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Tempo and mode of speciation in the sea
Jeremy B.C. Jackson
Alan H. Cheetham
The theory of punctuated equilibria proposed that most fossil species exhibit
morphological stasis for millions of years between geologically instantaneous shifts in
morphology associated with splitting of lineages by allopatric speciation. The theory
initially spawned more rhetoric than data, but the few sufficiently detailed studies
now available generally support the punctuated pattern. The realities of punctuation
and stasis need to be better incorporated into evolutionary studies. Punctuated
speciation does not contradict conventional neodarwinian mechanisms, but it does
constrain the range of probable evolutionary scenarios for speciation, evolution of life
histories and macroevolutionary trends.
Second, morphological stasis for millions
of years was unexpected9, despite revisionism to the contrary.
Unfortunately, much of the continuing debate ignores most of what we have
learned since 1972, and new reports of
gradual or punctuated speciation have
not been subjected to consistently rigorous critical evaluation despite great differences in the quality of evidence available. Here we assess the frequency of
well documented cases of punctuated
equilibria in the sea, with emphasis on
the kinds of evidence required to measure the tempo of morphological evolutionary change in the fossil record.
Requirements to test the theory
of punctuated equilibria
Jeremy Jackson is at the Center for Tropical Paleoecology and Archeology, Smithsonian
Tropical Research Institute, Box 2072 Balboa, Republic of Panama, and the Scripps Institution
of Oceanography, La Jolla, CA 92093-0220, USA; Alan Cheetham is at the Dept of Paleobiology,
National Museum of Natural History, Smithsonian Institution, Washington,
DC 20560, USA (jbjackson@ucsd.edu; cheetham.alan@nmnh.si.edu).
F
or all its problems, real and imagined,
the fossil record provides the only
long-term record of the tempo of morphological evolution and speciation based on
direct evidence. Direct biological measurements of evolution are limited by the
generation times of the organisms involved, which even for the bacterium
Escherichia coli works out to less than
13105 generations in a scientist’s lifetime1. However, most fossil species persist for 13106 to 13107 generations (with a
generation time of approximately one year
or more) without discernible change2.
Indirect biological measurements based
on molecular divergence among closely
related living species necessarily ignore
untold numbers of extinct species scattered across the phylogenetic landscape
and their unknown relations to those still
alive3. Moreover, based as they are on
the assumption of a molecular clock, molecular phylogenies can tell us little about
the tempo of evolutionary change without reliable, independent calibrations of
amounts of genetic divergence that have
occurred since geologically well dated
times of divergence based on the fossil
record4.
Despite the enormous potential, there
were few rigorous paleontological studies of speciation until 1972 when the controversial theory of punctuated equilibria5 stimulated a flurry of new work. The
theory was based on the twin empirical
observations that most fossil species
originate geologically instantaneously
during cladogenesis (branching of evolutionary lineages to produce one or more
new species), with the persistence of
ancestral species, and otherwise exhibit
72
morphological stasis over millions of
years with no net change. Much of the
subsequent controversy concerned skepticism about the universal importance of
natural selection for speciation by some
proponents of the theory6 and the inevitably critical neodarwinian response7,8.
But, despite the rhetoric, the theory was
revolutionary for two reasons. First, it
took the fossil record at face value for the
first time since Darwin, who had invoked
gaps in the record to explain away the
absence of intermediate forms in evolutionary lineages. Large gaps certainly
exist8 but can commonly be overcome by
replicate sampling in different places.
Morphological change can exhibit a
continuum of evolutionary patterns from
highly punctuated cladogenesis to gradual
anagenesis (change without branching of
lineages to produce additional species)10.
Thus, support for punctuated equilibria
requires that changes in morphology
within a species are so small and unsustained directionally that they cannot
account for morphological differences
between ancestors and descendants11.
This in turn requires rigorous taxonomy,
sampling, stratigraphy and phylogenetic
analysis12,13.
To compare the morphology of populations in space and time quantitatively,
taxonomic resolution must be sufficient
to discriminate species with confidence.
Consequently, good preservation of abundant, morphologically complex fossils
is necessary to obtain enough specimens and characters for biometrical discrimination of morphospecies (species
Box 1. Speciation of tropical American cheilostome bryozoans
Metrarabdotos and Stylopoma
Methods: taxonomy, distributions and phylogenies were completely revised11–13.
• Taxonomy: morphologically defined species (morphospecies) were discriminated using replicate measurements of 46 zooidal characters for Metrarabdotos and 12 for Stylopoma. Colonies were assigned
to morphospecies based on clustering and the classification function of discriminant function analysis.
• Sampling: more than 120 recent and fossil collections were made for each genus. New ages of fossils
were determined using planktonic foraminifera and nannoplankton.
• Phylogenies: cladistic methods were used for both genera using 33 qualitative and quantitative characters for Metrarabdotos and 14 for Stylopoma. Phylogenies for Metrarabdotos were also constructed
based on phenetic similarity and stratigraphic position (stratophenetics).
Results: speciation was punctuated in both genera11–13.
• Taxonomy: splitting morphospecies as finely as possible gave the best fit between morphologic and
genetic data for seven species of Stylopoma. Only one out of 237 colonies was classified incorrectly.
• Sampling: neotropical Stylopoma and Metrarabdotos originated 15 to 25 million years ago. Stratigraphic confidence intervals during the past 10 My (million years) are ,0.5 My for all but one species
of Metrarabdotos and about 1 My for Stylopoma. Most species had very narrow geographic ranges.
Species’ durations are positively correlated with geographic range, but several species persisted
within small regions for millions of years so that their narrow distributions are probably real.
• Phylogenies: phylogenies rooted to an outgroup were stratigraphically upside down by as much as
16 My. Phylogenies rooted to the oldest fossils of each genus were stratigraphically more consistent
but still included reversals of 7–8 My. Phylogenies constrained by stratigraphic information gave the
best correlations between morphologic, genetic and cladistic distances for Stylopoma.
0169-5347/99/$ – see front matter. Published by Elsevier Science Ltd. All rights reserved. PII: S0169-5347(98)01504-3
TREE vol. 14, no. 2 February 1999
PERSPECTIVES
Fig. 1. Four phylogenies for Metrarabdotos12. (a) Cladogram rooted on living outgroup (M. costifer). The tree is stratigraphically upside down, with the two oldest
species, M. micropora and Metrarabdotos n.sp.1 (new species 1), hypothesized to be derived from Metrarabdotos n.spp.5 or 10, although these putative ancestors first appeared 6–16 million years (My) after their putative descendants became extinct. Moreover, M. pacificum and M. unguiculatum were placed near the
base of the tree but originated only 2–3 million years ago (Mya). (b) Unmodified cladogram rooted on the earliest fossil species M. micropora. New species 7 and
8 are hypothesized to give rise to the right branch of the tree that originated 7 My before they first appeared, and ghost lineages are 10 My long. (c) Modified cladogram based on the rejection of the hypothesis that either new species 7 or 8 is the ancestor of new species 2. (d) Unmodified stratophenogram in which phylogenetic hypotheses are constrained by both the overall phenetic similarity and stratigraphic position of each species. Reproduced, with permission, from Ref. 12.
defined on the basis of morphology).
These requirements largely limit studies
to marine shelly invertebrates. Likewise,
genetic support for morphospecies is
necessary to have confidence in their
equivalence to recent biological species12,14. Genetic calibration effectively
limits studies to the past 25 million years
(Neogene and Quaternary), when most
modern clades originated.
To resolve biogeographic and stratigraphic ranges with confidence, the density and distribution of sampling must be
sufficient12,13,15. These sampling requirements also limit studies to shelly clades
that are common throughout most of
their history. Biogeographic resolution is
TREE vol. 14, no. 2 February 1999
necessary to distinguish ecophenotypic
change or biogeographic replacement
from evolution16. Stratigraphic precision
is required to constrain phylogenies
that are routinely plagued by extreme
problems of convergent evolution when
species of disparate geological age are
combined in cladistic analyses17. Well
determined ages of first and last occurrences of species are critical because
well resolved phylogenies are necessary
to establish ancestor–descendant pairs
of species with high confidence. Resolving these relationships depends at least
as much on the quality of taxonomy and
sampling as the method of phylogenetic
analysis.
Two bryozoan examples
The importance of meeting all of
these requirements as well as possible is
illustrated by our studies on two genera
of cheilostome bryozoans from tropical
America (Box 1). First, we were able to
discriminate morphospecies with high
statistical confidence and to substantiate
morphospecies genetically in every case
for which genetic data were available.
Thus, morphospecies of these bryozoans
are apparently good biological species12, and the same is true for various
snails14,18, corals19 and foraminifera20
when appropriately rigorous morphological analyses are used. This genetic
support allows paleontologists to study
73
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Table 1. Case studies of speciation and stasis in Neogene to recent marine and brackish water environments
Taxa
Benthos
Metrarabdotos
Stylopoma
Melanopsis impressa clade
Melanopsis bouei clade
Prunum
Amalda
bivalve species
Porites
Montastrea
Plankton
Globorotalia (Globoconella)
Globorotalia
Globorotalia (Fohsella)
Number of
species
Number of
characters
Cases of
stasis
Duration of
stasis (My)a
Cases of
cladogenesis
Duration of
cladogenesis (My)
Cases of
anagenesis
19
19
3
7
2
3
19
3
4
33–46
12–15
19
21
11
10
24
29
16
11
8
1
3
1
3
18
3
4
2–6
2–16
7
.1–5
11
.2
2–4
3
2–3
11
11
0
6
1
0
0
0
0
,0.16
,0.86
0
0
2
0
0
0
0
0
0
7
3
4 or 2?
14–18
34
eigenfunctions
eigenfunctions
32
3
2
0
0.6–3.5
1.5
2
2
1
1
3
0
2
.3
1
Globorotalia tumida
2
Pterocanium
2
aMy
0.01
0.3
0.01
0.05
Refs
0
11–13
12,13
24
24
25
14
26,27
28
28
3
3
1–3?
.2
0.5
0.5
31,32
33
30
1
,1
35
2
0.5
34
2
5 million years.
patterns of speciation in the fossil record
with more confidence. Second, differences between closely related recent and
fossil species of cheilostomes are much
smaller than realized previously, and the
same is true for marine invertebrates in
general21. This widespread cryptic diversity suggests that morphological differences associated with speciation should
be small.
Confidence intervals for ages of first
and last occurrences were constrained
sufficiently to choose easily among alternative phylogenies of the two genera
in most cases12,13. For both genera, conventional cladistic analyses rooted to
an outgroup produced trees that were
stratigraphically upside down (Fig. 1a) –
in some cases, by as much as 16 million
years (My)! Cladistic analyses rooted to
the oldest fossil Metrarabdotos (Fig. 1b),
rather than an outgroup, improved the
stratigraphic consistency of the cladogram but still reversed the whole right
branch of the tree by 6–7 My. These
‘ghost lineages’17 push back the radiation
of the genus by 10 My before the fossil
record of any of the species involved,
even though there are other abundant
fossil bryozoans known from the same
interval. Rejection of this ancestry gives
the tree in Fig. 1c. This tree is similar to
the stratophenetic tree (Fig. 1d), in which
stratigraphic position directly constrains
hypothesized relations among morphologically most similar species, but with
inevitably longer ghost lineages. Similarly, large stratigraphic inconsistencies
emerged from cladistic analyses of Stylopoma12,13. Nevertheless, highly significant positive correlations among cladistic, morphological and genetic distances
for Stylopoma strongly support the use of
74
0.03–0.3
0.073–0.275
Duration of
anagenesis (My)
stratigraphic information in the acceptance or rejection of phylogenetic
hypotheses.
In summary, results for Metrarabdotos
and Stylopoma fulfill reasonably well the
requirements for taxonomy, sampling
and construction of phylogenies to measure the tempo of speciation, and are in
excellent agreement with the theory of
punctuated equilibria. Eleven of 19 species, including all the abundant species,
persisted morphologically unchanged
for 2–16 My (Ref. 13). The same 11 species
also originated fully formed, with no evidence of morphologically intermediate
morphologies.
Other case studies of speciation
in the marine fossil record
By comparison with Metrarabdotos,
one of the most widely cited cases of
gradual evolution22 concerns a parallel
shift in only a single morphological character, the number of pygidial ribs used
to discriminate species in five genera of
trilobites. All the material comes from
a single volcanic inlier in southwestern
Wales (UK), so it is not possible to rule
out the alternative interpretation that
changes in numbers of ribs were a parallel ecophenotypic response to environmental change. The same criticism16
applies to Williamson’s23 example of synchronous, punctuated morphological
change in 13 lineages of freshwater gastropods. There are, however, a small
number of sufficiently comprehensive
and geographically extensive studies to
begin to have an idea of the prevalence of
punctuated speciation (Table 1). These
are divided into benthos and plankton
because of apparent differences in the
tempo of speciation between them.
Benthos
There are two other studies of Neogene benthos with phylogenetic data that
are important for demonstrating that it is
possible to document gradual morphological evolution in fossil species. The first
concerns two clades of the gastropod
Melanopsis that occur throughout the progressively isolated, marine to freshwater
Paratethyean basins of eastern Europe
and western Asia24. The oldest species of
the first lineage, M. impressa, persisted
without net morphological change for
7 My, until extinction of the last remaining
marine fauna. It then gave rise to two new
species by anagenesis over 2 My. At the
same time, however, six new species arose
abruptly in the second lineage by rapid
cladogenesis from a single persistent species without evidence of intermediates.
In the second example, the widely distributed marginellid gastropod Prunum
coniforme persisted unchanged for
11 My, both before and after it gave rise
to P. christineladdae by rapid cladogenesis25. However, the transition was gradual, with clear morphological intermediates, over a period of 73 to 275 thousand
years (Ky), which is only 0.6–2.5% of the
duration of the ancestral species.
Stasis has also been demonstrated for
three species of the gastropod Amalda14, 19
species of bivalves26,27 and 12 species of reef
corals28 (Table 1). Many of the coral species
exhibit significant but oscillating morphological change over a few million years, but
net change throughout the entire history
of the species is no greater than intraspecific variation within recent species.
Plankton
The tempo of speciation of plankton
is more variable than for benthos
TREE vol. 14, no. 2 February 1999
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because of the enormous abundance
and broad geographic ranges of planktonic species, and their close tracking of
changing oceanographic conditions29. The
great advantage of well preserved plankton, such as foraminifera and radiolarians,
for studies of speciation is the unprecedented spatial and temporal resolution
of sampling based on the more than one
thousand deep-ocean cores from around
the world made by the international Ocean
Drilling Program. Temporal resolution
is typically less than 10 Ky and sample
sizes are limited only by the patience
of the investigator. Disadvantages include ignorance of much basic biology
of planktonic protists and frequently inadequate traditional morphological characters for taxonomy and systematics,
as suggested by genetic20 and isotopic30
analyses, and by scanning electron
microscopy20.
The most detailed example is for the
foraminiferan Globorotalia (Globoconella)
clade from six cores in the southwest
Pacific, which exhibits both anagenesis
and highly punctuated speciation associated with cladogenesis at different times
and parts of its range31,32 (Fig. 2). Five
species replaced each other in a series of
chronological shifts in morphology in the
central, cooler water range of the clade
between New Zealand and eastern Australia. In contrast, the peripheral subtropical population gave rise to a single
new species by punctuated cladogenesis
less than 10 Ky after it was isolated from
the temperate populations by the migration of a thermal oceanographic front.
The ancestral and descendant subtropical species coexisted for another 200 Ky
before the ancestral species became
extinct. When the temperate and subtropical lineages met subsequently, they
maintained distinct morphologies in
sympatry. A final species then arose by
punctuated cladogenesis but with continued morphological divergence after speciation. These last two species coexisted
for about 1.2 My before the demise of the
ancestral species.
Two of the four other cases for plankton (Refs 33 and 34 in Table 1) show clear
cladogenesis over as little as 50 to 300 Ky,
but then continued divergence after speciation for another half million years.
Afterwards, both lineages exhibited morphological stasis. Globorotalia tumida
might have arisen by punctuated anagenesis, but there are methodological problems and possible bias in age dating
because of changes in rates of sedimentation35. Finally, overall shape of Globorotalia (Fohsella) appears to have evolved
continuously, but isotopic analysis suggests punctuated cladogenesis over only
10 Ky (Ref. 30).
TREE vol. 14, no. 2 February 1999
Fig. 2. Phylogeny and tempo of speciation of seven species of Neogene to recent Globorotalia (Globoconella) in the southwest temperate to subtropical Pacific31,32. There are five cases of anagenetic
replacement with intermediates and two cases of punctuated cladogenesis during seven million years.
Mya = million years ago. Redrawn, with permission, from Refs 31,32.
The case for punctuation
Overall, 29 out of 31 species of Neogene benthos for which phylogenetic data
are available (Table 1) exhibited punctuated morphological change at cladogenesis that is consistent with the theory
of punctuated equilibria. Cases of punctuation more than double if we include
extended morphological stasis. The two
exceptional cases of anagenesis in Melanopsis occurred during a progressive
environmental shift from brackish to fresh
water conditions isolated from the open
sea. In contrast, patterns of speciation
for planktonic protists are more variable,
although problems of cryptic species
raise doubts about some cases of anagenesis20,30. Thus, most but not all cases of
speciation in the sea are punctuated.
Evolutionary implications
Punctuated speciation in the sea constrains the possible range of underlying
evolutionary processes responsible.
Role of natural selection versus
random genetic change
Morphological changes at speciation
are small, which raises questions about
the relative importance of selection or
drift. We used two quantitative genetic
procedures to examine morphological
stasis and speciation of Metrarabdotos
and Stylopoma relative to expectations of
the neutral model13,36. The first is the
mutation–drift equilibrium model37, which
uses trait heritabilities that we calculated
for quantitative characters used to discriminate morphospecies. The second
approach is based on Lynch’s rate statistic D, which is the ratio of the betweenspecies to within-species phenotypic
variance divided by the number of generations38. Both approaches assume that
traits are polygenic with typical mutation
rates for quantitative characters of between 131022 to 131024 per generation.
Thus values of D within this range are
compatible with random genetic change,
whereas values .131022 imply directional
selection and values ,131024 imply stabilizing selection. The models also assume
gradualism, in that calculations are made
over the entire time since divergence.
Both methods gave the same results13,36. Values of D were always much
less than 131024, so that we had to reject
the neutral model and invoke stabilizing
selection to explain stasis. In contrast,
estimates of the decline in mean evolutionary rate, D, with divergence time
were never as high as 131022, even for a
75
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single generation. Thus, we could not
reject the neutral model for morphological divergence at speciation for these
cheilostomes. Failure to reject a null
model is weak evidence for anything.
Nevertheless, the results strongly suggest that if directional selection is important during speciation, it must act very
fast during intervals of intense ecological
release caused by geographic isolation,
invasion or climate change. Any of these
scenarios is consistent with the theory of
punctuated equilibria.
The genetic analyses help clarify
two important misunderstandings about
punctuation and stasis. First, stasis does
not imply lack of morphological evolution,
but lack of net morphological change.
Stabilizing selection is evolution. Second,
punctuation is not about the absolute
time required for a species to originate,
rather it is about the time required for a
species to originate relative to how long
the species persists with no new morphological change before it becomes
extinct. Speciation sometimes occurs extremely fast, as is the case for the more
than 300 species of cichlids that evolved
in Lake Victoria during the last 10 Ky
(Ref. 39). But that is not inconsistent with
punctuated equilibria.
Evolution of life histories
Small morphological changes at speciation do not necessarily imply small
changes in behavior, development or life
history. Subtleties of behavior are virtually impossible to study with fossils, but
we can infer much about the evolution
of life histories from fossilized growth
rates and larval shells. Larval development commonly differs greatly between
closely related species that are otherwise nearly identical in adult morphology40. For example, the sea urchin Heliocidaris tuberculata produces small eggs
that develop into typical swimming and
feeding larvae that spend weeks in the
plankton before settlement and metamorphosis. In contrast, sympatric H. erythrogramma produces eggs 100 times
larger in volume that develop into nonfeeding larvae that drift for only a few
days before metamorphosis into miniature adults40. The two species also differ
profoundly in cell lineages, formation of
the embryonic axis and patterns of gene
expression during development.
The fossil record of Heliocidaris is
apparently inadequate to estimate the
tempo of divergence in development and
speciation. Molecular genetic data suggest the two species diverged 5–8 million
years ago (Ref. 40), but this is greater
than the longevity of most species and is
sufficient for the radiation of entire Neogene clades (Table 1). Fortunately, many
76
closely related species of marine gastropods also differ in mode of larval development, and these differences are recorded
by the size and number of volutions of
the larval shell (the protoconch)41. Moreover, all well studied species of shelled
gastropods have only a single mode of
larval development42, and there are hundreds of Pliocene (i.e. 5.3–1.8 million years
ago) to recent pairs of species that differ
obviously only in morphology of their
protoconchs and inferred modes of development43,44. Limited morphometric and
genetic data suggest that these differences
in larval development arose during cladogenesis which, by analogy to the gastropods in Table 1, was probably punctuated.
species with particular traits2. Metrarabdotos offers a clear example concerning
colony form. Growth form of tropical
American cheilostomes shifted during the
Late Neogene from dominance by species
with erect colonies to encrusting species49. The same trend is apparent in
Metrarabdotos, but no species evolved
from erect to encrusting growth within
its history. Instead, encrusting growth
appeared suddenly at the first appearance of M. pacificum and M. unguliculatum, the two youngest species in the
clade (Fig. 1). This is not a taxonomic
artifact because growth form was not a
character used to discriminate species.
Conclusions
Timing of speciation and
extinction
The five great mass extinctions of the
past 500 My account for only a few percent
of total extinction during that time45. Extinction and speciation in between these
great events were not random but concentrated in pulses of typically a million years
or less that are correlated with major
changes in oceanography and climate46.
Neogene Caribbean reef corals had a
75% turn-over in species composition during the Late Pliocene47. Increased speciation was spread out over more than
1 My before the more concentrated burst
of extinction. Shallow reef communities
also changed profoundly during extinction
because of increased dominance by giant,
branching staghorn and elkhorn Acropora
corals instead of tiny finger corals, such
as Porites and Stylophora47. The Acropora
species originated 1–2 My before they became abundant. Ecologists attribute their
present success to rapid growth, gigantic
colony size and resistance to hurricanes,
but these are ‘exaptations’ (sensu Gould
and Vrba)48 that originated long before
under dramatically different environmental conditions than now.
Turnover of corals and other Caribbean taxa occurred during intensification
of Northern Hemisphere glaciation, global cooling and astronomically driven
fluctuations in sea level during the Late
Pliocene46. However, it is difficult to correlate biological change with any specific
climatic event, although this is what one
would expect if speciation and extinction
occur as threshold events as implied by
punctuated equilibria, rather than by
gradual change.
Macroevolutionary trends
If speciation were gradual, then macroevolutionary trends could result from
continuous evolution within species. However, if evolution is mostly punctuated,
such trends must result from differential rates of speciation and extinction of
Most cases of speciation in the sea
over the past 25 My show prolonged morphological stasis punctuated by geologically sudden morphological shifts at
cladogenesis. Exceptions increase confidence in our ability to detect different
patterns. Most speciation involves trivially small changes in morphology, although perhaps profound changes in life
history and development. Fortunately, we
can observe such developmental changes
in fossilized larval snails.
Prolonged stasis requires stabilizing
selection but causes of punctuated speciation are unresolved. We cannot reject
genetic drift for cheilostomes, so if directional selection is important for speciation
in these animals it must act extremely
fast. Pulses of speciation involving entire
regional biotas require external forcing,
which could be as simple as the breakdown of oceanographic barriers during climate change. Such events could provide
rare windows of opportunity for geographic isolation and speciation50. Alternatively, many planktonic species might
arise sympatrically or parapatrically during climate change29,32, so that geographic
isolation might not always occur. Finally,
granted the prevalence of punctuated
equilibria, macroevolutionary trends must
arise through differential rates of origination and extinction, and not by adaptive evolution within single species.
All of this is compatible with traditional neodarwinian evolutionary biology,
but was unexpected before the theory of
punctuated equilibria.
Acknowledgements
Our work on Metrarabdotos and
Stylopoma would have been impossible
without the technical assistance of
JoAnn Sanner, Amalia Herrera, Lee
Weigt, Gwen Keller, Javier Jara and Yira
Ventocilla. Ann Budd, Lee-Ann Hayek,
Nancy Knowlton, Russ Lande and John
Maynard Smith provided fruitful
discussion and criticism. Jeremy Jackson
is also grateful to Steve Stearns,
TREE vol. 14, no. 2 February 1999
PERSPECTIVES
Michi Doebeli and colleagues at
the Zoology Institute of the University
of Basel for their helpful skepticism
and discussions during a sabbatical
year at the Naturhistorisches Museum
in Basel, and to Jos Van Damme for
the invitation to present much of this
paper as a plenary talk at the 6th
Congress of the European Society
for Evolutionary Biology at Arnhem.
This work was supported by the
Smithsonian Institution.
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