Download Macroevolution: The Morphological Problem1

AMER. ZOOL., 32:106-112 (1992)
Macroevolution: The Morphological Problem1
KEITH STEWART THOMSON
Academy of Natural Sciences, Philadelphia, Pennsylvania 19103
SYNOPSIS. The central question of macroevolution concerns the evolution of major morphological innovations (and major taxonomic groups). It is a matter of scale rather than
simply rate of evolution or hierarchical level of mechanism. Through the history of microevolutionary theory there is a constant counterpoint of macroevolutionary questioning: are
current versions of microevolution sufficient to explain the data concerning origins of major
novelties? Thus, Simpson proposed the term "megaevolution" and "quantum evolution."
Mayr partially separated micro- and gradual aspects by proposing "genetic revolution" as
a mechanism for rapid change. Evolutionary theory in general suffers because different
concepts have incompatible frames of reference. Key innovation and correlated progression
are concepts that approach the problem in terms of evolutionary morphology. They have
in common with quantum evolution and genetic revolution features such as threshold effect,
preadaptation and smooth transition due to change in function and environment. One aspect
of paleontology and morphology, however, is to show that some morphologies can only
exist in binary states, with no intermediates. This leads us to look away from selection on
graded series of phenotypes to development (developmental cascade/threshold models) for
new levels of explanation.
I have been asked to discuss two interesting concepts of evolutionary morphology—key innovation and correlated progression—in the context of macroevolution
and development. The term macroevolution evidently means many things to many
people (often at the same time). I believe
that the key macroevolutionary question is
not rate of evolution but scale of evolution.
Macroevolution is a discussion of the origin
of major evolutionary (principally morphological) novelties and therefore the origins
of major taxonomic groups. It is not simply
a question of the hierarchical level of evolutionary process—for example species
selection for macroevolution and population genetics for microevolution. It is,
instead, about the scale of evolutionary
change that can be produced at any level.
This wall be the primary focus of the present
paper.
While the origins of major morphological
novelties remain unsolved, one can also view
the stubborn persistence of macroevolutionary questioning, and particularly its
1
From the Symposium on Development and Macroevolution sponsored by the Division of the History
and Philosophy of Biology of the American Society of
Zoologists and presented at the Annual Meeting of the
American Society of Zoologists, 27-30 December 1990,
at San Antonio, Texas.
revival in recent years, as a challenge to
orthodoxy: resistance to the view that the
synthetic theory tells us everything that we
need to know about evolutionary processes?
Similarly, the "success" of particular viewpoints has also depended on reaction against
another (heroes require villains). Therefore,
one can trace out in the history of evolutionary theory a sort of dialectic between
micro- and macro- questions and answers
while at the same time the scale of questions
has changed.
The sub-theme of the paper is that
throughout the history of the subject people
have argued about matters that correctly
belong to different frames of reference. They
then talk "past each other" rather than
together.
Starting with Darwin, all evolutionary
problems were redefined in terms of the
word gradual and it is this word that continually creates a problem. Within a "gradual" context, theories of mechanisms tend
to come and go, depending in large part on
what subject is currently most accessible to
research (i.e., giving decent hard data),
especially through the sine qua non of experimentation. For a hundred and fifty years
we have seen paleontology, morphology,
genetics, and development wax and wane
as dominant evolutionary disciplines (the
custodians of the answers, as it were), but
largely with no effect on the basic macro-
106
THE MORPHOLOGICAL PROBLEM
evolution problem of how major events that
appear to happen very quickly can also be
gradual.
In this discussion I will separately discuss
three aspects of the problem as it developed
in Britain and the United States, showing
the ways in which different approaches
reached their limits, and how they have converged upon a new kind of solution. (Unfortunately, space does not permit a thorough
discussion of antecedent intellectual
traditions, for example in the Soviet Union,
Adams, 1980.)
107
mechanisms of a macroevolution, which
require time on a geological scale." However " . . . we are compelled at the present
level of knowledge reluctantly to put a sign
of equality between the mechanisms of
macro- and micro-evolution, and proceeding on this assumption, to push our investigations as far ahead as this working
hypothesis will permit."
However, after this date we see the wagons formed into a circle at the center of
which is an extreme reductionist gradualism, as exemplified by Fisher's famous
microscope metaphor—his reductio ad
absurdum in which virtually any variant is
THE GRADUALIST'S DILEMMA
hopeless (Thomson, 1988a). In this view
The basic article of faith of a gradualist the organism-environment interaction is a
approach is that major morphological inno- passive filter, whereas, in principle, for any
vations can be produced without some sort individual there might somewhere be a
of saltation. But the dilemma of the New functioning match—in large part because
Synthesis is that no one has satisfactorily each individual creates as well as receives
demonstrated a mechanism at the popula- its own niche.
tion genetic level by which innumerable very
In the face of the strength of the explansmall phenotypic changes could accumulate atory power of the New Synthesis (to say
rapidly to produce large changes: a process nothing of the political power of its expofor the origin of the magnificently improb- nents), paleontologists and morphologists
able from the ineffably trivial. This leads to had by 1950 largely reframed their work in
skepticism about the microevolutionary microevolutionary terms. The watershed
approach. Perhaps, as Waddington (1967) event was the coming together of disciplines
put it: "the whole real guts of evolution— at the 1948 conference at Princeton (Jepson
which is, how do you come to have horses et al, 1949). In this new unity the work of
and tigers, and things—is outside the math- Simpson, especially his 1944 Tempo and
Mode In Evolution, was obviously critical.
ematical theory."
In looking back over the literature of the Simpson applies many of the quantitative
last 60 years, it is fascinating that through- approaches of the population geneticists to
out the whole grand development of the New the fossil record. For example there is his
Synthetic theory, the macroevolutionary classic figure superimposing survivorship
question remains as a constant undercur- curves of fossil pelecypod and carnivore
rent. It persisted even after the mutationism genera and Drosophila individuals. But at
of de Vries, Bateson and others had been the same time, Simpson is still obviously
thoroughly stamped out: "Bateson . . . was testing the premises of Dobzhansky's
unprepared to recognize the mathematical "working hypothesis." He not only distinor statistical aspects of biology, and from guishes microevolution ("changes within
this and other causes he was not only inca- potentially continuous populations" and
pable of framing an evolutionary theory macroevolution ("the rise and divergence
himself, but entirely failed to see how Men- of discontinuous groups"), but also defines
megaevolution which is not merely an etydelism supplied the missing parts of the mologically superior version of macrostructure first erected by Darwin" (Fisher, evolution (as many have supposed) but a
1929).
phenomenon of "major systematic disconIn one of the most influential books of tinuities in the record." It is, however, never
the New Synthetic approach as it evolved clear from Simpson's rambling discussion
in Britain and the United States, Dobzhan- of megaevolution whether he is talking about
sky (1937) was quite circumspect about "the
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KEITH STEWART THOMSON
a pattern to be explained or a special process
that produces discontinuous patterns.
Simpson explores the subject of rates of
evolution in searching for a way in which
evolution could, as it were, "change gears"
while still remaining a gradual process in
the analysis of rates of evolution. But he
still could not completely surrender to the
view that all change must be "micro."
Finally in his Major Features of Evolution
(1953) he declares that "all three monstrous
terminological innovations (micro-, macro-,
and mega-evolution) have served their purpose . . . clarity might now be improved by
abandoning t h e m . . . it is better to recognize
that. . . there are innumerable levels of evolution."
Here Simpson not only rejects an exclusively "micro" theory, which he refers to as
"a myopic outlook"; in invoking different
levels of mechanism, he reverts to another
view of Dobzhansky's. In 1937 Dobzhansky suggested that evolution operates at three
levels: "gene changes . . . the dynamic regularities of the physiology of populations
. . . and the realm of fixation of the diversity
already attained on the preceding two levels." This is the predecessor of the modern
hierarchical approach of Vrba and Eldredge
(1984), Salthe (1986) and others.
Paradoxically however, Simpson embraces
the term quantum evolution, which seems
to be nothing but our old friend macroevolution in disguise—"a particular set of
evolutionary events.. . that involves an allor-none reaction. They are changes of adaptive zone such that transitional forms
between the old zone and the new cannot,
or at any rate do not, persist . . . (this is) a
special, more or less extreme and limiting
case of phyletic evolution" (1953; cf. Simpson, 1944).
By this time, of course, Goldschmidt had
set himself up as the new "mutationist to
be against," supplanting de Vries and Bateson. It is important to note Goldschmidt's
definitions of micro- and macroevolution.
For him, the former is entirely an intraspecific set of processes and macroevolution
starts with the origin of new species. In the
process of rejecting Goldschmidt, microevolutionists closed ranks against any
broader viewpoint. Goldschmidt's The
Material Basis of Evolution (1940) is, however, more interesting than its usual caricature. More revealing than "hopeful monsters" per se is his summary of the
mechanism by which he thought "species
and the higher categories originate in single
macroevolutionary steps as completely new
genetic systems." Goldschmidt's mechanism depends on chromosomal rearrangement producing a new "genetic system which
may evolve by successive steps of repatterning until a threshold for changed action
is reached, (this) produces a change in development which is termed a systemic mutation. Thus selection is at once provided with
the material for quick macroevolution. The
facts of development, especially those furnished by experimental embryology, show
that the potentialities, the mechanics of
development, permit huge changes to take
place in a single step" (1940). The developmental evidence on which he concentrates includes the now very fashionable
homeotic mutations in insects. Goldschmidt was trying to place the question in
a different framework. He failed, but that
does not mean the attempt itself was invalid.
Of all the opponents that Goldschmidt
summoned up, none was more virulent than
Mayr. His Animal Species and Evolution
(1963) is the apotheosis of the New Synthesis and yet even here there is a curious
difficulty in dealing with the small remaining resistant germ of macroevolution. Mayr,
like Simpson, proposes a "micro" exception
to "gradual," this time as genetic revolution.
The language sounds familiar: although set
in terms of population genetics, it is the
same language as Simpson used in the paleontological/morphological context. Thus,
under particular circumstances of population biology, "the mere change of the genetic
environment may change the selective value
of a gene very considerably . . . the most
drastic genetic change (except for polyploidy and hybridization) which may occur
in natural populations, since it may affect
all loci at once. Indeed, it may well have the
character of a veritable 'genetic revolution'
. . . may well have the character of a chain
reaction . . . until finally the system has
reached a new state of equilibrium" (1963).
THE MORPHOLOGICAL PROBLEM
The punctuated equilibrium model of
Eldredge and Gould (1972) was an attempt
to trace out the effect of Mayrian genetic
revolution in peripheral populations at the
finest scale resolvable (rarely) in the fossil
record. Unsuspectedly, this then set off a
renewed interest in matters such as "species
selection" that fall outside the generally
defined mainstream of the New Synthesis,
much to the distress of Mayr and the skepticism of J. M. Smith, and reopened the
question of different levels of evolutionary
mechanism.
In hindsight, the obvious flaw of the otherwise superb population genetics of the New
Synthesis is that it failed to take into account
the fact that morphologies have their origins
not only in evolutionary lineages but in
developmental pathways (see later). The
other, less obvious, drawback is that
hypotheses expressed in the framework of
quantitative genetics of populations produced predictions that are difficult to test at
the morphological, functional, or developmental level. This does not mean that questions posed at those levels are not valid.
However, it should cause us to question
whether "micro" and "macro," as applied
to evolution, really are opposing or alternative terms, and therefore whether one
could ever be explainable in terms of the
other. Both might be explainable in terms
of something else.
109
seemed to be through functional morphology. Experiments with living organisms can
illuminate the evolutionary histories of fossil forms, for example through analysis of
feeding mechanics, thence to adaptation and
from there to ecology and selection, all
changing over broad sweeps of evolutionary
time. Surely analysis of changes in function,
which should be smoothly continuous,
would illuminate such questions as the origin of tetrapods, birds, or mammals.
In modern approaches to functional morphology we see explicit attempts to translate
the concepts of Simpson, Goldschmidt and
Mayr into mechanisms (scenarios) of major
morphological (macro) change.
The concept of the "key innovation"
(Bock, 1965) seems to have arisen as the
functional morphologists' answer to an
interesting aspect of the gradual origin of
major morphological novelties—the question of the adaptiveness of intermediates.
Although Darwin in Chapter 6 of the Origin
deals squarely with the question of the
adaptiveness of "intermediate" stages (for
example in the evolution of the vertebrate
eye), the question is constantly raised: How
can the initial phenotypes of quantum evolution or genetic revolution be adaptive?
This is an obvious legacy of Fisher's extreme
microevolutionary approach.
Miller (1949) and Simpson (1953) proposed that morphological transition might
be facilitated if some "key" change occurs
THE PALEONTOLOGIST'S DILEMMA
that unlocks a suite of following changes.
While the gradualist cannot show how you The key innovation need only be small, if
get there from here, paleontologists and it is (in hindsight) the right one, but it could
evolutionary morphologists have been the open up the possibility of exploitation of a
principal custodians of the view that there new habitat. That would largely take care
might be a "there" to get to. The two-edged of the "gradual" requirement of any model.
problem paleontologists have always had is Bock (1965) added to this the essential (key?)
their data (the very rapid appearance of ingredient of preadaptation. According to
morphological novelties and major groups) this view, the breakthrough event is not the
reveal phenomena that appear to be quan- change in phenotype per se, but a subtle shift
titatively different from those observed on in function as Darwin himself proposed.
the ecological time scale but at the same Then the change would be accomplished
time their research methods and data do not without genetic-developmental/phenotypic
fit the experimental scale of population biol- disruption and without loss of adaptation.
ogy. They cannot fully test, therefore, Morphological change could be accumuwhether there are quantitative, let alone lated slowly either side of the functional shift.
qualitative, differences between underlying
An example of such a shift would be the
evolution of the feather from a modified
mechanisms.
One possible way out of this dilemma has scale, the shift in function of a fish airblad-
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KEITH STEWART THOMSON
der to the tetrapod lung, or the different use
of the lobed fin of an osteolepiform fish in
water compared with the same structure on
land in a tetrapod.
Bock (1965) proposed the key innovation
model in order to account for the origin of
major groups. It is a model in which major
morphological/taxonomic novelty arises
through a succession of key (preadapted)
innovations, each followed by a "period of
postadaptational adjustment associated with
a minor adaptive radiation."
The key innovation model works very well
to describe adaptive radiations at low taxonomic levels. An excellent example appears
to be Liem's analysis of cichlid fish radiations around a subtle change in pharyngeal
jaw mechanics. However, a crucial postulate of key innovation in explaining the origin of a major group (for example the wholesale revision of morphology between
tetrapods and fishes) is the coincidence not
only of a whole host of preadaptive features
in the ancestral group that change one after
the other in rapid succession, but also of
successive diversifications (adaptive radiations) of groups showing "mosaic" features
at each intermediate stage. The fossil record
may show the former, but not the latter. In
fact, the record often shows the opposite for
any major transition—very low taxonomic
diversity.
The "correlated progression" model
(Thomson, 1966) is the complement and in
many ways the opposite of "key innovation." Key innovation isolates phenotypic
features and proposes that they change independently and serially. Correlated progression starts with the fact that features may
be functionally connected and proposes that
if any one changes, all the others will be
affected and may (note, not all must) change
also at the same time. The first example was
taken from the origin of the tetrapod skull
and ear. It proposed that a group of features
that are mechanically linked—jaw mechanics, hyoid arch, middle ear, opercular system—responded in concert to a single environmental context. The result is a positive
feedback that reinforces change. Preadaption is still important in preserving continuity of adaptiveness. An essential part of
the model is that change in any part of the
functional complex without change in the
others would bar, rather than facilitate, further change of the sort that (in hindsight)
produces the major new group. For example
if the respiratory mechanism were changed
alone, leading to loss of the hyomandibular,
then that element would not be available
for future transformation into a stapes. Such
independent changes would occur in some
lineages, of course, leading to a mosaic effect
in the record of diversity. In this case the
mosaic lineages are dead ends rather than
stages in a trend.
Both models have the advantage that
change is integrated by external environmental factors and adaptiveness is ensured.
Neither requires any sort of hopeful monster.
Neither key innovation nor correlated
progression is strictly a model of evolutionary mechanism. They are models of patterns of evolutionary change in morphology
and function. They are useful as interpretations of the fossil record and produce predictions that can be tested against fossil data,
for example in predicting the coupling (key
innovation) or uncoupling (correlated progression) of taxonomic and morphologic
rates. To the extent that they are corroborated or falsified, they then challenge us to
find parallel underlying patterns, and eventually mechanisms, in development and
genetics. But they belong in a different intellectual framework from models of population genetics. They share with the scenarios
of Simpson or Mayr (and Goldschmidt) the
notion of threshold, preadaptation, and
smooth transition based on change in environment and function.
There is, however, a development in paleontology that challenges both models and in
a most productive way. Because of the lack
of fine temporal resolution in most of the
fossil record, it is difficult to know just how
quickly most changes in phenotype actually
have occurred. But it is beginning to be clear
that some "major" phenotypic conditions
can only exist in binary states—with no
graded intermediate stages at all. Perhaps
the best example is in the ankle of some
reptiles, where the astragalus or both astragalus and calcaneum have moved from the
distal to the proximal side of the ankle joint.
No intermediate states are possible. Each
bone can only exist on one side of the joint
THE MORPHOLOGICAL PROBLEM
or the other. And the shift is not a simple
one because it is accompanied by matching
changes in ligaments and muscle insertions
to maintain a functioning joint. Once the
shift is made it becomes the basis of a broad
diversification (in dinosaurs and birds, for
example). The sort of muscle and joint
changes that Liem envisages for cichlid
"innovation" must involve the same sort
of binary state.
At one level, such a binary shift in bone
arrangement could be considered a "key
innovation" because it then allows new
adaptive radiations in limb morphology. But
it is also an example of correlated progression because functionally it must involve
not merely bone arrangement but the simultaneous repatterning of ligaments and muscles.
Numerous (now comical-seeming) scenarios have been proposed to explain the
situation away in terms of the astralagus
slowly becoming vestigial until it can slip
unnoticed to the other side of the joint and
then swell back up again. Of course, the
truth is that such a gradual-morphological
explanation is bound to fail because it is set
in the wrong terms. The transitional states
never existed as adult phenotypes in large
(or small) populations acted upon by strong
selection for different running mechanisms
acting on astragalus size. The transition
occurred at an early stage of development
and required no intermediates at all, just a
single mistake (probably of timing) in the
maturation of various blastemata. Here is
a case where intermediates would all have
been selected against. Is this a case of hopeful monsterism?
THE EMBRYOLOGIST'S DILEMMA
I conclude that quantum evolution,
genetic revolution, key innovation, correlated progression, even hopeful monster, are
all ways of trying to solve (or even to state)
the same problem, only in different frames
of reference. An obvious place to try to find
an underlying unity is in development,
development being a major site of the causation of phenotypes.
The decline of developmental biology as
a force in mid-twentieth century evolution
has two sources. The first is the emptiness
111
of the ontogeny-phylogeny connection. Here
is a wonderful dilemma, in that von Baer's
laws appear to reflect a set of fundamental
facts of nature that has no causal explanatory power. The second factor was the petering out of experimental approaches in the
1950s. They revealed no data useful to the
framing of hypotheses about evolution.
However, in recent years this has completely turned around, with the result that
developmental biology is now a major player
in the subject.
The field of development offers great
opportunity for macroevolution because the
sorts of features that morphologists attempt
to link in models such as key innovation
and correlated progression are integrated in
a more profound way than mechanical functional. Some, at least, are linked morphogenetically and so development ought to
have a great deal to say about models like
key innovation and correlated progression.
Because of the hierarchical nature of the
developmental process, all structural features are basically linked together. Therefore it has been hard to see how major
changes in any part of the phenotype could
be produced with disruption of all—leading
to hopeless monsters. One way around the
problem would be that at very early stages
in development, when pattern formation is
initiated (say the preblastema stage of the
tetrapod limb), only a very tiny subtle change
could produce a significant change at the
end of the developmental pathway (see
Waddington's various models of the developmental landscape).
However, it has long been a major tenet
of evolution (and development) that any
change to early developmental stages would
be extremely disruptive or lethal. This then
is another "embryologist's dilemma."
The problem may have been reinforced
by the false view (see Muller, von Baer, and
Haeckel) that early ontogenetic stages within
a major group are all very similar. In fact,
early stages in development in different lineages differ markedly, and that is where
divergences in phenotype are set in place.
But the features by which they differ are not
a matter of gross morphology but occur at
the gene and cell level. They have to do with
gene expression and genetic and epigenetic
driven processes involving relatively undif-
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KEITH STEWART THOMSON
ferentiated cells. So the dilemma is easily
resolved: because early stages have changed,
they must be capable of change (Thomson,
1988). As such they are available for the
creation of new phenotypes.
Much has been written about new models
for the introduction of evolutionary innovation at the developmental level. Such
models depend on the basic integration of
development and threshold effects in developmental cascades. I have proposed
(Thomson, 1988Z?) that while minor changes
in phenotype might be caused at any stages
in development, major changes must be
caused at early stages. But developmental
cascade/threshold models, as we may call
them, do not require that major changes in
phenotype be caused by drastic revolutions
in development. On the contrary, the new
models can be paraphrased in terms that
rather well parallel the key innovation
model: a small change early in the developmental pathway unlocks a series of subsequent changes. And there are strong
elements of correlated progression: developmental pathways are functionally linked,
therefore changes caused early in development have the chance to affect several phenotypic features simultaneously. Further,
because of the buffered and integrated nature
of development (see Waddington's canalized landscape model), it is by no means
inevitable that new phenotypes will be dysfunctional.
Thus, different variants on the reptilian
ankle must be caused developmentally by a
relatively small shift in timing of blastema
maturation at the prechondrogenic stage of
limb development. They could not be caused
at the late ossification stage because ligament associations are already set in place.
The result is not a monster, but a workable
joint that turns out to have possibilities for
adaptive radiation of limb function. It was
not automatically selected against because
it found/created its own functional niche.
The most exciting possibilities for evolutionary theory now will exist in attempting to find models that provide a feedback
between the functional integration of the
phenotype, and the capacity of the underlying developmental systems to change in
an integrated pattern. (After all, as Louise
Roth once observed [personal communication] Waddington's canalized developmental landscape model is Wright's adaptive landscape seen from underneath!) Then
it would be necessary to find the correct
linkages of those "causes" to the mechanisms of population biology, species selection, and no doubt other factors yet to be
discovered. That would truly be a synthesis.
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