Greg Detre
Saturday, 11 May, 2002
The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme
Stephen Jay Gould and Richard C. Lewontin
in PROCEEDINGS OF THE ROYAL SOCIETY OF LONDON, SERIES B, VOL. 205, NO. 1161 (1979), PP. 581-598
An
adaptationist programme has dominated evolutionary thought in England and the
United States during the past forty years. It is based on faith in the power of
natural selection as an optimizing agent. It proceeds by breaking an organism
into unitary "traits" and proposing an adaptive story for each
considered separately. Trade-offs among competing selective demands exert the
only brake upon perfection; nonoptimality is thereby rendered as a result of
adaptation as well. We criticize this approach and attempt to reassert a
competing notion (long popular in continental Europe) that organisms must be
analyzed as integrated wholes, with Baupl� so constrained by phyletic
heritage, pathways of development, and general architecture that the
constraints themselves become more interesting and more important in delimiting
pathways of change than the selective force that may mediate change when it
occurs. We fault the adaptationist programme for its failure to distinguish
current utility from reasons for origin (male tyrannosaurs may have used their
diminutive front legs to titillate female partners, but this will not explain
why they got so small); for its unwillingness to consider alternatives to
adaptive stories; for its reliance upon plausibility alone as a criterion for
accepting speculative tales; and for its failure to consider adequately such
competing themes as random fixation of alleles, production of nonadaptive structures
by developmental correlation with selected features (allometry, pleiotropy,
material compensation, mechanically forced correlation), the separability of
adaptation and selection, multiple adaptive peaks, and current utility as an
epiphenomenon of nonadaptive structures. We support Darwin's own pluralistic
approach to identifying the agents of evolutionary change.
in the great central dome of St. Mark's Cathedral in Venice:
�spandrels-the tapering triangular spaces formed by the intersection of two rounded arches at right angles are necessary architectural byproducts of mounting a dome on rounded arches. Each spandrel contains a design admirably fitted into its tapering space�
�The design is so elaborate, harmonious, and
purposeful that we are tempted to view it as the starting point of any
analysis, as the cause in some sense of the surrounding architecture. But this
would invert the proper path of analysis. The system begins with an
architectural constraint: the necessary four spandrels and their tapering
triangular form�
we do not impose our biological biases upon such architectural constraints
Dr. Pangloss: "Things cannot be other than they are... Everything is made for the best purpose. Our noses were made to carry spectacles, so we have spectacles. Legs were clearly intended for breeches, and we wear them."
�Yet evolutionary biologists, in their tendency to focus exclusively on immediate adaptation to local conditions, do tend to ignore architectural constraints and perform just such an inversion of explanation�
or alternatively:
Harner proposed (1977) that Aztec human sacrifice arose as a solution to chronic shortage of meat (limbs of victims were often consumed, but only by people of high status)
Gould & Lewontin suggest that cannibalism is rather a secondary epiphenomenon representing a fruitful use of available parts, not a cause of the entire system
�To put it crudely: a system developed for other
reasons generated an increasing number of fresh bodies; use might as well be made
of them. Why invert the whole system in such a curious fashion and view an
entire culture as the epiphenomenon of an unusual way to beef up the meat
supply.�
Moreover, as Sahlins argues, it is not even clear that human sacrifice was an adaptation at all
if these had been biological systems, would we not, by force of habit, have regarded the epiphenomenal adaptation as primary and tried to build the whole structural system from it?
the adaptationist/Panglossian programme (Wallace and Weismann, but not Darwin) = �the near omnipotence of natural selection in forging organic design and fashioning the best among possible worlds. This programme regards natural selection as so powerful and the constraints upon it so few that direct production of adaptation through its operation becomes the primary cause of nearly all organic form, function, and behavior�
usually proceeds by two steps:
1. �An organism is atomized into "traits" and these traits are explained as structures optimally designed by natural selection for their functions�
deciding what a trait is is not as simply as it�s made out to be either
e.g. If we regard the chin as a "thing,"
rather than as a product of interaction between two growth fields (alveolar and
mandibular), then we are led to an interpretation of its origin
(recapitulatory) exactly opposite to the one now generally favored (neotenic).
Organisms are integrated entities, not collections of discrete objects
2. �After the failure of part-by-part optimization � The notion of "trade-off' is introduced, and organisms are interpreted as best compromises among competing demands�
�Any suboptimality of a part is explained as its contribution to the best possible design for the whole. The notion that suboptimality might represent anything other than the immediate work of natural selection is usually not entertained�
The adaptationist programme is truly Panglossian.
Our world may not be good in an abstract sense, but it is the very best we
could have. Each trait plays its part and must be as it is.
in response to evolutionists� protests that they admit �genetic drift, allometry, and a variety of reasons for nonadaptive evolution�, Gould & Lewontin �maintain that alternatives to selection for best overall design have generally been relegated to unimportance by this mode of argument.�
e.g. genetic drift: it can only be important in
populations so small that they are likely to become extinct before playing any
sustained evolutionary role (but see Lande, 1976).
The adaptationist programme can be traced through common styles of argument:
1. If one adaptive argument fails, try another
�The eskimo face, once depicted as "cold engineered" (Coon, et al., 1950), becomes an adaptation to generate and withstand large masticatory forces (Shea, 1977).�
2. If one adaptive argument fails, assume that another must exist; a weaker version of the first argument
3. In the absence of a good adaptive argument in the first place, attribute failure to imperfect understanding of where an organism lives and what it does
4. Emphasize immediate utility and exclude other attributes of form
e.g. the explanatory information accompanying the full-scale Fiberglass Tyrannosaurus at Boston's Museum of Science reads: �Front legs a puzzle: how Tyrannosaurus used its tiny front legs is a scientific puzzle; they were too short even to reach the mouth. They may have been used to help the animal rise from a lying position.�
�We don't doubt that Tyrannosaurus used its diminutive front legs for something. If they had arisen de novo, we would encourage the search for some immediate adaptive reason. But they are, after all, the reduced product of conventionally functional homologues in ancestors (longer limbs of allosaurs, for example). As such, we do not need an explicitly adaptive explanation for the reduction itself.�
�It is likely to be a developmental correlate of
allometric fields for relative increase in head and hindlimb size. This
nonadaptive hypothesis can be tested by conventional allometric methods (Gould,
1974, in general; Lande, 1978, on limb reduction) and seems to us both more
interesting and fruitful than untestable speculations based on secondary
utility in the best of possible worlds. One must not confuse the fact
that a structure is used in some way (consider again the spandrels, ceiling
spaces, and Aztec bodies) with the primary evolutionary reason for its
existence and conformation�
(???)
the rejection of one adaptive story usually leads to its replacement by another, rather than to a suspicion that a different kind of explanation might be required
but the range of adaptive stories is as wide as our
minds are fertile, new stones can always be postulated
the criteria for acceptance of a story are so loose that many pass without proper confirmation
but often, evolutionists use consistency with
natural selection as the sole criterion and consider their work done when they
concoct a plausible story. But plausible stories can always be told.
�The key to historical research lies in devising criteria to identify proper explanations among the substantial set of plausible pathways to any modern result�
considers Barash�s conclusions about the adaptiveness of anti-cuckoldry behaviour in mountain bluebirds�
�Although Darwin regarded selection as the most important of evolutionary mechanisms (as do we), no argument from opponents angered him more than the common attempt to caricature and trivialize his theory by stating that it relied exclusively upon natural selection�
�We
do not now regard all of Darwin's subsidiary mechanisms as significant or even
valid, though many, including direct modification and correlation of growth,
are very important. But we should cherish his consistent attitude of pluralism
in attempting to explain Nature's complexity�
an incomplete hierarchy of alternatives to immediate adaptation for the explanation of form, function, and behavior:
�population geneticists are sharply divided on the question of how much genetic polymorphism within populations and how much of the genetic differences between species is, in fact, the result of natural selection as opposed to purely random factors�
genetic differentiation independent of natural selection is especially likely for small populations where the selection intensity is low
(�as a result of this restriction in population
size, frequencies of alleles change by genetic
drift, a kind of random genetic sampling error�)
pleiotropy, allometry, "material compensation"� (Rensch, 1959, pp. 179-187) and� mechanically forced correlations in D'Arcy Thompson's sense (1942; Gould 1971)
here we come face to face with organisms as
integrated wholes, fundamentally not decomposable into independent and
separately optimized parts (???)
see question below on example
A wide spectrum of "good design" may be purely phenotypic in origin
the different means/causes for an organism being well-suited to its environment get masked by using the same term, �adaptation�, for all of them
i.e. over-extending Darwinian adaptation to the other two types
Physiological adaptations are not heritable, though the capacity to develop them presumably is
cultural adaptation is a �heritable� (by learning) form of non-Darwinian adaptation in humans (and, in rudimentary ways, in a few other advanced social species)
Much confused thinking in human sociobiology arises
from a failure to distinguish this mode from Darwinian adaptation based on
genetic variation
e.g. when a species produces more than one equally adaptive solution to a problem
The immediate utility of an organic structure often says nothing at all about the reason for its being (see spandrels, spaces, and cannibalism)
e.g. �if blushing turns out to be an adaptation affected by sexual selection in humans, it will not help us to understand why blood is red�
�In continental Europe, evolutionists have never been much attracted to the Anglo-American penchant for atomizing organisms into parts and trying to explain each as a direct adaptation�
a) strong form
�natural selection under the adaptationist programme
can explain superficial modifications of the Bauplan that fit structure to
environment: why moles are blind, giraffes have long necks, and ducks webbed
feet, for example. But the important steps of evolution, the construction of
the Bauplan itself and the transition between Baupl�, must involve some other
unknown, and perhaps "internal," mechanism. We believe that English
biologists have been right in rejecting this strong form as close to an appeal
to mysticism� (???)
b) weak form
�the argument has a weaker -- and paradoxically
powerful -- form that has not been appreciated, but deserves to be. It also
acknowledges conventional selection for superficial modifications of the
Bauplan. It also denies that the adaptationist programme (atomization plus
optimizing selection on parts) can do much to explain Baupl� and the
transitions between them. But it does not therefore resort to a fundamentally
unknown process. It holds instead that the basic body plans of organisms are so
integrated and so replete with constraints upon adaptation (categories 2 and 5
of our typology) that conventional styles of selective arguments can explain
little of interest about them. It does not deny that change, when it occurs,
may be mediated by natural selection, but it holds that constraints restrict
possible paths and modes of change so strongly that the constraints themselves
become much the most interesting aspect of evolution�
considers at least two categories of constraints on
evolutionary change:
phyletic constraints � ???
developmental constraints
basically, they seem to be saying that �architectural constraints can exert a far-ranging [but underestimated] influence upon organisms as well�
�We do not offer a council of despair, as adaptationists have charged; for nonadaptive does not mean nonintelligible. We welcome the richness that a pluralistic approach, so akin to Darwin's spirit, can provide�
�Under the adaptationist programme, the great historic themes of developmental morphology and Bauplan were largely abandoned: for if selection can break any correlation and optimize parts separately, then an organism's integration counts for little�
where does Sahlins� comment on cannibalism fit in with Gould & Lewontin � do they agree or not???
�Sahlins (1978) has argued that human sacrifice represented just one part of an elaborate cultural fabric that, in its entirety, not only represented the material expression of Aztec cosmology, but also performed such utilitarian functions�as the maintenance of social ranks� and systems of tribute among cities�
I think they are disagreeing with him too
orthogenesis /<revc>:T<schwa>(U)"dZEnIsIs/ n.L19. [f. ORTHO- + -GENESIS.] Biol. Evolutionary change in one direction, esp. as supposedly caused by internal tendency rather than external influence.
allometry /<schwa>"lQmItri/ n.M20. [f. ALLO- + -METRY.] Biol. Growth of a part at a rate different from that of the body as a whole (or of some other standard). Cf. ISOMETRY 2.
�(3) The decoupling of selection and adaptation. / (i) Selection without adaptation. Lewontin (1979) has presented the following hypothetical example: "A mutation which doubles the fecundity of individuals will sweep through a population rapidly. If there has been no change in efficiency of resource utilization, the individuals will leave no more offspring than before, but simply lay twice as many eggs, the excess dying because of resource limitation. In what sense are the individuals or the population as a whole better adapted than before? Indeed, if a predator on immature stages is led to switch to the species now that immatures are more plentiful, the population size may actually decrease as a consequence, yet natural selection at all times will favour individuals with higher fecundity."
surely though, such a species will be less adaptive, if only because it has to carry around twice as many (twice as heavy) eggs, waste the resources needed to grow twice as many eggs, and the survival rate for an individual egg may even be lower???
Bauplan = a creature's overall architectural scheme. For example, a jellyfish or five-pointed starfish has a very different Bauplan from you and me