Punctuated equilibrium excused: the original examples fail to support it

Biological Journal o f t h e Lznnean Sociep (1987) 31, 383404. With 3 figures
Punctuated equilibrium excused: the
original examples fail to support it
W. L. BROWN JR.
Department of Entomology, Cornell University, Comstock Hall, Ithaca, New York
148550999, U.S.A.
Received 22 October 1986, accepted f o r publication 28 April 1987
The four original stipulations of the definition ofpunctuated equilzbrium (PE; Eldredge & Gould, 1972)
are shown to be unsupported, and even contradicted, by the two evidential examples these authors
supplied: the trilobite Phacops rana of the Middle Devonian, and the land snail Poectlozonites
bermudensis of Late Pleistocene Bermuda.
The additional, much-discussed example of Bellamya unicolor and the accompanying suite of
molluscan species of Pliocene-Recent African Lake Turkana, studied by Williamson, also fails to
exemplify PE. I n particular, the data produced for these cases appear to represent counterexamples to Mayr’s paradigm of peripatric speciation, embraced by Eldredge and Gould as the central
effective mechanism of PE. The data actually illustrate instead a prevailingly centrifugal pattern of
speciation that will accommodate and explain episodic aspects of microevolution in a more realistic
way.
KEY WORDS:-Centripetal
centrifugal speciation - Middle Devonian
Pleistocene - Poecilozonites bermudensis Turkana molluscs.
~
-
Phacops trilobites
~
CONTENTS
Introduction .
PhflC0p.T rQnQ .
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Phacops sch[ofhetmi and relatives.
Phacops rana in North America .
Poecilozonites bermudensis .
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The Lake Turkana molluscs . .
Discussion . . . . . . .
Summary of the evidential audit
Is evolution centripetal? . .
Acknowledgements
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References
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383
385
387
388
392
396
398
399
401
403
403
INTRODUCTION
No critique is so damning as the sequential removal of examples, one
after the other . . .-Stephen
Jay Gould ( T h e New York Reuiew,
25 September 1986, p. 48.)
The most recent flush in episodic evolution traces to a primary source: the
1972 paper by Eldredge & Gould entitled, “Punctuated equlibria: an
002&4066/87/080383
+ 22 $03.00/0
383
0 1987 The Linnean
Society of London
384
u’.L. BROWN
alternative to phyletic gradualism”. ‘The term punctuated equilibrium
(abbreviated to PE in the discussion to follow) was there proposed for a specific
model of the speciation process that boils down to four essential steps:
1. Species tend to persist with slight or no change through time. (They are in
“eqilibrium”.)
2. Changes occur a t speciation events that are instantaneous in geological
time. (These are the ‘punctuations’.)
3. New species originate each in a small, peripherally isolated part of the
ancestral species’ range. (This is the “peripatric speciation” of Mayr, 1954,
1982.)
4. The new species subsequently replaces its ancestral species. ( I call this
‘centripetal speciation’.)
l’his model is framed in stricter terms that most of its subsequent discussants
apparently have noticed, and it seems clear that even some of its most ardent
proponents (including at times, Eldredge and Gould themselves) have allowed
more or less loosening and slippage in interpreting and applying the separate
tenets of the original paper, as well as the overall definition of PE. Thus in some
versions, PE is expanded almost to broad synonymy with episodic evolution, so
that many of the casual aficionados of modern evolutionary thought may be
hard put to say how they differ.
It is my intention here to make this distinction as clear as it can be made. To
do this, I shall examine the three prime examples of PE cited repeatedly from
the palaeontological literature. Two of these are the very ones offered by
Eldredge & Gould as their primary evidence in their original (1972) PE paper,
taken from their respective monographs on the Middle Devonian trilobite
Phacops rana (Eldredge, 1972) and the Quaternary land snail Poecilozonites
bernudensis (Gould, 1969). T h e third (multiple) example is the controversial
Pliocene-Pleistocene assemblage of molluscan species from African Lake
Turkana studied by Williamson (1981a).
Other examples were put forward by Gould & Eldredge (1977) in their
second general PE paper and in various contributions by other authors claiming
to support PE. I have not dealt with all of these in detail, but it does seem to me
that most if not all of them suffer from one or more deficiencies of the kind I
shall examine below.
In addition, there has been no lack of counter-examples and theoretical
criticism of PE (see, for example, the cogent review of Levinton, 1983).
In evaluating case histories claiming to represent PE, it would be well to heed
the strictures of Lister (1984):
In particular, for an observed ‘punctuation’ to be direct evidence of
rapid evolution, it must be explicitly demonstrated to be not merely the
result of a break in deposition, nor the sudden immigration of a form
which had evolved (at a n unknown rate) elsewhere, but a genuine in situ
transformation. This requires sampling of finely stratified correlated
geological sequences over a wide geographical area.
. . . many examples of ‘punctuation’ are still lacking the geographical
control to distinguish rapid evolution from rapid immigration.
PUNCTUATED EQUILIBRIUM
385
PHACOPS R A N A
The enrolled fossils of this trilobite species are familiar to several generations
of North American geology students because they resemble the head of a small
frog. T h e family Phacopidae includes a number of genera, some of them very
poorly defined, and even possibly polyphyletic, extending from the earliest
Silurian to the end of the Devonian.
Although it has been considered a relatively well-known taxon, the
systematics of the family is actually in a very incomplete state, and there exists
much disagreement among the few phacopid specialists about the limits,
relationships and distinctions among the subfamilies, genera, species and
subspecies, and even about what genera belong to the family (Chlu’pac, 1975).
Application of cladistic methods to limited numbers of species of the type genus,
Phacops, has not led to a consensus about their taxonomy, and serious
disagreement exists (Campbell, 1975) about the relationships of the very Phacops
‘species’ (P. rana, P. iowensis) treated by Eldredge in his monograph of 1972, as
well as doubts about the connections between P. rana and the (now) European
P. schlotheimi and relatives (Burton, 1969). I t seems obvious that a careful
monograph done on a world scale is necessary before Phacops can make a
reliable base exemplar €or evolutionary conclusions, and even if that is done,
there will doubtless remain many serious gaps in the evidence due to the loss of
fossils through normal taphonomic, erosional and tectonic processes. With these
caveats in mind, let us look at Eldredge’s world of Phacops rana.
The species spans some eight or ten million years of the Middle Devonian,
and is most characteristic of the shallow epeiric sea that alternately flooded and
dried up over a wide area of (the present) eastern North America, stretching
from the Acadian Mountain Range of the present New England area west at
times to Iowa, into Ontario, and deep into present-day Appalachia and beyond.
I n Mid-Devonian times, one embracing land mass was the continent of
Laurussia, including much of the present North America, coextensive with what
is today much of northwestern Europe and Russia (Fig. 1). The trend of the
southern coastline of Laurussia in Mid-Devonian times lay east and west, more
or less along the Equator; the corresponding coast of the modern North
American fragment of the former Laurussia is now prevailingly north and south,
or at least northeast and southwest.
There was, of course, no Atlantic Ocean then. T h e Acadian Range, indicated
as the hatched area at the southern edge of Laurussia (black arrow in Fig. 1 )
extended from about the present level of Delaware (at least) on well into the
region now separated as northwestern Europe. T h e belt of sea extending east
and west, separating Laurussia from Gondwana to the south, has been called
‘Tethys’. I prefer to call it Proto-Tethys. We shall return to this
palaeogeography below.
The story of the Phacops rana complex opens, according to Eldredge (and
Burton & Eldredge, 1974), at the very beginning of the Middle Devonian, a t
the end of the Eifelian Stage of Europe and North Africa, when two ‘subspecies’
of P. rana, P. r. tindoufensis and P. r. africunus, occurred in what is now the
northwestern Sahara (southern Morocco and northern Mauritania). At about
the same time (Lower Cazenovian) in the present North America (then
Laurussia) , two similar ‘subspecies’ existed: P. r. milleri compares closely with
the African P. r. tindoufensis, while P. crassituberculatus pairs with P. r. africanus,
W. L. BROWN
386
Figure 1. A rough m a p of the world as it was arranged in the Middle Devonian, interpolated from
Late Early Devonian and Early Carboniferous maps, highly simplified from Ziegler et n l . (1979).
'l'he seaway between Gondwana on the south and Laurussia, Kazakstania, etc. on the north, is thr
abyssal Proto-Tethys Sea, which closed during the Late Carboniferous (in Pangaea), and was
constituted anew afterwards. I have indicated the Acadian Range with a black arrow. T h e actual
Mid-Devonian extent of this mountain-peninsula to the (palaeo-)west, and its status as a
(sometime) barrier us. 'permeable' island arc, are highly problematical. M a p simplified by David
Grimaldi, redrawn by Steven Horn.
though perhaps less closely. All four forms have 18 dorsoventral files of eye
lenses, which is the primitive count for the whole P. rana complex. The total
number of lenses per eye is larger in the tindoufensis-milleri pair than in africanus
and crassituberculatus, the difference being due to the number of lenses per file
Table 1. Rough equivalence of Middle Devonian
stages in what is now Europe and New York State,
modified from Eldredge (1972) and other sources. The
Atlantic Ocean did not yet exist, and the (partial?)
barrier between the shallow seas then covering them
was the rising Acadian Mountains, symbolized by the
column of ( - ) marks, beginning perhaps with the close
of the Eifelian. IsoIation probabIy ended or decreased
as the Taghanic opened
Central New York
Europe
Taghanic
Frasnian
'I'ioughniogan
Givetian
Cazenovian
-
Eifelian - _ _
-
PUNC‘IXJA’TEDEQUILIBRIUM
387
(maximum 6-7 us. 8-9). T h e smaller-eyed forms are contemporaneous with the
larger-eyed ones, but in both North America and Africa they seem to occur in
different facies. The larger eyes are associated most often with shales and marly
limestones, suggesting turbid waters, while the smaller-eyed africanus and
crassituberculatus are found in the pure and sandy limestones indicating clear
water conditions. The taxonomic relationships between the subspecies pairs on
each continent, and between continents, are enigmatic and may never be
completely understood. The three leading possibilities are that the differences
between P. r. crassituberculatus and P. r . milleri were ( 1 ) ecophenotypic, due to the
influences of different environments on the ontogeny of what Eldredge reports
are indistinguishable young stages; (2) ecotypic, due to different genotypes
being selected in a mosaic pattern for advantages gained in, say, turbid us. clear
water habitats; and (3) separate species with different ecological preferenda. I
reject the possibility that these two forms are subspecies in the old, standard
sense of geographical races, and indeed I think that the burden of proof falls
upon those who believe that the subspecies system is an efficient method of
describing and understanding geographical variation (Wilson & Brown, 1953).
Eldredge is using the subspecies designation as a matter of convenience in an
unresolved situation. Perhaps not too many readers are misled by this
convention. At any rate, we can label the 18-file form m-c (for millericrassituberculatus) for the rest of this discussion.
Something should be said about the relationship of the ‘European’ Phacops
schlotheimi complex to P. rana. Phacops schlotheimi is said by practically everyone
concerned to be especially close to P. rana, but the variation of the P. schlotheimi
group, a very complicated matter, is hidden in the Burton (1969) MS., still
unpublished, and available to me only in awkward microfiche.
Phacops schlotheimi and relatives
The P. schlotheimi group has its main distribution in the Rhenish Seaway,
ranging from Brittany and southwestern England (therefore east of the Acadian
Range) eastward to Poland. A few representative ‘subspecies’ of P. schlotheimi
come from southern FrQnce and even Algeria. The species is divided by Burton
into six or seven named and unnamed “subspecies” with eye lens file counts
ranging from 15 to 19, and sometimes varying from 16 to 18 or 17 to 18 in single
nominal forms. O n top of this, some subspecies display ‘polymorphism’ in eye
lens arrangement, etc. Some of these forms may represent true,
contemporaneous geographical variants in, for example, Brittany us. Poland, but
of course there is no way at present to confirm such a supposition.
I n addition, there are recognized some more or less closely related ‘species’
from the Lower Mid-Eifelian ( P . brongniarti, with 18 lens files, Rhenish; and its
possible equivalent, P. sobolemi, with 17-18 files, from Poland); and Upper MidEifelian (P. latifons, with 15 files, from Germany eastward, possibly into Asia).
Phacops papulatus, a supposedly related species, is also listed, as are members of
the related P. turco and other Bohemian and Proto-Tethyan groups.
Although my blurred copy of a copy of Burton’s thesis includes a great deal of
quantitative data describing many of these forms, plus photographs of features
of a series of them, I found it essentially impossible to sort out
biological-taxonomic relationships among them. Most importantly for the
388
W. L. BROWN
present discussion, the differences between the P. schlotheimi group and the
northwestern African members of the P . rana group remain unclear. The Gees
samples of P. schlotheimi and the African P. rana are compared as typical
members of their respective groups, and the latter are said to be much larger
than P. schlotheimi, with different ‘packing’ of the middle eye lenses. But
P. shlotheimi schlotheimi and the African P. rana subspecies are really only taken as
kinds of ‘taxonomic centroids’ for their respective groups, so that form for form,
the similarities and differences between the two groups remain, at least for me,
yet to be clearly spelled out.
One reason for better understanding the P. schlotheimi group is the
assumption, implicit in various older discussions, that it is somehow ancestrally
related to the P. rana group. It should be borne in mind, however, as Burton has
shown, that P. schlotheimi and P. latzfrons have long been taxonomically
confounded. The Devonian phacopids really cry out for a global revision!
Phacops rana in North America
Eldredge’s 1972 paper is not easy to follow. Not only is it framed in terms of
taxonomic units called ‘subspecies’ of each of the two ‘species’ discussed there:
P. rana and P. iowensis, but this convention, and Eldredge’s failure adequately t o
describe and discuss in plain words variation within locality samples
(populations?) forces the reader to appeal to factor analysis plots that seem to
fog description more than they clarify it. I n this connection, it should be noted
that for P. rana variants, plot samples listed in his table 5 (Eldredge, 1972: 74)
total 303 specimens from 27 “localities” in 28 “formations”. His table 8
(pp. 94-95) lists “Distribution of the subspecies of Phacops rana and Phacops
iowensis” by geographical region and geological formation, but we are left t o
guess which of these entries represent samples also included in table 5, or even
whether they represent specimens Eldredge examined (us. literature records or
other evidence). Table 5 also reveals that 14 of the 27 P. rana samples listed there
consist of less than 10 specimens each, a datum of interest in assessing probable
population variation.
In general, it is very difficult to compare entries between tables 5 and 8 and
appendix 2 (Locality List), partly because of uncertainties, overlaps, and
duplications involving formations and their stratigraphic subdivisions, and the
geographic locations of the samples.
At any rate, the cast of P. rana subspecies considered by Eldredge numbers
five:
P. r. crassituberculatus
P. r. milleri
P. r. paucituberculatus
P. r. rana
P. r. norwoodensis
He also dealt with three subspecies of Phacops iowensis:
P. i. iowensis
P. i. alpenensis
P. i. southworthi
Eldredge’s table 5 lists only 26 specimens of the rare P. iowensis lineage, which
differs from P. rana most strikingly in having 13 dorsoventral (vertical) files of
PUNCTUATED EQUILIBRIUM
389
eye lenses, us. 18 to 15 files in P. rana. Eldredge found only one sample of
P. iowensis that was certainly intimately sympatric and synchronic with a P. rana
population (Hungry Hollow Formation, southern Ontario). He found there
only one specimen (of P. iowensis southworthi) that he could measure, but claimed
that character displacement may be indicated between P. iowensis and P. rana
co-occurring in this sample. Although this claim probably has little direct
relevance to the total pattern discussed here, it needs to be mentioned now that
Eldredge (1972: 86) says, of P. rana:
. . . six major trends involving ornamental and other features are
thought to be progressive, i.e., gradual, involving a linear shift a t a more
or less constant rate from the initial character state in all Cazenovian
populations through to the final character state exhibited by populations
in the Taghanic. I t is essential to emphasize that “gradual” and
(6
progressive” imply changes in “mean” character state; no claim is
made that these changes can be documented stratigraphically, e.g.,
within successive samples of P . rana rana within the Tioughniogan. T h e
significance of most of these trends lies in the gradual approximation of
P. iowensis characteristics by the entire P. rana complex through time.
In other words, P. rana shows character release after the extinction of
P. iowensis. Gradualness is not reflected, in Eldredge’s analysis, by the rate of
change in compound eye morphology, upon which he so largely bases his
taxonomic division of the P. rana lineage into separate subspecies. Instead, he
emphasizes the abruptness of the transitions in dorsoventral eye lens file counts
from the ancestral 18 (P. r. crassituberculatus and P. r. millerijointly through the
earliest, or Cazenovian, stage) to 17 ( P . r. rana from the middle of the
Cazenovian, through the succeeding Tioughniogan Stage and some way into
the final, or Taghanic Stage), to 15 (P. r. norwoodensis, branching from P. r. rana
in the early Taghanic).
A possible separate development was the earlier appearance, in the Upper
Cazenovian of northeastern Michigan and northern Ohio, of a form with 15 lens
files, which Eldredge named (1972: 80) P. r. paucituberculata (rightly,
paucituberculatus), though from his fig. 16, the glabellar tubercles hardly seem
appreciably sparser than those of P. r. norwoodensis shown (fig. 15) on the facing
page. Also, P. r. paucituberculatus was described from a series of only five
specimens, one of which, from a sample collected away from the type locality, is
said not to share the sparse glabellar tuberculation. Eldredge says that the
“Remainder of features [of P. r. paucituberculatus are] as in P. rana
crassituberculata.” The remaining features are few and perhaps less compelling
taxonomically, and are left to be evaluated through the abstracted conversions
of Eldredge’s morphometrics. But let us suppress all nagging doubts in his
favour and press on with a description of the P. rana history.
The focus is on the nature and abruptness of the change in numbers of
vertical eye lens files and their constancy through periods between the changes.
We have already noted above that Eldredge’s table 5, listing samples for factor
analysis plots, includes 303 P. rana complex specimens from 28 formations.
About half (14) of these samples comprised fewer than ten specimens (mean
4.3), but one of them (Cardiff-Solsville) contained five specimens with either 18
or 17 lens files, thus bridging the gap between P. r. crassituberculatus and
390
W. L. BROWN
P. r . rana. What are the chances that some of the other small samples also
represented transitional populations at present undetected? And later, in the
Tully Formation of the Taghanic Stage, four or five separate localities marking
a wide area of central New York have yielded samples each varying through
17-16-15 lens files, thus running through the change from P. r. rana to
P. r . norwoodensis. I t is worth noting that the Tully is coextensive with the
Taghanic Stage in central New York and eastern Pennsylvania. The transitional
( 1 7-15 files) populations may thus have lived for a long time; in fact, only one
reasonably extensive P. r . norwoodensis sample ( 1 1 specimens), that from the
Milwaukee Formation ( U M M P G), was safely regarded to be “pure” at 15 eye
lens files. One also wonders about the nine Cedar Valley (Iowa) examples,
labelled CRVN for P. r. norwoodensis and CRVR for P. r. rana; it is hard from the
locality list to be sure that some of them were not part of mixed populations.
Perhaps we should regard doubts about the representativeness of the samples
as mere quibbles; but in view of the rather sparse overall sample, both in terms
of number of individuals and of populations that they may represent, and
considering the rather expansive model of the origin of species based on this
series, augmentation of the data would be very welcome.
The allopatric model, where new character states arise on the
periphery of the range of a species, is directly applicable to the history of
P. rana. Important changes in eye morphology originate in the
exogeosyncline to the east, and subsequently spread through the epeiric
seas on the cratonal interior [Eldredge, 19721.
Eldredge’s “periphery . . . to the east” and his “cratonal interior” must be
examined in the light of what is known about the lie of the Phacops rana-group
habitat (shallow seas) during Middle Devonian times. Reconstructions of landand-sea distributions of this time vary greatly, partly because the relevant time
interval lasted so long (8 to 10 million years, depending on dating methods and
on differences in criteria for deciding when it began and ended), but even more
so because the geological information about the situation is so very scrappy and
vague, both as to geomorphology and chronology.
Anyone speaking about “marginal seas”, as Eldredge calls them, is obliged to
say what they are marginal to, and when. Obviously in this case, he means
marginal to the epeiric sea. But he has already told us that the P. rana complex “is
morphologically closest to P. schlotheimi of the Eifelian of Europe and Africa and
probably migrated to Devonian North America together with [the nonphacopid] trilobite Greenops boothi . . . In view of the striking similarity between
P. rana and European-African species of Phacops, a European-African ancestry
for Phacops rana seems well founded.”
I agree. Reconstructions of the world’s land masses and seas are replaced
frequently as geological information accumulates. The highly speculative and
imprecise nature of this information as it now exists cannot be overemphasized,
but there are a few grand features of Mid-Devonian geomorphology on which
tentative agreement seems to have been reached.
The map I offer (Fig. 1 ) is an interpolation of those given by Ziegler, Scotese,
McKerrow, Johnson & Bambach (1979) for the late early Devonian and the
early Carboniferous, and it vastly underrepresents a period in which drastic
changes in epeiric sea invasion and re-invasion of the continents, continuing
PUNCTUATED EQUILIBRIUM
39 1
continental drift, and mountain-building were changing the face of the earth.
Alternative schemata are offered by, for example, Livermore, Smith & Briden
(1985), especially their map, fig. 7, recostructing Eifelian-Tournaisian time,
about 387-352 Ma, in which the Proto-Tethys Sea forms a narrowing channel
as it closed before the assembly of Pangaea. They find “no serious disagreement
exists between the present maps and those of Scotese (1984).” Another view of
the world pattern is offered by Boucot (1985a,b) in the same volume.
The outlines on which agreement now seems broad are that an abyssal but
straitening sea roughly paralleled the Equator in a belt separating Laurussia
(and other land masses) to the north from massive Gondwana to the south. The
southern coast of Laurussia, corresponding in part to what is now the Eastern
Seaboard of North America, lay on or near the Equator. T h e Atlantic Ocean
had not yet opened up, and much of what is now northern and central Europe,
then fused with North America on the east, was covered by shallow seas
stretching at their broadest a t least to the site of the present Urals. The Acadian
Orogeny, raising the mountain range with the same name, apparently
commenced in early Middle Devonian. The Acadian Mountains, indicated by a
black arrow in Fig. 1, rose on the southern edge of Laurussia and extended in a
moderately oblique bearing eastward into that part of the palaeocontinent now
separated as northern Europe.
The Acadian Range left the signs of its presence in deep structures, but
evidence appreciated earlier came in the form of stratigraphic features
widespread in New York and Pennsylvania, indicating that high mountains to
the south (present east), close to the much earlier Taconic Orogeny of the Late
Ordovician, sent down eroded materials that formed the ‘Catskill Delta’ and
similar sedimentary gradients deposited in the shallow seas to the palaeo-north
(now west), particularly in central and western New York.
There exists a widely held belief that the Acadian Chain may have existed in
the form of an island arc. This arc would have afforded continuous or
intermittent access of the Proto-Tethys Sea, separating Laurussia from
Gondwana, to the epeiric American Sea. Eldredge has, as we have seen,
proposed the immigration of 18-file species from the direction of Gondwana
near the beginning of the Middle Devonian. Whether this avenue was closed or
restricted during most of the (Cazenovian and Tioughniogan) rest of the span is
not known, but apparent endemism in corals and some other fossil taxa is
suggestive of a n interval of relative isolation. I n the Taghanic, the appearance of
a suite of twenty-odd brachiopod species signals an influx that, according to
Dutro (1981: 76) “correlates with high Givetian faunas elsewhere in the world
and reflects a major episode of marine onlap onto the North American craton”,
appears to coincide with the onset of instability of the P . rana stock in eye lens
file counts (17-15).
South of Delaware-Pennsylvania, the thickness of sediments derived from the
Acadian Range or its possible extensions to the palaeo-west (present south)
declines markedly. The ‘range’ may well have been represented or penetrated
by seaways during part of the Middle Devonian, leaving the American epeiric
arm of the sea open to Proto-Tethys and the immigration of its faunal elements.
Both coasts of the Proto-Tethys Sea harboured ram-complex Phacops, and they
were probably evolving around the roots of any emergent land, including the
Acadian Chain and nearby islands, during the entire time we are considering.
392
W. L. BROWN
Though they are often linked as “The European-North African” faunal
region, it should be remembered that most reconstructions place northwestern
Europe as an extension of Laurussia during the Middle Devonian, whereas
North Africa was a part of Gondwana. Their respective Phacops-bearing shallow
seas were, of course, separated by the Proto-Tethys Sea, which would scarcely
have been much of a barrier to the movement of marine organisms, even
presumed shallow-water forms such as Phacops.
Burton (1969) and other European authors do leave the impression that
during the Eifelian and Givetian (North American Cazenovian through
Tioughniogan) the palaeo-eastern, now northeastern, part of the Acadian
Mountains separated the shallow sea-and-island stretches now represented by
‘Rhenish’ northern Europe and the shallow sea beyond the Catskill Delta. The
connection between the Phacops seaways lay elsewhere, to the palaeo-west.
Wherever and whenever they existed, these ‘marginal seas’ were in effect
ingresses to the large, shallow embayment we know as the American epeiric sea.
From all of this, it can be seen that Eldredge (1972: 105, and later writings)
himself describes the evolutionary dynamics of the P. rana complex in terms of
changes introduced in the (present) East, then sweeping in time westward. Thus
the evolutionary centre for this time span and this trilobite complex lay somewhere
to the (Devonian) south. The instability of the epeiric sea, the outer margins of
which suffered some periods of broad regression in places, must have helped to
constrain this directionality. Eldredge says ( 1972: 55), “The stratigraphic
section, then, is far more complete in the exogeosyncline on the eastern margin
of the continent than it is on the cratonal interior to the west. The rock
sequences in the east are also much thicker and evidently represents a more
complete record of time.”
Since we are in apparent agreement that the Michigan Basin and adjacent
areas are not the evolutionary seedbed of the P. rana subspecies, it seems
appropriate to discount them as evidence for ‘punctuation’. When we look to
the East, what we have, with anything like significant samples, is central and
western New York. I n this area, the only samples of the 18-file m-c stock are in
the Solsville, which is mixed 18 and 17-file forms, and thus ‘transitional’.
After this, 17-file samples held sway in the area, as listed in Eldredge’s table 8
(p. 94) “Formations”, through the greater part of the Middle Devonian, in the
Cazenovian through Tioughniogan Stages. I n the third and final stage of the
Middle Devonian, the Taghanic (that is to say, the Tully), New York rana
samples are characteristically mixed, with 17, 16, and 15 files in all or most. It is
stretching the rather skimpy eastern evidence to call the sequence thus
exemplified “punctuated”. As to whether it shows ‘stasis’, one must weigh
Eldredge’s remarks, cited above, about the trends showing convergence toward
the P, iowensis lineage following the disappearance of the latter species. How
much of a trend do we allow without negating the very meaning of stasis?
POECII.OXOMTES BERMUDENSIS
Gould’s (1969) treatment of the Poecilozonites land snails of Bermuda is
restricted to the four nominal species recognized by him in the typical subgenus
Poecilozonites. The genus belongs to the pulmonate family Zonitidae, and is
thought to have originated as an oversea propagule from a North American
PUNCTUATED EQUILIBRIUM
393
stock of subfamily Gastrodontinae, which arrived at an unknown (late
Tertiary?) time in Bermuda and radiated luxuriantly to give rise to at least 15
nominal species distributed among three subgenera. T h e 15 species have
accumulated a flock of subspecies, many of them known from a very few
examples, but a few of these may possibly represent real biospecies.
Despite Gould’s essay (1969: 444ff.) on the “geographical” nature of
subspecies of these snails, the size and provenance of the available samples raises
doubts about their status as “neontological subspecies”. Since even the species in
subgenus Poecilozonites produced apparent hybrids when their ‘ecological zones’
touched one another, we may in this case again find our threshold of acceptance
raised against arguments that seek to invoke shadowy fossil taxonomies as
evidence for any particular phylogenetic scenario.
Be this as it may, we focus on the morphospecies P. bermudensis, which is the
only member of its subgenus surviving today. “During the last 300,000 years,
the central stock of the species, P. bermudensis Zonatus . . . has branched a t least
four times and has itself undergone fluctuating alterations of morphology that
correlate with ice age climatic oscillations” (Gould, 1969: 469).
These oscillations produced drastic sea level changes for such a small oceanic
land patch as Bermuda. During the later Pleistocene, the glaciers at peaks
appear to have tied up so much of the earth’s water that the Bermuda gauge
read 120+60m lower than it does today. High-water intervals brought sea
levels nearer present values, and at times even topped them by as much as 23 m.
Each time the ice advanced and sea levels fell accordingly, the exposed soils of
Bermuda were leached of lime, and when warmer periods brought the sea back
up, lime-rich dunes built up on the shrunken land areas of the platform left
unflooded by the sea, particularly on the southwestern rim facing the prevailing
winds.
The “fluctuating alterations of morphology” that correlate with the climatic
oscillations involve variation in the thickness of the shell, which of course
changes shape to some extent. For the “central stock”, Gould confesses:
In P. b. zonutus, temporal variation of environments elicits an adaptive
response, and I am equally unable to assess the contributions of genetic
and nongenetic factors to these adaptations.
U p to a date very late in the time series under consideration, the
P. 6. zonatus populations of the present main island of Bermuda were
divided almost equally between ‘East zonatus’ and ‘West zonatus’, color
forms that apparently were sharply parapatric and distinct for
100,000-1 50,000 years (Shore Hills to Southampton times).
The presence of these two forms, very possibly distinct as species, is certainly a
daunting complication, but Gould presses on.
I assume that genetic exchange between eastern and western snails
was eliminated or greatly curtailed during the whole Shore HillsSouthampton interval and that P. bermudensis zonutus was evolving as two
parallel stocks during that time. At some time after the end of the
Southampton deposition, P. bermudensis zonatus became extinct. Its
primary range on the main island was quickly repopulated by
P. bermudensis bermudensis, which had been evolving in isolation on St.
394
W. L. BROWN
George’s Island since St. George’s time; the color pattern of all modern
snails is therefore 123 [i.e. banded as in fossil East zonatus].
This extinction, completely undocumented in the preserved record, is
itself one of the most fascinating events in the history of P. berrnudensis.
Since it occurred after the latest deposition of Southampton dunes, the
extinction is a very recent event, probably attributable to human
disruption of the native biota. Isolated on St. George’s Island,
P. bermudensis bermudensis survived the plagues of rats, pigs, and snails
that ravaged Bermuda during the early settlements and later
repopulated the whole island complex.
Two points whould be noted here: ( 1 ) T h e Zonatus extinction, including its
precise timing, is “completely undocumented”; (2) the isolation of St. George’s
Island during Southampton times, and even up to a few thousand years ago, is
entirely unlikely in view of the evidence I offer below.
Gould’s “central stock” of the species, P. 6. zonatus, was plump and rounded
as an adult, but had lower, more disciform juvenile stages, with the
circumference of the disc acutely marginate rather than rounded. Forms judged
to tie fully adult, but up-scaled paedomorphic copies of zonatus juveniles, were
recorded four times by Gould. Three of these samples are represented each by a
very few specimens from widely separated sites at earlier (Shore Hills to
Pembroke) fossil horizons; they are regarded as separate paedomorphic offshoots
of (both East and West) zonatus. T h e fourth such paedomorph turned up on St.
George’s Island in the interval of the same name, and it is this form, in a slightly
less extreme version, that we know today as the surviving, Bermuda-wide,
P. bermudensis bermudensis. T h e sequence is illustrated in Gould (1969: fig. 20 and
pl. 4);fig. 20 is repeated as figs 5- 4 in Eldredge & Gould (1972: 102).
Gould (1969: 492) says each of the four paedomorphs “is a distinct genetic
entity, not a mere phenotypic response to a recurrent set of environmental
conditions. Each paedomorph has the geographic distribution of a peripheral
isolate, and each has lost genes for previously adult features that could never be
brought to phenotypic expression.” Then (same page): “It is one of the
frustrations of such work that, while we can affirm the adaptive nature of a
correlation between climate and morphology, we cannot tell whether we are
dealing with genetically determined changes or purely phenotypic responses.”
Although these two quotations apply respectively to “phyletic branching in
P. bermudensis” and “phyletic evolution of P. berrnudensis ,gnatus”, there are no
substantial grounds for believing that the two processes are always separate and
readily distinguishable in these and other molluscs. Work such as that of Kemp
& Bertness (1984) on the development of the marine snail Littorina littorea should
serve as a warning that even slight perturbations of the environment can affect
growth rates and through them the allometric constant k for individuals. Kemp
& Bertness found rather different shell shape extremes: broader, more globose
shells in faster-growing, uncrowded populations, and taller, more slender shells
in dense populations-differences
involving some of the same important
parameters discussed for Poecilozonites by Gould. Kemp & Bertness refer to other
published evidence for their view.
Bermuda sits atop an isolated pillar of rock rising from the ocean floor. Its
truncated tip is the Bermuda Platform, now mostly drowned by the sea, with
PUNC'IUATED EQUILIBRIUM
395
only a ridge near its southwestern rim exposed as a long, narrow, curved strip of
land, with some adjacent smaller islands. At least during the last few million
years, the pillar appears to have been stably rooted and has therefore borne
eustatic rises and falls of sea level like a gigantic tide gauge. Parts of the record
for the Pleistocene and Recent sea level changes can be read in the stratigraphic
system outlined by Gould and predecessors, and can be inferred from what is
known of the geomorphology of the eastern North American coast. Aided by
radiocarbon and other kinds of dating done on the Bermuda Platform, a
detailed plot of the depth curve has been drawn for the last 10,000 years. The
curve shows that at the beginning of this time span, sea level was at about 30 m
below its present level, and rising fast as the glaciers melted (Neumann, in
Bloom, 1977; Lighty, Macintyre & Stuckenrath, 1982). Earlier still in the
Pleistocene, it is believed that the sea a t Bermuda was 120+60 m below its
present level.
The surface relief of the platform, now mainly under water, features an
elliptical atoll-like reef structure surrounding a lagoon, the bottom of which is
now only at -20 m. From this, it is clear that even only 10000 years ago, the
exposed Bermuda land surface must have been several, perhaps ten or more,
times its present area, and in a totally different configuration from what it is
now (Figs 2, 3 ) . At 10 000 B P , St. George's Island was only a small eastern sector
of the then main land mass of Bermuda. During that extended time, we have
scant direct evidence of what the Poecilozonites fauna of the platform may have
been like, but it does seem worth consideration that the extant P. 2 . bermudensis,
Years x lo3BP
0
5
10
E
.-f
a
15
'0
0,
-
E
ul
2c
25
3c
Figure 2. Sea-level curve for Bermuda over the last 10 000 years, based mainly on radiocarbon dates
gathered by Neumann (1971j and others. The curve used here is simplified from Neumann's revised
plot, published by Adey in 1975, Macintyre & Glynn in 1976 (references and additional curves for
the western Atlantic in Lighty et al., 1982) and most recently by Bloom (1977).
W. L. BROWN
396
/i
reef
I”
Bermuda
Island
NORTH
ATLANTIC
OCEAN
BERMUDA IS1.ANDS
Figure 3 Bermuda as it exists today (in black), and as it was 6-7 thousand years ago (in grey),
based on chart depths of 10 metres and less. Based on the sea-level curve in Fig. 2, the central
lagoon, which is mostly less than 20 m deep a t present, probably would have been eliminated at 8-9
thousand years B P . At 10 thousand years B P the exposed surface of the platform, with the sea level
30 m lower than it is now, would have been still larger in all directions. Map drawn by Steven
Horn.
the so-styled paedomorphic variant of St. George’s, could in fact have been the
prevalent form of the species on Bermuda Extensa of 10000 BP. If so, the
St. George’s population may well at some point have represented the expanding
peripheral front of a central Bermudian population of bermudensis.
This palaeoscenario is certainly a t least as valid as the one presented by
Gould, whose account curiously seems to constrain the history of this taxon
within a geographical model based on the remnant rim of land we know as
Bermuda today, even though, in an early paragraph of his 1969 paper (p. 408)
he mentions “times of low sea level when modern Bermuda was at the interior of
a land mass ten times its present size.” I would exchange the word “interior”
for ‘at one edge’.
THE LAKE TURKANA MOLLUSCS
Yet another example has attracted attention as a model case of PE. P. G.
Williamson (1981a) has described a 3-4 million year sequence of fossil
molluscan assemblages from the Pliocene-Pleistocene of Lake Turkana Basin in
northeastern Africa. Each species in this suite of more than a dozen lineages is
PUNCTUATED EQUILIBRIUM
397
reported to have maintained a relatively stable morphology over most of that
span of time, with the notable exception of three separate relatively brief periods
when a majority of the samples show changes away from their long-term
morphotypic track. Williamson assumed and then tenaciously defended ( 1981b,
1982, 1983, 1985a, 198513) the thesis that the intervals of change were
predominantly ecotypic (genetically based).
Because of its relevance to early human evolution, the stratigraphy of this
section has received detailed scrutiny, and the most recent corrections to the
sequence are outlined in Williamson (1985a).
Unfortunately, Williamson’s fully detailed taxonomic and biometrical
analysis of the Turkana shells has not yet seen print, but in his summary
description he tells us that five species of gastropods and eight of bivalves
displayed marked changes during one to three relatively brief (5-50 thousand
year) intervals. His best examples, or at least those on which he focuses, are
those of Bellamya unicolor, Melanoides tuberculatus, and Cleopatra ferruginea.
The pelecypods receive scant attention, perhaps, say his critics, because the
taxon is so notoriously variable.
What appears to have been recorded is that each of the different lineages
(species) in question seems, more or less concurrently, to have experienced a
change in form in Lake Turkana during each of the brief intervals in question.
At least some of these changes are represented as genetically based, and possibly
even as at species level, thus involving reproductive isolation. Critics (Fryer,
Greenwood & Peake, 1983, 1985; Kemp & Bertness, 1984; among others) have
questioned this interpretation, and it must be said, in view of the uncertain and
incomplete nature of the taxonomy of the gastropod genera concerned, that
Williamson’s convictions about the status of, for instance, the Turkana
morphotypes of Bellamya unicolor, rest on rather mushy grounds.
The authority to whom he makes basic taxonomic appeal (D. S. Brown,
1980) says of the viviparid genus Bellamya:
In Africa, Bellamya occurs from Egypt southwards to Zululand,
Rhodesia and northern Namibia . . . Of about fifteen species which may
be tenable, most are confined to large lakes and only B. unicolor and
B. capillata are widespread. A group of poorly known species of
uncertain status found in central and western Africa is segregated at the
end of the present account . . . Mandahl-Barth . . . distinguishes
[B. unicolor] from B. capillata . . . according to its more numerous and
smaller embryos. However, few populations of either nominal species
have been studied adequately in this respect.
Brown (all on p. 37) then lists “Nominal species . . . which seem to be merely
lacustrine forms [of unicolor].” T o me, this does not add up to confidence about
the circumscription and intraspecific variation of Bellamya unicolor, especially as a
standard against which to measure the apomorphy of its Turkana
representatives. Similar difficulties haunt the other three gastropod examples
offered by Williamson, though their detailed description remains to be
published.
A central problem in the controversy is, then, whether Williamson really has
a case in the Turkana fossils for genetically based us. merely ecophenotypic
variation away from the ‘norm’. As important as this question is (and I certainly
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W L BROWN
do not belittle it), it does not seem possible to settle it conclusively with the kind
of evidence Williamson has offered us to date.
What does seem conclusive is that Williamson’s Turkana fossil assemblages
cannot be taken to support the theory of punctuated equilibria. For the sake of
argument, let us grant that Bellamya unicolor or Melanoides luberculata ( a
parthenogenetic form) evolved in true genetic style to the point where they
attained species status in Lake Turkana once, or twice or even three times.
What does that exemplify? O n each occasion, all of the Lake’s molluscs
apparently reached extinction, probably because the habitat disappeared into
an unconformity (that is, it dried up, or came close to it). The record, following
each episode after the lake filled again, shows that the apomorphs, whatever
their nature, were replaced by good old morphotypic Bellamya unicolor,
Melanoides tuberculata, and so on. Remember that, even then, Lake Turkana was
a peripheral body of water for these mollucs. We know that the African
environment fluctuated, wet periods following dry, while the wet forests, dry
forests, savannas and deserts waxed and waned accordingly, through the
Pliocene and Pleistocene. Aquatic animals such as snails and bivalves became
isolated in peripheral lakes and rivers, and some of them differentiated and
probably reached species level in the particular environments in which they
were trapped. Most or all of them were undoubtedly snuffed out as lake
conditions reached extremes, or possibly as their parent stocks re-expanded with
the return of more favourable periods. But the aberrant populations did not
come back to invade the evolutionary core areas of their respective parent
stocks. They were cornered and died in place. They did not live up to the fourth
stipulation of punctuated equilibrium. In fact, they seem in every respect to
fulfill the expectations of the centrifugal imperative (W. L. Brown, 1957)
dictating that generally-adapted ‘central stocks’ will tend to dominate and
replace populations left marginally stranded from previous expansion fluxes of
the same stock.
DISCUSSION
So we see that the concept of PE was unsupported from the start, if not
contradicted outright, by its own cardinal examples. Phacops rana, Poecilozoniles
bermudensis, and later Hellamya unicolor and the other Turkana fossil molluscs, just
do not tell the story as Eldredge and Gould modelled it. But what about all the
evidence, gathered by many people before and after 1972, that numerous fossil
lineages stayed more or less morphologically constant over long periods, and
then were succeeded, often abruptly, by different but related forms?
In a great many cases, as has long been recognized, these shifts correspond to
discontinuities in the fossil record: periods of transformation of the habitat;
subsequent obliteration of strata by erosion, vulcanism, subduction and other
physicalLgeologica1 processes; or (not least) simple failure of discovery. In other
cases, the changes apparently coincide with episodes of invasion by replacement
species, and very often whole faunae. T h e last--faunal shifts-suggest relatively
sudden structural changes in the earth’s surface-orogeny;
subsidence; the
incursion of seas; division, assembly, and migration of land masses, and the like.
Indeed, we often find that faunal change and crustal structural change clearly
coincide and reasonably seem to be causally related. One of the greatest
PUNCTUATED EQUILIBRIUM
399
difficulties of the punctuation argument is its frequent failure to eliminate
structural geographical changes somewhere as underlying causes of the shifts
they detect at species level in another place. Are we dealing with a rapid shift at
a local speciation event, or are we seeing species replacement from elsewhere?
There is a difference, an important difference, and few if any of the claimed
examples yield information to allow a confident distinction between the two
alternatives. Lister’s call for “geographical control” is clearly in order.
But even if we make due allowance for the flimsy and often contradictory
palaeontological evidence for PE, it seems that fossil as well as neontological
findings suggest significant and often intermittent variation in the rates of
evolution of different lineages. The works of Simpson and many other
palaeontologists before and since have made this a truism. The phenomenon of
character displacement, invoked by both Eldredge and Gould, also implies this
intermittency (but following speciation) in organisms living right now or in the
very recent past. However, discontinuity or sharp variation in rates of
morphological evolution do not constitute PE, because (let us recall) PE comes
born with a set of four conditions.
Summary of the evidential audit
At the risk of asking the reader to attend the pitiless flogging of a dying horse,
let us review these four classical requirements, however they may have been
breached in the application.
1. Species persist with slight or no change through time
The data from the fossil history of Phacops rana are equivocal on this point. For
the principal diagnostic character distinguishing the subspecies, eye lens file
number, the few useful samples from nearest the (palaeosouthern) evolutionary
centre-represented
by remnant deposits surviving mainly in the present
central-western New York area-the
evidence is for alternating periods of
instability (18-17, 17-15 files) and stability (17). During the long, middle 17-file
period, however, other parameters of eye size appear to have waxed and waned.
Still other characters show ‘major trends’ of convergence towards P. iowensis.
Poecilozonites bermudensis zonatus, the ‘central stock’ of the P. bermudensis
morphospecies, showed substantial fluctuation in shell thickness and relative
whorl proportions through its long span in the Pleistocene. Though the adaptive
connections of shell traits to alternating lime availability in successive palaesoils
is easy to accept, “we cannot tell whether we are dealing with genetically
determined changes or purely phenotypic responses” (Could, 1969: 500). Thus,
the fossil record of P. bermudensis provides no comfortable support for the
occurrence of stasis.
I n the case of Bellamya unicolor, the best-known example among the Turkana
assemblage, morphotypic stasis may have prevailed, though the details are still
incomplete. After all, what we have in Lake Turkana today is B. unicolor.
2. Changes occur at speciation events that are instantaneous in geological time
T h e Phacops rana and Poecilozonites bermudensis accounts d o not even address the
question of ‘speciation events’, since both Eldredge and Gould, separately and
together, discuss only ‘subspecies’. Eldredge later (1985) made a gesture towards
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W. L. BROWN
remedying this situation in his popular account of the PE story when he offhandedly bestowed nomenclatorial species status upon the m-c, rana, and
norwoodensis morphotypes.
The subspecies category as a palaeotaxonomic convention is a vestige of the
bad old days before most of us read Mayr and Rensch. Nowadays, nearly all
systematists use the subspecies, if they use it a t all, to denote geographical races.
Formerly, taxonomists of living groups, as well as palaeontologists, used
subspecies to indicate samples (populations, mutants, pathotypes, etc.) that
were ‘just a little bit’ different us. ‘a lot different’, or ‘distinct’. I t was a category
within or ‘below’ the species, but even species had not yet been defined in the
modern senses of population biology. Sadly, many palaeontologists still resort to
subspecies as taxonomic units in the old, undefined sense-even palaeontologists
who know better when they wear their neontological hats.
In any event, it will not do to beg the question of ‘speciation events’ by using
these vague and insubstantial pre-Mayrian subspecies to stand in for real
biological species.
Suppose, though, that we consent to keep debate alive by granting species
status to some or all of the Eldredge and Gould subspecies. Then in the trilobite
case we are faced with evidence for gradual transition in the first half, and
afterwards again in the last half, of the Middle Devonian in the evolutionary
spawning ground fringing the Catskill Delta, at the western foot of the Acadian
Mountains. For the Bermuda land snail, despite the relatively minute temporal
and spatial scale of the system, the evidence for punctuational change in the
evolutionary source area (wherever that was!) is hardly compelling.
As for Williamson’s Lake Turkana molluscs, the controversy continues about
whether, and how often, true speciation may have taken place in this restricted
area. The matter requires further analysis, including a thorough taxonomic
revision of Bellamya and the other genera involved, and genetic comparisons of
living populations.
As a crucial comment on the PE speciation requirement, the obvious may be
noted: very significant adaptive changes, ‘instantaneous in geological time’, but
intrinsically undetectable by any palaeontological methods now known, have
swept through historical populations, and have given rise to true biological
species in some cases. Examples such as those from Drosophila pseudoobscura,
Perornyscus, and the Heajdepta moths of Hawaii will undoubtedly be multiplied
many times as our knowledge expands. And let’s not forget the testimony of
existing geographical variation, often dramatic, in thousands of living plant and
animal species.
3. N e w species originate each in a small’peripherall3, isolated part o f theancestralspecies’ range
For the evolving metapopulations of Phacops rana, Eldredge read the centre
and the periphery of the geographical range in the reverse direction, even
though he acknowledged that the morphotypic changes progressed from the
direction of Proto-Tethys and the Acadian Range toward the margins (his
“cratonal interior”) of the epeiric sea. From the evidence available, scrappy as it
is, the populations to the east (palaeosouth) seem to have been relatively
continuous and gradually changing, while repeated invasions of new stocks from
there produced the apparently discontinuous succession toward the only
intermittently present outer reaches of the unstable shallow continental sea.
PUNCTUATED EQUILIBRIUM
401
The same applies to Gould’s interpretation of the microcosmic Bermuda scene
of the Late Pleistocene. Although he plays out the evolutionary dynamics of
Poecilozonites hermudensis on the stage of the present disposition of the Bermuda
Islands, with St. George’s Island separated by a narow sea channel from the
main island, we have solid evidence that only 6 or 7 thousand years ago, at
most, these islands were parts of a much larger land mass in which St. George’s
was anything but ‘peripherally isolated’ from the present Bermuda. Where then
was the centre, and where the periphery, of the range of the P. bermudensis
metapopulation? I think Gould has it backwards.
The Lake Turkana molluscs, on the other hand, may well have met all
reasonable criteria as peripheral isolates. The lake is in some sense marginally
isolated in the dry Horn of Africa neighbourhood even today. If Bellamya unicolor
and any of its fossil associates did in fact reach the status of separate species, we
would have to admit the likelihood of their having met the third condition of
the punctuational model.
4. T h e new species subsequently replaces its ancestral species
For Phacops rana and Poecilozonites bermudensis, as we have seen, the ‘central
stock’ may well have evolved and replaced the peripheral stock, and not the
reverse. In some sense, the central stock is the ancestral stock, the on-evolving
mainstream stock. Finally, the Turkana situation exemplifies the centrifugal (us.
centripetal) model in the most direct way. Whatever may have been the nature
of the brief excursions away from the mainstream morphotype of Bellamya
unicolor during the life of Lake Turkana, it is clear that these events occurred
peripherally to the main range of the central Afrotropical stock of the
morphospecies, that this mainstream stock re-populated Lake Turkana
repeatedly in replacement of its evanescent, divergent derivatives, and that after
two or three million years, it triumphantly occupies the lake today.
Is evolution centripetal?
The four conditions just assayed are so integrally a part of the concept of PE
that evidence against any one of them weighs against employment of the term
punctuated equilibrium itself. But the term has had a life of its own, no matter
how many of those who use it misunderstand, or have forgotten, its full
implications about how microevolution works. These implications can be
separated into two main themes, each embracing two of the four conditions we
have just reviewed.
The first theme is the episodicity, or discontinuousness, of evolutionary
processes, involving mainly the first two conditions listed for PE. For many
practitioners and spectators of evolutionary biology, conditions 1 and 2 have
boiled down to a rather vague and variable notion that the evidence of the fossil
record shows ‘stop-and-start’, or even just ‘slow-down-and-speed-up’ of
evolutionary change in some range of actual cases. Rarely are the strictures of
Lister and others about ‘geographical control’ seriously heeded, and some
palaeontologists have found that their particular data call for such modified
applications of the concept as “punctuated gradualism” (Malmgren, Berggren
& Lohmann, 1983), or even its falsification (e.g., Kellogg, 1983).
It seems to me that it would help us all if we would call phenomena involving
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W. L. BROWN
apparent discontinuity or sudden changes in rate of evolution by the relatively
noncommittal descriptor episodic. I don’t know how this term was introduced
into evolutionary studies, but it is in use and has a common, if flexible, meaning
in several languages. I would include George Simpson’s term quantum evolution
here, though he largely confined its use to macroevolutionary processes.
Although most of the palaeontologic evidence for episodic evolution at the
speciation level is by its very nature inconclusive, I certainly do not want to give
the impression by my criticism of PE that episodicity is precluded by that
evidence. But the real proof of its importance in microevolution comes from
observations on living populations. A sequel to this paper will cite examples in
taxa ranging from prokaryotes to man.
The second theme-a
vitally important one, even if neglected by many
partisans of PE--is the particular geographic mode of microevolution explicitly
outlined by Eldredge and Gould in what I have listed as conditions 3 and 4.
They call this “the allopatric model”, but it turns out (as I have verified by
separate private conversations with each of them) that in 1972 they did not
mean by this term what most students of speciation processes do-broadly, the
rise of reproductive isolation in geographically separated (allopatric)
populations. Rather, they were referring to the special subset of allopatric
speciation models that Mayr (1954, 1982) described as peripatric speciation.
Mayr’s scenario, widely heralded but unsupported by convincing examples,
describes the situation in which a propagule from a ‘central’ population reaches
a peripherally isolated site, undergoes a ‘genetic revolution’ there, resulting in a
new species that re-immigrates to replace the ancestral species in the old home
range. The genetic revolution part of the model is hotly disputed even today,
but Eldredge and Gould did not dwell on that aspect. However, the peripheral
speciation and subsequent return-and-replacement of the parental by the
I have called ‘centripetal speciation’ in the
derived species--what
Introduction-was nuclear in the original concept of PE. “The central concept
of allopatric speciation is that new species can arise only when a small local
population becomes isolated at the margin of the geographical range of its
parent species. Such local populations are termed peripheral isolates.”
(Eldredge & Gould, 1972: 94). This limited and idiosyncratic version of
‘allopatric speciation’ was also followed in their respective taxonomic
monographs, analysed separately above. Gould ( 1982: 383) writes, “Indeed,
Eldredge and I originally proposed punctuated equilibrium as the expected
geological consequence of Mayr’s theory of peripatric speciation”. Latterly,
under challenge in private conversation, and perhaps to some extent in more
formal writings, they have tended to play down, or even wave aside, the
peripatric component of their PE megadigm, yet they continue to include it
affirmatively, and sometimes prominently, in semipopular articles, books and
lectures (e.g. Eldredge, 1985; Eldredge & Cracraft, 1980: 126-143; Gould, 1982:
383).
I think that it is important, in view of the resulting wide currency of these
representations, and the uncritical assumption of their validity as a basis for
further evolutionary speculation, to declare and to emphasize the lack of solid,
or even strongly suggestive, neontological or palaeontological, evidential
support for the notion of centripetal speciation. I don’t say that it never occurs,
but I will state my conviction that, if i t does, it must be a relatively rare event,
PUNCTUATED EQUILIBRIUM
403
running counter to the prevailingly centrifugal pattern amply documented for
micro- as well as macroevolutionary processes.
And what is PE without centripetal speciation? It is good old episodic
evolution, probably mainly centrifugal in geographic orientation, based
ultimately on centrifugal speciation.
(The centrifugal theory, updated in concept and supported by new examples,
will be reviewed in a sequel to the thoughts presented above.)
ACKNOWLEDGEMENTS
I should like to thank my intellectual opponents, Steve Gould, Niles Eldredge
and Peter Williamson, for their reprints and several illuminating discussions,
most of which were even amiable, The Cornell geologists, especially John Cisne,
Arthur Bloom, John Wells, Douglas Nelson, Jo Ann Nicholson, Ray Gildner,
and others, patiently responded to my numerous importunate inquiries for basic
knowledge of Middle Devonian and Pleistocene geology and its literature. Frank
Ramberg shared his incomparable bibliographic expertise. Susan Pohl mastered
the disking of the fragmented manuscript.
Before and during submission, the manuscript was read and improved after
suggestions from Doris Brown, John Cisne, Ross Crozier, Daniel Goodman,
David Grimaldi, James Liebherr, Jeffrey Levinton, Amy McCune, Karl Niklas,
William Provine, and Frank Ramberg; acknowledgement does not necessarily
imply their approval of my argument.
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