Punctuated Equilibrium and
Phyletic Gradualism
Peter R Sheldon, The Open University, Milton Keynes, UK
Secondary article
Article Contents
. Punctuated Equilibrium and Phyletic Gradualism:
Competing Models
. The Perception of Microevolutionary Patterns
Punctuated equilibrium and phyletic gradualism are contrasting patterns of evolution
among a spectrum of patterns found in the fossil record. In punctuated equilibrium, species
tend to show morphological stasis between abrupt speciation events, whereas in phyletic
gradualism species undergo more continuous change.
Punctuated Equilibrium and Phyletic
Gradualism: Competing Models
In 1972, Niles Eldredge and Stephen Jay Gould stimulated
controversy by publishing their model of punctuated
equilibrium, according to which most evolutionary change
in the history of life is concentrated in brief speciation
events. The belief that fossil species remained in morphological stasis throughout their existence had prompted
them to propose this pattern as an alternative to what they
saw as the pervasive paradigm of gradual evolution within
lineages, which they called phyletic gradualism. In phyletic
gradualism, evolution occurs slowly and steadily, without
the particular association of morphological change with
speciation; as much or more evolutionary change takes
place between times of branching as at the branching
points themselves (Figure 1a). By contrast, in punctuated
equilibrium a new species originates rapidly (a punctuation) and then remains virtually unchanged (in equilibrium
or stasis) for the rest of its existence (Figure 1b). The new
species might itself bud off another one, but the parent
species remains in stasis until its extinction. Thus, in
punctuated equilibrium, significant evolutionary change
occurs at events of branching speciation (cladogenesis) and
not during the in toto transformation of lineages (anagenesis). Eldredge and Gould maintained that the pattern of
punctuated equilibrium was more or less universal,
whereas phyletic gradualism was rare or absent.
Eldredge and Gould based their model on observations
of fossils, especially Devonian trilobites and Pleistocene
land snails, and on modern speciation theory. They
invoked Mayr’s concept of peripatric speciation, a form
of allopatric speciation involving the geographic isolation
of small, localized populations peripheral to the main
species range. Such founder populations, often with a
somewhat random or unusual sampling of the parental
gene pool, would undergo rapid selection to local
conditions, and soon become reproductively isolated from
the parent population. Eldredge and Gould argued that
various ‘constraints’, especially those on genetic variability
and developmental pathways, maintained stasis in the
parent population. These constraints, however, became
. Examples from the Fossil Record
. Recent Developments
. Conclusion
broken down in small, isolated populations, allowing rapid
change by natural selection and genetic drift, possibly
facilitiated by macromutations. The possible genetic
mechanisms involved were then, and remain, controversial.
Eldredge and Gould argued that, if peripatric speciation
is the norm, the rise of new species is expected to be
episodic, local and rapid – as opposed to continuous,
widespread and slow. The chances of finding in the fossil
record intermediate forms recording a speciation event is
therefore bound to be low. Previously, such a lack of
intermediates had largely been accounted for by incompleteness of the record, or by grossly inadequate sampling.
But if peripatric speciation prevails, the fossil record can be
taken more at face value; it would explain as real
phenomena both the sudden appearance of new fossil
species and the rarity of intermediate forms ‘caught in the
act’ of speciation.
Eldredge and Gould stated that the species-forming
event would normally take less than 1% of that species’
later existence in stasis. This means, for example, that for a
new species subsequently lasting a million years, reproductive isolation and associated morphological evolution
might take the order of 5000–10 000 years – ample time for
these two processes to occur from the perspective of a
neontologist. This is an awkward timespan: too long for
direct observation, yet normally too short for details to be
resolved in the fossil record. Net, long-term stasis, by
contrast, can readily be verified simply by finding very
similar morphologies separated by extensive time intervals;
its demonstration does not require a high-resolution
record.
In punctuated equilibrium, fossiliferous strata should
typically show a species in stasis, with sharp morphological
change recording the migration of the descendant species
from the peripheral, isolated area in which it developed.
Cases of introduced species in modern times (e.g. the
starling in North America) demonstrate the rapidity with
which a species may spread across large areas. In the strict
version of punctuated equilibrium, the presumed ancestor
is then expected to persist for a while alongside its
descendant species. In the toned-down version – not
ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net
1
Time
Punctuated Equilibrium and Phyletic Gradualism
Morphology
(a)
Morphology
(b)
Figure 1 The contrasting idealized evolutionary patterns of phyletic gradualism (a) and punctuated equilibrium (b). The top of each evolutionary tree
represents the most recent time depicted, such as the present day. The horizontal distance between lineages (lines of descent) indicates in a general way the
degree of morphological difference. In punctuated equilibrium (b), all morphological change is concentrated in branching events, with stasis thereafter.
Note that, for comparative purposes, the two patterns have been drawn with a similar number of branching points. In addition, if the patterns are
superimposed on each other, corresponding lineages have the same morphology at the latest occurrence (i.e. at their extinction or at the top of the
diagram), but the morphological routes by which lineages reach those points are very different in each pattern. Fine-scale zig-zags in morphology will occur
in both phyletic gradualism and punctuated equilibrium, but are not shown in these simplified depictions.
strictly punctuated equilibrium – punctuated change may
occur within a lineage, but without lineage branching
(speciation); this is sometimes referred to as punctuated
anagenesis or punctuated gradualism (see below).
Eldredge, Gould, Stanley and others claimed that stasis
was a major, hitherto largely unrecognized or ignored,
feature of the fossil record, and should be studied in its own
right. ‘Stasis is data’, became the punctuationists’ motto.
As Gould and Eldredge (1993) wrote, ‘palaeontologists
never wrote papers on the absence of change in lineages
before punctuated equilibrium granted the subject some
theoretical space’.
Punctuated equilibrium differs radically from traditional neo-Darwinism, not in claiming that stasis is
common, but mainly by proposing that (1) species
generally evolve new morphologies only when a small
population becomes a new, reproductively isolated species,
and (2) long-term trends in morphology among taxa above
the species level are often the consequence not of natural
selection favouring genotypes within species, but of a
higher-level selection process among species (see below).
The concept of punctuated equilibrium has certainly
exerted a strong influence on the investigation and
interpretation of evolutionary patterns (see review by
Gould and Eldredge, 1993). Many supposed textbook
cases of gradualism dissolved in the wake of punctuated
equilibrium and in the climate of more rigorous analysis
that it produced. Some have argued, however, that
Eldredge and Gould’s version of phyletic gradualism was
to some extent a straw man, because they identified one of
its tenets as being that ‘transformation is even and slow’.
Punctuationists have also been criticized as portraying the
modern synthesis as more reductionist, i.e. denying
2
hierarchical processes, than it in fact was. Although it is
true that the prevalence of morphological stasis in fossil
species was not widely predicted by the modern synthesis,
many earlier authors, such as Simpson, believed that
evolution embraced a wide spectrum of rates. Darwin’s
position in The Origin of Species was essentially gradualistic, especially in explaining the evolution of complex
adaptations by the step-by-step accumulation of relatively
small changes. Interestingly, however, in response to
criticism by Hugh Falconer, a palaeontologist, Darwin
inserted a passage concerning the stability of species in
later editions, and in chapter 15 there is this statement:
‘Many species when once formed never undergo any
further change but become extinct without leaving
modified descendants; and the periods during which
species have undergone modification, though long as
measured by years, have probably been short in comparison with the periods during which they retain the same
form.’ Darwin was thus not a caricature phyletic gradualist
in that he did not insist there was constant, steady
evolution within lineages between speciation events.
The Perception of Microevolutionary
Patterns
The ideal requirements for establishing microevolutionary
patterns include large numbers of complete fossil specimens from successive, small time intervals; a precise and
accurate framework of absolute ages; samples that span
the entire geographical and temporal range of all closelyrelated species; a correct understanding of phylogenetic
Punctuated Equilibrium and Phyletic Gradualism
relationships; and statistical data on many characters from
different ontogenetic stages of each species. Some of these
requirements are rarely if ever met, and many relevant
hypotheses, such as ancestor–descendant relationships,
are never truly verifiable; as usual, we must assess the
relative probabilities of competing hypotheses. A knowledge of geographical variation is highly desirable because
spurious patterns of change may arise in local areas
owing, for example, to the immigration or emigration of
intraspecific variants tracking their favoured environment.
The critical test of punctuated equilibrium (sensu stricto)
is whether morphological change is usually accompanied
by lineage splitting, i.e. cladogenesis or true speciation – as
opposed to chronospeciation, in which sufficient anagenetic evolution occurs within a single lineage for forms to
be considered different species and given a different name.
One obvious problem is that speciation cannot be
recognized in the fossil record by the aquisition of
reproductive isolation, and is instead necessarily equated
with the initiation of considerable morphological difference in preserved features. The most that palaeobiologists
can do is try to make species they describe live up to a
definition such as ‘Species are morphologically distinct
groups within which variation is of the magnitude expected
in interbreeding populations, and between which the
differences are of the kind and degree expected to result
from reproductive isolation in natural populations.’
There are actually very few indisputable cases of lineage
splitting observed in the fossil record, and sister species
have to be inferred but cannot be proven. Demonstrating
stasis in a species is not the same as demonstrating
punctuated speciation. In theory, for example, a peripheral
isolate could evolve gradually (and undetected) to a new
species which, on migration, appeared abruptly alongside
its parent species, thereafter remaining in stasis.
Darwin recognized that gaps in the geological record,
both vertically (in time) and laterally (in space), can
generate an impression of abrupt evolutionary change
(though not stasis). In addition, various biases can arise
during the collection and description of fossils that
generate an impression of punctuation and stasis, irrespective of the real pattern (Sheldon, 1996). These biases
include the requirement to apply binominal taxonomy (i.e.
discrete Linnean names) to fossils as well as living
organisms; the amalgamation of fossils collected from
strata of different ages, thereby submerging any evidence of
change within one large sample; a natural reluctance to use
terms implying uncertainty when identifying species; an
absence of formal nomenclature to signify small morphological differences between samples of different age; the
plotting of species time ranges in charts as vertical lines;
and cladistic methods of phylogenetic analysis.
The almost universal textbook depiction of gradualistic
evolution as straight-line change (as in Figure 1a) reflects
another bias that has hindered understanding of microevolutionary patterns. This is the predisposition not to
expect evolutionary reversals or to dismiss them as
ecophenotypic effects, the migration of ecotypes, genetic
drift or sampling problems – all of which are possible
influences on individual patterns. The simplest, most
tempting way of connecting successive data points – a
straight line – is, in fact, highly unlikely in nature. Straightline evolution, that is prolonged unidirectional evolution at
a constant rate, is, in fact, a very special case, like drawing
one random walk and expecting evolution to follow that
particular path (Sheldon, 1996). Reversals can be expected
to occur in any long record of a single character, as long as
that character maintains some variation. Cases of gradualism will indeed seem rare if the theoretical pattern is set
up as unidirectional change. High-resolution fossil records
show that patterns generally become increasingly dynamic
(fluctuating, with reversals) as finer timespans are resolved;
how patterns are perceived can thus depend on sampling
strategy and the extent to which samples from different
timespans have been amalgamated. The pervasiveness of
reversals also explains the general observation of an inverse
relationship between measured rates of evolution and the
timespan under consideration.
Examples from the Fossil Record
In a survey of 58 detailed studies made since 1972, covering
a wide range of taxa and different geological periods, Erwin
and Anstey (1995) concluded there was no overwhelming
signal, but a wide variety of patterns. These certainly
include punctuated equilibrium and phyletic gradualism,
but also many cases of punctuated anagenesis (stasis and
abrupt change in lineages without branching), with some
cases of gradual multiple branching, punctuated multiple
branching, and so on. Patterns can be complex within
individual lineages, let alone groups. For example, Geary
(in Erwin and Anstey, 1995) describes a Cenozoic
gastropod lineage that underwent stasis for 7 million
years, followed by prolonged directional change to a new
species over about 2 million years.
One of the most convincing examples of punctuated
equilibrium, with stasis and with abrupt speciation, is that
of the bryozoan genus Metrarabdotos described by
Cheetham in 1986 and subsequent papers (see Cheetham
and Jackson in Erwin and Anstey, 1995). Cheetham
measured up to 46 characters in colonies of this sessile
marine organism from Cenozoic strata in the Dominican
Republic, and applied a method of discriminant analysis to
determine patterns of morphological change in the
numerous samples. Most of the 19 species concerned
appeared suddenly, changed very little (if at all) through
some millions of years, and tended to persist alongside their
inferred parent species. This case was lent extra weight by
studies of Cheetham and Jackson (see their paper in Erwin
and Anstey, 1995) on living species of a closely related
3
Punctuated Equilibrium and Phyletic Gradualism
genus, Stylopoma. They investigated the extent to which
species of Stylopoma that had been differentiated according to the morphology of their hard parts alone corresponded with genetic differences determined by protein
electrophoresis. There was an excellent correspondence
between morphology and genetics, so that in Stylopoma –
and by inference Metrarabdotos species (which are either
extinct, or rare and hard to study) – equating morphospecies with true species seems justified.
In a remarkable demonstration of stasis, several species
of living bivalves studied in great morphological detail by
Stanley and Yang (1987) show no more evolutionary
change over the past 17 million years than there is
geographic variation within each species at the present
day. Brett and Baird (in Erwin and Anstey, 1995), studying
shallow marine Devonian faunas, found a pattern of
species occurring in discrete packages that appeared and
disappeared more or less in synchrony, a phenomenon they
coined ‘coordinated stasis’. Once originated, new biotas
persisted more or less intact in both morphologies of
constituent species and ecological associations for several
million years, despite environmental fluctuations. Only the
occasional, more extreme perturbations produced change
when thresholds were exceeded.
Other clear examples of at least approximate stasis have
been documented in invertebrate groups such as ostracods,
brachiopods, gastropods, echinoids and corals (see references in Erwin and Anstey, 1995). The average duration of
marine invertebrate species is about 5 million years. In
general, notwithstanding the biases mentioned above, the
fact that Linnean taxonomy can be applied more or less
successfully to such fossils, as well as to living forms, is itself
good evidence for the prevalence of approximate stasis.
A study by Sheldon of about 15 000 Ordovician
trilobites from central Wales revealed a pattern of broadly
parallel, gradualistic evolution in eight lineages from a
moderately deep, relatively stable marine setting (see
references in Sheldon, 1996). Over a period of about 2
million years, a variety of changes took place at different
times in different lineages, with patterns far closer to
phyletic gradualism than to punctuated equilibrium. There
was no evidence of lineage branching. The ‘missing links’
between previously described, successive trilobite species
were no longer missing: intermediate horizons yielded
specimens of intermediate morphology. The end members
of most lineages had been assigned to different species and,
in one case, to different genera. The apparent success of
earlier Linnean nomenclature (with its implications of
discrete species) could easily have been misinterpreted as
evidence of punctuation and stasis. Many of the other
biases mentioned above could be seen in retrospect to have
obscured evidence of gradual evolution, and temporary
reversals made it impractical to subdivide each lineage into
discrete successive chronospecies.
A wide spectrum of patterns has been observed in
vertebrates, as documented in a comprehensive survey by
4
Carroll (1997). The average duration of mammal species in
the fossil record is about 0.5–1 million years, i.e. shorter
than for marine invertebrates. There seems to be a
tendency for mammals to show more continuous evolution
than many invertebrate groups, but there are numerous
exceptions. Prothero and Heaton (1996) document the
prevalence of stasis in some North American mammals
during a period of intense climatic change in the mid
Cenozoic. Unlike many Quaternary organisms, the mammals apparently did not even migrate in response to
climatic changes. Badgley and Behrensmeyer (1995, for
reference see Sheldon, 1996) found that, at another time
in the Cenozoic, the mammals of Pakistan underwent
their greatest faunal turnovers, with considerable gradual
turnover too, during a long interval in which no climatic
change is discernible. They argue that these results, and
others showing persistent evolution of early Cenozoic
mammals during stable or gradually changing climates,
support the Red Queen model of evolution driven by
continuous biotic interaction, but they emphasize that
major climatic change also initiated substantial faunal
change.
There are few high-resolution data on land plants, but
stasis appears to be common in the deltaic floras of the
Carboniferous tropics. Quaternary beetles are well known
for showing remarkable stasis throughout glacial–interglacial cycles. Marine microorganisms such as planktonic
foraminiferans and radiolarians show a tendency for slow,
gradualistic evolution, but again a wide spectrum of
patterns has been discovered.
Although some correlations are emerging between
individual groups of organisms and particular evolutionary patterns, most patterns will have to remain suggestive
at best until many more high-resolution studies are
undertaken.
Recent Developments
A major criticism of punctuated equilibrium is ample
evidence that populations can evolve in response to
selection without speciating. Many fossil sequences reveal
evidence of stasis within species, but not direct evidence for
the association between morphological change and speciation. Gould and Eldredge (1993) agree that speciation does
not cause morphological change, but they maintain that
the two are strongly associated with each other in the fossil
record. To explain this, they accept the suggestion of
Futuyma (see also Futuyma, 1998) that speciation facilitates long-term morphological divergence not by accelerating morphological change but by preventing such
changes being broken down by recombination when
divergent populations eventually come into contact with
ancestral phenotypes. Reproductive isolation allows
changes to be ‘locked up’; speciation thus provides
Punctuated Equilibrium and Phyletic Gradualism
morphological changes with enough permanence to be
registered in the fossil record.
There are several possible mechanisms by which stasis
could be maintained, including stabilizing selection, the
close tracking of favoured habitats by migration, and
genetic constraints, but their relative contributions are not
yet understood. According to Eldredge (1995), stasis is an
outcome of both habitat tracking and, more importantly,
the organization of species in the wild as envisaged by
Sewall Wright in his ‘shifting balance theory’. Where
species are split up into so many local populations in
slightly different habitats, evolution cannot proceed in a
lock-step, gradualistic transformation of the entire species
because local populations are evolving in different directions. Eldredge places great importance on factors that
affect the probability of survival of fledgling species, and
argues that the rate of successful speciation is higher in the
tropics because, being easier to specialize, it takes less
adaptive change at speciation for a newly evolved species to
establish ecological distance from its ancestor.
Given the variety of patterns reported above, the
challenge is to find a better synthesis incorporating
elements of both punctuated equilibrium and phyletic
gradualism, and encompassing evidence from the spectrum
of patterns obtained so far.
There is now a wealth of evidence that contradicts the
widely held, intuitive expectation that evolution is the
normal response of species to environmental change, and
that stable environments lead to stable morphology.
According to the Plus ça change model (Sheldon, 1996),
the more that the physical environment changes, the more
that species stay the same. Morphological stasis over
geological time scales thus tends to arise not from the
stability of physical environments, but from their instability.
In this model (Figure 2), stasis is the usual response to
widely fluctuating physical environments over geological
time scales, until thresholds are reached (when punctuational change or extinction occurs). The lineages that
survive in an environment of widely fluctuating sea levels,
climate, substrate, salinity, etc., become relatively inert to
each environmental twist and turn, in contrast to more
evolutionarily sensitive lineages in less changing environments. Generalists in this long-term sense – i.e. most
species found in the fossil record – are species with
properties that enable them to survive throughout wide
environmental fluctuations for millions of years; they are
distinct from ecological generalists (eurytopes). The
relationship between evolution and environmental change
is thus far from simple; the impact of an event is contingent
on the long-term history of the system. If a species has been
subjected many times to large physical disturbances, yet
another such disturbance might yield no response at all (or
might send it over a threshold). Conversely, smaller, less
easily detectable, events following a long stable interval
may often promote evolution. So, in this perspective, small
Figure 2 The Plus ça change model, which combines aspects of
punctuated equilibrium and phyletic gradualism. It proposes that, over
geological time scales (e.g. a million years), gradualism (right) is
characteristic of narrowly fluctuating, relatively stable environments such
as a tropical rainforest and the deep sea. By contrast, punctuated
equilibrium (left) is expected to prevail in the physically more unstable
environments that dominate the fossil record, especially shallow seas. In a
widely fluctuating environment (left), a threshold might occur not only
when some environmental variable exceeds wide reflecting boundaries,
but also when it contracts to become narrowly fluctuating (dotted line).
‘Environment’ indicates some long-term physical aspect of environment,
e.g. mean temperature, sea level or type of sediment forming the sea floor.
‘Morphology’ indicates some aspect, or aspects, of morphology. Highfrequency environmental oscillations such as annual cycles are not shown.
From Sheldon (1996), modified after Sheldon (1990).
abiotic events and biotic interactions tend to drive
evolution, but so also do some major perturbations.
The Plus ça change model includes the following
predictions: (i) continuous, gradualistic evolution tends
to occur on land in the tropics (e.g. tropical rainforest), in
the deeper sea, in settings with many biotic interactions,
and at times of global climatic stability; (ii) stasis, and
occasional punctuation, tends to occur in shallow marine
settings, in temperate latitudes, in settings subject to wide
abiotic fluctuation, and at times of global climatic
instability. Any such model will have many exceptions
(among which appears to be the evolution of Homo
sapiens).
If true as a tendency, the model has some important
implications (Sheldon, 1996). Punctuated equilibrium may
be being mistakenly perceived as the overwhelming pattern
in the history of life because the environments in which
gradualism is expected to prevail (e.g. a tropical rainforest
and the deeper sea) are rarely represented in the fossil
record. The vast majority of fossils come from dynamic
shallow marine environments, so it is not surprising that
many fossil lineages show approximate stasis and occasional punctuations. Another implication is that many of
today’s species, especially in temperate latitudes, may be
relatively inert to environmental twists and turns, having
lived through 2 million years of Quaternary climate
5
Punctuated Equilibrium and Phyletic Gradualism
upheavals, and that, except for human-influenced environments, relatively little microevolution may be occurring
globally compared with more stable times. The last 10 000
years of relative climatic stability in the Recent may be an
order of magnitude too short an interval to relax the
influences promoting net stasis.
The claim of punctuated equilibrium that there are
higher-level selection processes among species has proved
highly controversial. Is microevolution really decoupled
from macroevolution? Do species behave as units, having
properties that influence large-scale patterns of evolution
independently of natural selection acting on individuals in
populations? Can a group of species show variation that
affects the probability with which they speciate, and that
can be inherited by daughter species? The answer to the
latter is yes, in principle. Despite much early confusion in
the literature on hierarchical thinking, it seems increasingly
likely that species-wide properties such as population size,
geographical range and dispersal mode can and do
influence the probabilities of speciation and extinction.
Stanley (1998) gives some cases in which species have
apparently been ‘selected’, or at least sorted over geological
time intervals. In general, however, it seems that trends
may have multiple causes. For a useful discussion of these
issues and the causes of evolutionary trends, see Futuyma
(1998).
Conclusion
Punctuated equilibrium and phyletic gradualism have both
been documented in the fossil record, as have a variety of
other patterns spanning many different groups of organisms. Morphological changes, whether abrupt or gradual,
need not be associated with branching speciation, but
reproductive isolation may confer permanence on such
changes. Approximate morphological stasis is commonly
seen in many fossil species, especially the shallow marine
invertebrates that dominate the fossil record, and this may
be related to the instability of shallow marine environ-
6
ments over geological time scales. Conversely, gradualism
may prevail in more stable, though rarely (if ever)
preserved, environments such as a tropical rainforest.
The claim of punctuationists that higher-level selection
processes can act among species remains controversial, but
species-wide properties can probably influence the probabilities of speciation and extinction.
References
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Further Reading
Clarkson ENK (1998) Invertebrate Palaeontology and Evolution, 4th edn.
Oxford: Blackwell Science.
Kemp TS (1999) Fossils and Evolution. Oxford: Oxford University Press.
Ridley M (1996) Evolution, 2nd edn. Oxford: Blackwell Science.
Skelton PW (ed.) (1993) Evolution – A Biological and Palaeontological
Approach. Wokingham: Addison Wesley.