populations and species,
then it must be formulated quite differently as herein and would probably lose
most of its usefulness in evolutionary theory.
Adaptation and adaptive evolutionary
change have often been discussed under
the heading of teleology or teleonomy (Pittendrigh, 1958; Ayala, 1970; Curio,
1973). Although the idea of teleology (or
teleonomy) in its original sense is still of
great value in biology (Mayr, 1974) as a
goal-directed activity or behavior that is
223
BIOLOGICAL ADAPTATION
External Environment
External Selection
B
Environment
Selection Force A
Other Selection Forces
Organism
Internal
Environment
Internal
Other Features in
structural network
plus
all mutual interactions between
them
tructural Network.
FIG. 2. Schematic models to contrast the notion of "internal selection" (A) with the idea of mutual interactions
between features comprising a structural network (B). In both models, feature A is the one under consideration and is judged to be an adaptation. In model A, the feature is an adaptation with respect to the total
selection arising from the external and the internal environments; I reject this model because the internal
environment is a feature of the organism. In model B, the feature is an adaptation to selection forces A, but
its morphology is also influenced, and hence must be explained by the mutual interactions it has with other
features in the structural network. But the features in the structural network are ultimately under the control
of selection arising from the external environment.
controlled by a program, teleological ex- misconstrued as "internal selection" arisplanations are not synonymous with adap- ing from the "internal environment" and
tive explanations (Bock and von Wahlert, independent of external selection from the
1965; Mayr, 1974); teleology and adapta- external environment of the organism
tion must be carefully separated and their (Fig. 2/4). Rather the total influences on the
relationship to one another clarified as adapted feature, be it the selection force
acting directly on it or the mutual interMayr did.
Lastly the analysis of adaptation is action with other features of the structural
closely intertwined with general phenom- network, always relate back to selection
ena such as mechanisms of internal ad- demands on the organism arising from the
justments (the basis of physiological ad- external environment (Fig. W). Adaptaaptation), developmental mechanisms, tions are judged only with respect to selecpleiotrophic effects, etc., that result in mu- tion forces no matter how complex the intual modifications (plasticity) between fea- tervening system of modifiers between the
tures in a network (see Dullemeijer, 1958, environment and the adapted feature may
1974). Of these diverse mechanisms of be. Features are not adapted to internal
mutual modification, those of internal ad- selection of the internal environment or to
justment are of particular interest to mor- demands of the structural network nor do
phologists because they interpose a com- they arise in response to these demands.
plex system of modifiers between the Adaptations are complex systems involvselection force and the adapted feature ing far more than the selection force and
and/or between the feature under selec- the adapted feature as argued by Frazzetta
tion and the remaining features in the (1975), Gutmann (1977 and elsewhere)
structural network. Moreover these mech- and other morphologists. It will not be
anisms of internal adjustment are amena- possible to analyze adaptations to any exble to study by methods used by morphol- tent until the mechanisms of internal adogists. Unfortunately they have been justment and other modifiers are under-
224
WALTER J. BOCK
stood. This is one of the major tasks facing arise even when the several species are unmorphologists, but one that is still largely der the control of the same selection force.
ignored.
They have nothing to do with adaptation
and therefore are termed paradaptive with
Associated concepts
the prefix "para" signifying "besides adThe concept of adaptation does not ex- aptation." The fact that morphologically
plain everything about features and their different adaptations in different species
evolution that is generally considered to be are diverse paradaptations evolving under
under the general heading of adaptive the control of the same selection force has
evolution. At least two associated concepts no bearing on the analysis of adaptation,
must be mentioned, that of preadaptation a spurious problem raised by many evoand that of paradaptation which is inter- lutionists.
locked with the concept of multiple pathways of adaptation.
Independence of adaptation as a state of
Preadaptation refers to features that being and a process
have acquired the necessary properties of
form and function to be adapted to a parWhen considering any individual featicular environmental demand (selection ture, its adaptation (state of being) is inforce) before that selection force has acted dependent of whether it evolved adaptiveon the feature (see Bock, 1959, 1979; Bock ly (process) to the same selection force.
and von Wahlert, 1965 for a more pre- Adaptive features do not have to evolve by
cise definition). The evolution of a fea- adaptive evolution. And features that have
ture to a preadapted state is under the evolved adaptively may no longer be adapcontrol of selection forces associated with tive. These statements must, of course, be
the existing biological roles of the fea- made with respect to specified selection
ture. Or it may be the result of one of the forces. Thus a feature that has just shifted
mechanisms of mutual modification men- from a preadapted to an adapted stage
tioned above. Most important is that evo- would be an adaptation, but has not
lution of a feature to the preadaptive stage evolved adaptively under the control of seis strictly fortuitous with respect to the new lection forces to which it is now adapted.
selection forces. When the new selection Moreover a feature may undergo a seforce interacts with the preadapted fea- quence of adaptive evolutionary changes
ture, the resulting new adaptation is usu- under the control of one set of selection
ally a poor one and it undergoes a subse- forces so that the successive morphological
quent period of postadaptive improvement. stages are also differential adaptations to
The second concept is that of paradap- a second set of selection forces. In parrots,
tation (Bock, 1967) which is associated with the morphology of the tongue appears to
the concept of multiple evolutionary path- have evolved adaptively under the control
ways (Bock, 1959). Adaptations do not of selection associated with feeding. The
evolve only via the mechanism of selection, several different morphological types of
but also depend on the genetic makeup of tongues seen in parrots are also adaptaeach species and the genetic modifications tions to drinking. But their evolution, i.e.,
that happen to arise. These genetical the different tongues and drinking methmechanisms are accidental with respect to ods, is simply the byproduct of adaptive
the current and future selection forces act- evolutionary changes in feeding behavior
ing on the species; they comprise the ac- and is not adaptive in itself, i.e., has not
cidental component of evolutionary change evolved under the control of selection
(Mayr, 1962). Not all differences between forces associated with drinking (Domispecies are adaptive, i.e., the result of dif- nique Homberger, personal communicaferent selection forces acting on the several tion). Thus the demonstration that a feaspecies; many are the results of the genet- ture is an adaptation does not necessarily
ical mechanisms giving rise to different mean that it evolved adaptively under conheritable variation. These differences mav trol of the same selection forces.
BIOLOGICAL ADAPTATION
The assessment of adaptations
Given the theoretical framework for the
concept of adaptation as presented above,
a major question exists of how a feature
can be judged to be an adaptation to a particular selection force. Two questions exist.
The first is which general methods are valid. The second is how accurately can adaptations be determined with valid methods. Herein the problem in ascertaining
adaptations arises from the many mechanisms of mutual modification that may influence the feature under study and from
the difficulty of pinpointing those selection
forces that form a synerg with the feature.
At the onset I would like to emphasize that
it is far more difficult to demonstrate adaptations than supposed by most biologists; we are only fooling ourselves when
reaching adaptive conclusions on the basis
of superficial study.
Methods for showing adaptations can be
grouped under three main headings—the
comparative, the correlative, and the synthetic—of which only the last one is valid.
Each will be treated briefly.
The most commonly used method to
determine adaptations ever since Darwin
is the comparative approach. It is based on
the theory that only one adaptive answer
or only one optimum adaptation exists for
each selection force. Thus if the morphology of feature A of unknown adaptive significance in one species is similar to the
morphology of feature B of known adaptation in a second species, then feature A
has the same adaptive significance as feature B. If feature A differs morphologically from feature B, then its adaptive significance differs from that of feature B.
This method must be lawlike to be applicable. However, it is not because of exceptions arising in two possible ways (Bock,
19776). The first is morphologically different adaptations to the same selection
force—multiple pathways of adaptation
(Bock, 1959, 19776). The second is that
morphological convergences need not be
adaptively convergent—that is, morphological convergences need not evolve under the action of the same selection force.
These categories of exceptions do not
225
mean that adaptations arising in different
lineages under the same selection force
cannot be morphologically similar—many
are, or that morphological convergences
cannot also be adaptively convergent—
many are. The argument is that the existence of these exceptions means that the
comparison method of judging adaptations is not lawlike and hence invalid.
The second method is the correlation
approach; it can be subdivided into two
quite different methods. One is the use of
various multivariate and other statistical
morphometric analyses (e.g., Oxnard,
1972). It is based on the assumption that
close statistical correlation between features or attributes of the same feature indicates functional association and/or adaptive significance. Although these methods
are valuable in many facets of morphological analysis, they simply do not provide
valid approaches to determining or testing
adaptations for two reasons. Statistical
analyses of features omit consideration of
the environment, and adaptation does not
include the fit or adjustment of one feature to another feature of the same organism.
The second correlation approach is to
establish a statistical correlation between
the morphological variation in a feature
and the variation in some environmental
factor (e.g., Lister, 1976). This method suffers mainly from the shortcoming that a
correlation is just that and does not necessarily indicate a cause-effect relationship
between the two correlated variables. Adaptation is a cause-effect relationship with
the cause being the selection force and the
effect being the adaptation—the adaptive
modification of the feature. This method
provides an excellent first step but it is not
sufficient to ascertain the adaptive nature
of a feature. It does provide a valuable basis on which to narrow the scope of the
difficult task of showing the cause-effect
relationship between the environment and
the feature essential for demonstrating adaptations.
The last method and the only valid one
known to me is the synthetic approach
(Bock, 1977a, b) in which the adaptation is
determined by a direct analysis of all of the
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WALTER J. BOCK
aspects of the feature and of the environment illustrated in Figure 1. The synthetic
approach requires a proper study of the
form and functions of the feature as well
as observations of the biological roles and
the selection forces arising from the environmental factors of the umwelt based on
studies made on the organism living free
in its natural environment. It must be
stressed that even with a thorough and
properly conducted study of all factors in
the laboratory and in the field, it is possible
to reach erroneous conclusions about the
adaptive significance of a feature because
the correct selection force was not ferreted
out or because an incorrect cause-effect
relationship was established.
Two common shortcomings of current
morphological studies should be mentioned because these influence the data
available on which to judge adaptations.
The first is the frequent omission of proper description of anatomical form in studies of functional morphology. Generally
the available descriptions in the morphological literature are not sufficient for
functional analysis and the essential correlation between the details of the properties of form and function. The trend has
been toward an excellent functional analysis of morphological black boxes. This can
and should be avoided because description
of morphological form can be done with
the methods available to morphologists.
What is critical is knowing which details of
the form must be described; herein the
essential clues are best provided by the
functional studies.
The second problem is the almost complete omission by morphologists of including field observations in their studies. Ecological morphology is all but nonexistent
in spite of the strong advocacy by Boker
(1935-7) and a few other workers. It is valid to argue that progress must be made
one step at a time and that a development
of functional morphology had to be done
before useful advances in ecological morphology could be made. I have no argument with this position, but I disagree
strongly with the widespread belief that
thorough functional analysis coupled with
proper description of morphological form
is sufficient for the judgment of adaptation. And I disagree with the research
strategy of overstress on functional morphology to the virtual exclusion of ecological morphology. It is simply not possible
to develop a proper foundation on which
to further our understanding of biological
adaptation and of evolutionary morphology without a proper blend of the two approaches.
I have argued earlier (Bock, 1977a) that
the best method to study adaptations is by
a team effort with a minimum of two workers—one a morphologist to do the laboratory analysis and the second an ecologist
to do the field studies—because of the difficulties of one person learning all of the
techniques and background information
to do the entire study alone. Moreover, I
stress the importance of continued feedback between the two workers throughout
the study. Successful study of adaptations
will be achieved rarely on the basis of correlating the results of independently conducted laboratory and field investigations.
My stress on the need for the development of an ecological morphology is for
reasons other than my interest in adaptation and adaptive evolutionary change.
With the extensive modification and destruction of natural environments in the
modern world it may not be possible to
study adaptations even with an excellent
basis of descriptive and functional morphology. This is already true for a number
of species exhibiting interesting facets of
adaptive evolution such as the Hawaiian
honeycreepers (Drepanididae). Moreover
for many others species that are not endangered, their environment has been so
modified that it may no longer be possible
to investigate the organism-environment
relationships responsible for the adaptive
evolution of existing features, a concern
already voiced by Lack (1965). It is for
these reasons that I urge the development
by morphologists of a broader based analysis of biological adaptation using the synthetic approach briefly outlined here.
ACKNOWLEDGMENTS
Most of the background research on
which this analysis is based was done under
BIOLOGICAL ADAPTATION
227
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date evolution. In M. K. Hecht, P. Goody and B.
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Hecht (eds.), Major patterns in vertebrate evolution,
paper was written under the tenure of
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