22. J. M. Weisberg, J. H. Taylor, in Binary Radio Pulsars,
F. Rasio, I. H. Stairs, Eds. (Astronomical Society of the
Pacific, San Francisco, 2005), pp. 25­31.
23. J. H. Taylor, A. Wolszczan, T. Damour, J. M. Weisberg,
Nature 355, 132 (1992).
24. M. Lyutikov, Mon. Not. R. Astron. Soc. 362, 1078
(2005).
25. T. Damour, J. H. Taylor, Astrophys. J. 366, 501 (1991).
26. J. M. Cordes, T. J. W. Lazio, http://arxiv.org/abs/astro-ph/
0207156 (2002).
27. K. Kuijken, G. Gilmore, Mon. Not. R. Astron. Soc. 239,
571 (1989).
28. T. Damour, G. Esposito-Fare`se, in preparation.
29. W. A. Coles, M. A. McLaughlin, B. J. Rickett, A. G. Lyne,
N. D. R. Bhat, Astrophys. J. 623, 392 (2005).
30. C. Lange et al., Mon. Not. R. Astron. Soc. 326, 274
(2001).
31. S. M. Ransom et al., Astrophys. J. 609, L71 (2004).
32. R. R. Rafikov, D. Lai, Phys. Rev. D 73, 063003 (2006).
33. E. Pfahl, S. Rappaport, P. Podsiadlowski, H. Spruit,
Astrophys. J. 574, 364 (2002).
34. T. Piran, N. J. Shaviv, Phys. Rev. Lett. 94, 051102 (2005).
35. I. H. Stairs, S. E. Thorsett, R. J. Dewey, M. Kramer,
C. McPhee, Mon. Not. R. Astron. Soc., in press (2006).
36. T. Damour, R. Ruffini, C. R. Acad. Sci. Paris Ser. I Math.
279, 971 (1974).
37. B. M. Barker, R. F. O'Connell, Astrophys. J. 199, L25
(1975).
38. N. Wex, Class. Quantum Grav. 12, 983 (1995).
39. I. A. Morrison, T. W. Baumgarte, S. L. Shapiro,
V. R. Pandharipande, Astrophys. J. 617, L135 (2004).
40. J. M. Lattimer, B. F. Schutz, Astrophys. J. 629, 979
(2005).
41. E. M. Standish, Astron. Astrophys. 336, 381 (1998).
42. We thank T. Damour and N. Wex for useful discussions.
The Parkes radio telescope is part of the Australia
Telescope, which is funded by the Commonwealth of
Australia for operation as a National Facility managed by
CSIRO. The National Radio Astronomy Observatory
(NRAO) is a facility of the NSF operated under
cooperative agreement by Associated Universities Inc.
GASP is funded by a Natural Sciences and Engineering
Research Council of Canada (NSERC) RTI-1 grant (I.H.S.)
and by NSF grants to D. Backer and D. Nice. We thank
P. Demorest, R. Ramachandran, and J. van Leeuwen for
their contributions to GASP hardware and software
development. Supported by an NSERC University Faculty
Award (I.H.S.), by the Italian Ministry of University and
Research under the national program Cofin 2003
(M.B., A.P., and N.D'A.), and by NSF, NASA, and NRAO
(F.C.). Pulsar research at the University of British
Columbia is supported by an NSERC Discovery Grant.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1132305/DC1
SOM Text
Figs. S1 to S4
Tables S1 and S2
10 July 2006; accepted 7 September 2006
Published online 14 September 2006;
10.1126/science.1132305
Include this information when citing this paper.
Out of the Tropics: Evolutionary
Dynamics of the Latitudinal
Diversity Gradient
David Jablonski,1
* Kaustuv Roy,2
James W. Valentine3
The evolutionary dynamics underlying the latitudinal gradient in biodiversity have been
controversial for over a century. Using a spatially explicit approach that incorporates not only
origination and extinction but immigration, a global analysis of genera and subgenera of marine
bivalves over the past 11 million years supports an ``out of the tropics'' model, in which taxa
preferentially originate in the tropics and expand toward the poles without losing their tropical
presence. The tropics are thus both a cradle and a museum of biodversity, contrary to the
conceptual dichotomy dominant since 1974; a tropical diversity crisis would thus have profound
evolutionary effects at all latitudes.
T
he most striking large-scale pattern in
biological diversity is the dramatic increase
in the number of species and
higher taxa from the poles to the tropics. This
taxonomic trend, commonly called the latitudinal
diversity gradient (LDG), has been documented
in the multicellular biotas of forests,
grasslands, wetlands, continental shelves, the
open ocean, and even the deep sea; it characterizes
plants, fungi, marine and freshwater invertebrates,
and all of the vertebrate classes (1).
The history of the LDG extends back through
the Mesozoic into the Paleozoic (2­7), although
the slope of the gradient has varied over time
and the trend might even have disappeared for
a time if any of the mass extinctions were disproportionately
severe in the tropics (8).
Although the existence of the LDG has been
known for more than a century (9, 10) and has
been quantified hundreds of times (1), it remains
the Bmajor, unexplained pattern of natural
history[ ERicklefs in (11)^, with Ban
astonishing lack of consensus about the mechanisms
leading to this variation in diversity[ (1).
Recent work has focused primarily on ecological
explanations for the LDG (9, 12­15), and
although these analyses have found interesting
correlations between diversity and environmental
variables, they reveal little about the evolutionary
dynamics of the species and lineages
that established and maintain the LDG (16, 17).
Because virtually all possible combinations of
the key evolutionary parameters have been proposed
to shape the LDG (table S1), progress in
this area depends on empirical data that can
falsify alternatives. Here we (i) outline a framework
for evaluating the spatial and temporal dynamics
that underlie the present-day LDG, (ii)
synthesize previous work from this perspective,
and (iii) present paleontological analyses that
falsify the classic portrayal of the tropics as either
a cradle or a museum of biodiversity (18).
Cradles and Museums
From an evolutionary perspective, large-scale
spatial patterns of biodiversity depend on three
variables: origination rates (O), extinction rates
(E), and changes in geographic distributions
(expressed here as I, for immigration into a
latitudinal bin) of taxa. For a simple two-box
model, with the tropics and extratropics denoted
as subscripts, diversity in the tropics (DT)
is determined by OT ­ ET  IT, and diversity
in the extratropics (DE) by OE ­ EE  IE (Fig. 1).
With this notation, it can easily be seen that a
latitudinal gradient in richness, with DT 9 DE,
can result from many different combinations of
these variables. Theoretically, the extinction terms
could represent either true global extinction of
taxa, local extinction for a particular spatial
bin, or a combination of the two. Estimating
local extinction rates using paleontological data
is generally difficult owing to incomplete spatial
sampling, and even more difficult using phylogenetic
information. In addition, our empirical
results suggest that the effect of local extinction
is much smaller than that of range expansion, at
least for marine bivalves. Thus, as in most
previous studies (table S1), our discussion of the
role of extinction in shaping the LDG focuses
primarily on global processes.
The simplest evolutionary models for the
LDG assume that taxa are static in their geographic
distributions (IT 0 IE 0 0) and treat the
greater number of species and higher taxa in
the tropics as the result of either a higher rate
of origination of species and lineages (OT 9
OE) or lower extinction rates as compared to
extratropical regions (ET G EE). For example,
Wallace (19) attributed high tropical diversity to
a more stable climatic history, which allowed
more time to accumulate taxa (ET G EE), and
this view has found proponents ever since (20)
(table S1). Others have argued that extinction
rates are high in the tropics but are outstripped
by even higher origination rates (ET 9 EE, OT d
OE) (21). The importance of origination and
extinction in generating the LDG was highlighted
in Stebbins' (18) famous metaphor of the
tropics as a cradle or a museum, and this memorable
dichotomy has been the dominant paradigm
ever since.
1
Department of Geophysical Sciences, University of Chicago,
5734 South Ellis Avenue, Chicago, IL 60637, USA. 2
Section
of Ecology, Behavior and Evolution, University of California,
San Diego, 9500 Gilman Drive, La Jolla, CA 92093­0116,
USA. 3
Department of Integrative Biology, University of
California, Berkeley, Berkeley, CA 94720, USA.
*To whom correspondence should be addressed. E-mail:
djablons@uchicago.edu
RESEARCH ARTICLES
6 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org102
onMay23,2007www.sciencemag.orgDownloadedfrom
Distinguishing evolutionary cradles from museums
requires separate estimates of origination
and extinction rates. Such estimates are currently
unavailable even for most large groups with a
good fossil record and may not be feasible for
groups lacking a fossil record without assuming
stochastically constant extinction rates (22), an
assumption often violated over the past 15 million
years of Cenozoic history (23, 24). Consequently,
attempts to quantify the evolutionary
underpinnings of the LDG have focused mainly
on latitudinal differences in net diversification
rates of living taxa [the composite value (O ­ E)],
a parameter more readily estimated from phylogenies
of extant organisms (table S1) (25, 26).
Such differences in net diversification rates are
valuable for investigating many questions (26),
but their application to the cradle/museum problem
is again limited by the many combinations
of O and E that can produce a given net value.
Realistically, areas with high net diversification
rates are more likely to be evolutionary cradles,
but those where such rates are low could have
experienced either high or low extinction rates.
Rate Differences and Range Shifts
The cradle/museum dichotomy, and the more
general hypothesis that attributes high tropical
diversity to higher net diversification rates, implicitly
assume that the LDG derives largely from
differences in in situ origination and extinction
(16, 25, 26). However, this simplifying assumption
is contradicted by biogeographic data showing
that (i) many taxa shift their geographic range
limits substantially in response to climatic
changes [they have moved across latitudes to
track changing climates (27, 28)], and (ii) many
taxa have geographic distributions that encompass
both tropical and extratropical regions
[assuming origination in a single climate zone,
they have expanded across latitudes in the face of
climate differences (9, 29)]. Thus, the dynamics
underlying the LDG must involve not only latitudinal
differences in origination and/or extinction
rates but also extensive changes in spatial
distributions of taxa over time.
Although most analyses of the LDG based on
present-day biogeography have ignored the role
of past distributional changes, the notion that
shifts in latitudinal distributions of taxa play an
important role in shaping the LDG is not new
(20, 30­33). Scenarios in which taxa preferentially
originate in tropical regions and spread
out from there (IT G IE) or the reverse (IT 9 IE)
have both been advocated (33), but attempts to
separate the contributions of O, E, and I to the
shape of the LDG have been undermined by a
lack of basic information on the time and place of
origin for the vast majority of living taxa. Instead,
taxa occurring in both tropical and extratropical
regions are generally handled either by (i) including
each taxon in rate calculations for all
latitudinal bins within its geographic range (34)
or (ii) including each taxon only in the bin
corresponding to the center of its latitudinal
range (25, 26). Neither approach can separate
the effects of past distributional shifts from
those due to changes in diversification rates
with latitude, however. Protocol (i) is analytically
problematic (owing to the autocorrelation
imposed by counting each taxon in multiple
bins) and allows a widespread taxon to influence
the age distributions of more latitudinal
bins than a restricted taxon does, even though
each should contribute only to its latitude of
origin. In contrast, protocol (ii) makes the unrealistic
assumption that taxa originate near the
midpoint of their present-day geographic ranges.
The asymmetry of range expansion from the
true place of origin is likely to increase with the
geographic range of a taxon (26), and even
narrow-ranging taxa may abandon ancestral
distributions in response to large climatic
changes such as occurred during the Pleistocene
(27). Some progress has been made recently in
estimating origination, extinction, and immigration
rates from the shapes of taxon age distributions,
but such models also make a number of
important simplifying assumptions about the
underlying dynamics (35). Thus, direct tests of
the role of large-scale range expansion in shaping
the LDG are needed, and the fossil record
remains the best source of data for such tests.
Out of the Tropics: A Dynamic Model
One potential reason why published studies
have failed to produce a consensus on whether the
tropics are a biological cradle or museum (table
S1) is that this dichotomy is misleading. The
tropics could be a cradle, a museum, or both;
theoretically, so could the polar regions; and taxa
could predominantly remain in place or either
expand or contract their distributions (Fig. 1). We
suggest that the available data are most consistent
with an ``out of the tropics'' (OTT) model, in
which the tropics are both a cradle and a museum,
with taxa preferentially originating in the tropics
and expanding over time into high latitudes
without losing their initial tropical distributions.
Thus OT 9 OE, ET e EE, and IT G IE.
Until now, direct empirical tests of this model
have been lacking, although one biogeographic
model suggests that such a dynamic could explain
the age-frequency distributions of bivalve genera
found in polar oceans today (35), and some
Fig. 1. Simple hypothetical scenarios
illustrating the cradle, museum, and OTT
models. Red denotes lineages that originated
in the tropics; blue denotes lineages
that originated outside the tropics. The
horizontal lines connecting sister lineages
also represent geographic distributions;
those extending from tropics to extratropics
denote clades that originated in
the tropics but have subsequently extended
their ranges into extratropical
regions while retaining a tropical presence.
Many other combinations of these
parameters are possible. The dashed horizontal
line indicates the present day.
TROPICSEXTRATROPICS EXTRATROPICS
TROPICSEXTRATROPICS EXTRATROPICS
TROPICSEXTRATROPICS EXTRATROPICS
Tropics as Cradle
Tropics as Museum
OTT Model
OT > OE, ET = EE, IT = IE = 0
OT = OE, ET < EE, IT = IE = 0
OT > OE, ET < EE, IT < IE
TIMETIMETIME
RESEARCH ARTICLES
www.sciencemag.org SCIENCE VOL 314 6 OCTOBER 2006 103
onMay23,2007www.sciencemag.orgDownloadedfrom
phylogenetic analyses find extratropical taxa to be
derived from tropical lineages (29, 36). Here we
test the OTT model using paleontological and
present-day distributions, and frame testable
predictions for groups lacking a good fossil record.
Testing the OTT Model
The marine Bivalvia currently provide one of the
few systems that can address each of the OTT
predictions directly. As a group, bivalves exhibit a
strong LDG, not only for species but also at the
level of genera and subgenera (henceforth simply
termed genera) (37, 38), which have been the
preferred units for large-scale paleontological
analyses owing to their taxonomic stability and
the robustness of the patterns to sampling artifacts
relative to species-level data. The fossil record of
marine bivalve genera is rich and densely sampled,
with a ``pull of the Recent'' (the artifact that can
arise via strong differences in the sampling of
present-day and geologic time intervals) of less
than 5% (39). Remaining preservational effects
are increasingly well understood (39­42), so that
artifacts can be avoided or minimized. Bivalves
occur at all latitudes in the modern oceans, and
sampling of their fossil record is almost as
widespread, although it is not unbiased spatially
(43).
Taxonomic standardization, a prerequisite
for rigorous analysis of the spatial and temporal
patterns of biodiversity, although not fully complete,
has been undertaken for many late Cenozoic
occurrences (39, 40). Accordingly, marine
bivalves are becoming a model system for
macroecological and macroevolutionary analysis
(40, 44, 45), allowing us to test the predictions
of the OTT model with data on the
modern latitudinal distributions of bivalve genera,
the geologic ages of those taxa relative to
their present-day distributions, the spatial pattern
of the first occurrences of those taxa, and
post-origination changes in their latitudinal
range limits.
OT 9 OE. Testing this prediction for genera
requires spatially explicit data on their first occurrences
in the geologic record, which must be
treated cautiously because of biases toward heavier
sampling in temperate latitudes (40, 43, 46­48).
One approach to this problem is to use the
proportion of living genera known from the
fossil record within each bivalve family as a
sampling gauge (49). For the past 11 million
years (from the beginning of the late Miocene
to the Recent), the proportion of living taxa that
first occur in tropical deposits is positively related
to the proportion of taxa known from the
fossil record: The better the fossil record of a
family, the higher the proportion of its genera
that first occurs in the tropics (Fig. 2).
We can also tie a more detailed analysis of the
geography of origination to the quality of each
family's fossil record (49). Restricting analyses to
families having Q75% of their genera known as
fossils, tropical first occurrences of those bivalve
taxa significantly exceed extratropical ones in each
of three successive geologic time intervals leading
up to the present day (late Miocene, Pliocene, and
Pleistocene; Fig. 3, A, C, and E). Summing over
the entire 11-million-year interval, we record 117
tropical and 46 extratropical first occurrences (a
significant difference, P 0 2.543 Â 10­8),
indicating that the overall pattern will be robust
to any error in the assignment of individual
stratigraphic units to our three time bins. And
because sampling is strongly biased in the opposite
direction (so that some genera originating
in the tropics will not be recorded paleontologically
until they expand into the bettersampled
extratropical zones), these data are
0
10
20
30
40
50
60
0
5
10
15
20
25
0 10 20 30 40 50 60
0
5
10
15
20
25
30
Tropical Extratropical
0
5
10
15
0 10 20 30 40 50
Tropics Extratropics
0
10
20
30
40
Tropical Extratropical
Tropical Extratropical
Tropics Extratropics
NumberofTaxaNumberofTaxaNumberofTaxa
NumberofTaxaNumberofTaxa
N = 53
N = 25
Latitude (N or S)
Latitude (N or S)
A B
C D
E
Modern Poleward limits of taxa with tropical
origins
First occurrences
0
5
10
15
0 10 20 30 40 50 60
NumberofTaxa
Tropics Extratropics
N = 39
F
Latitude (N or S)
N=38
N=73
N=52
Fig. 3. Latitudinal differences in originations (left) and present-day range limits of marine bivalve
genera first occurring in the tropics (right), using only families with Q75% of their living taxa known as
fossils. (A and B) Genera first appearing in the Pleistocene. (C and D) Genera first appearing in the
Pliocene. (E and F) Genera first appearing in the late Miocene. N indicates the total number of genera
in each analysis. For (C) and (E), tropical first occurrences are significantly more frequent than
extratropical ones and marginally so for (A), despite the sampling bias favoring extratropical
occurrences [(A), P 0 0.07; (C), P 0 0.0001; (E), P 0 0.0004; exact binomial test]. These results are not
sensitive to the cutoff value: For example, for the Pliocene, if we use 80% having a fossil record, we
find 39 tropical versus 18 extratropical first appearances (FAs); using 70% having a fossil record, we
find 52 tropical versus 22 extratropical FAs. Similarly, for the late Miocene, if we use 80% having a
fossil record, we find 26 tropical versus 8 extratropical FAs; using 70% having a fossil record, we find
35 tropical versus 9 extratropical FAs. If we treat the data in Fig. 2 as two discrete populations and
thus set a 60% cutoff value, we find 38 tropical versus 11 extratropical late Miocene FAs and 56
tropical versus 25 extratropical Pliocene FAs.
0
20
40
60
80
100
0 20 40 60 80 100
PercentStartinginTropics
Percent with Fossil record
Fig. 2. Sampling quality versus tropical originations
in marine bivalve families since the start of
the late Miocene (11 million years ago) for families
having three or more first occurrences within that
interval. Families having more complete fossil
records [measured as the proportion of living genera
known as fossils (40)] tend to show a significantly
greater proportion of first occurrences of their
constituent taxa in the tropics (simple linear regression,
R2 0 0.560, P 0 0.0001).
RESEARCH ARTICLES
6 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org104
onMay23,2007www.sciencemag.orgDownloadedfrom
almost certainly underestimates of the tropical
predominance of first occurrences. The latitudinal
difference in originations extends across the
Bivalvia and is not just restricted to the heteroconch
clade (table S2), which has been the most
prolific diversifier through the Cenozoic (50).
ET e EE. How extinction rates vary with
latitude remains poorly known. Taken at face value,
the bivalve data show substantially higher extinctions
at high latitudes over the past 11 million years;
only 30 exclusively tropical genera go extinct as
compared to 107 extratropical and cosmopolitan
ones. Factoring in the much greater taxon richness
in the tropics suggests an even higher differential in
per-taxon rates. These data must again be treated
cautiously, owing to the severe undersampling of
the tropics, but the presence of so many last
occurrences at high latitudes constrains potential
patterns and suggests that tropical extinction rates
are unlikely to be substantially higher than
extratropical ones. These results are also qualitatively
consistent with previous studies that have
found either little variation in species extinction
rates with latitude (51) or higher extinction rates of
genera and subgenera in polar oceans relative to
lower latitudes (35). Further analyses of latitudinal
trends in extinction rates are needed.
IT G IE. The bivalve data indicate that genera
originating in thetropics tend toextend theirranges
to higher latitudes over time, as predicted by the
OTT model (49). For each of the time bins in Fig.
3, assuming the tropics to be between 25-N and
25-S latitude, Q75% of the taxa that occur first in
the tropics also occur extratropically today; only 2
of those taxa have left the tropics entirely (Fig. 3,
B, D, and F; the proportions are 980% if 23- is
taken as the edge of the tropics). Again, because
the number of taxa known to start in the tropics is
undersampled, these values of IE are almost
certainly underestimates.
Insights from Modern Biogeography
The direct tests listed above require temporal and
spatial data on ancient distributions that are not
available for many important groups of organisms.
In such cases, biogeographic data from living taxa
can be tested for consistency with the OTT model,
although they will not be definitive tests of the
model for the reasons outlined above.
Endemism versus latitude. If genera primarily
originate in the tropics and expand into
extratropical regions, then the simplest biogeographic
prediction is that endemism today
should decrease with latitude. This prediction is
clearly supported for present-day marine bivalves
(49), in which the LDG persists if we simply
exclude all genera restricted to extratropical latitudes:
Most of the diversity of extratropical regions
comes from taxa shared with the tropics [(49) and
fig. S1]. However, this is strictly a consistency test,
evaluating the tendency of taxa to expand outside
of their initial geographic distributions (assuming
that each taxon starts with a single species within a
single climate zone), without establishing the
direction of those expansions.
Age versus latitude. If living genera preferentially
originated in the tropics and subsequently
expanded into higher latitudes, their average ages
should increase with latitude, with the tropics
harboring both old and young taxa and higher
latitudes progressively lacking in younger taxa. For
marine bivalves, both mean and median geologic
ages of genera occurring in 10- latitudinal bins
increase from the equator to the poles (49) (fig.
S2), and the age-frequency distributions of
tropical and polar assemblages differ significantly
(fig. S3). However, such trends suffer from the
problem of spatial autocorrelation (the right tails
of the histograms in fig. S3 share many taxa) and
cannot separate the OTT model from the more
traditional ``tropics as cradle'' hypothesis. A better
approach is to test for spatial differences in the
shapes of taxon age distributions, derived paleontologically
or from well-calibrated molecular
phylogenies, against predictions of models that
incorporate originations, extinctions, and range
expansions of taxa (35). Alternatively, reconstructing
ancestral geographic ranges of individual
taxa from well-supported phylogenies of living
species (52), in conjunction with biogeographic
data, should permit indirect tests. Finally, the
finding that the steepest latitudinal gradients
occur in the geologically youngest clades of
bivalves (50, 53) is also consistent with a dynamic
involving preferential origination at low
latitudes and poleward expansion over time.
Conclusion
Our goal here has not been to formulate yet another
hypothesis about the evolutionary dynamics
underlying the LDG; most possible combinations
of origination, extinction, and spatial shifts have
already been proposed. Instead, we suggest that the
long-standing ``tropics as cradle or museum''
paradigm is not supported by paleontological data
or present-day biogeographic patterns [also see
(29)]. The OTT alternative posits that lineages not
only preferentially originate in the tropics but also
persist there as they expand poleward; it does not
preclude extratropical speciation, of which there
are many examples (54), but predicts that most
extratropical species belong to lineages that
originated in the tropics. Thus, the OTT dynamic
is likely to be strongest at the level of lineages
(for example, genera and families), and we view
this model as providing a framework for understanding
latitudinal patterns of speciation. Preferential
origination of taxa in the tropics
followed by range expansion into high latitudes
has been proposed on biogeographic and phylogenetic
grounds (34, 36, 55, 56), and the
dynamic is consistent with previous paleontological
analyses (46). The OTT model is similar
to the niche conservatism model (29) in that
both view the tropics as a cradle and a museum
of diversity [see also (57, 58)], but our model
differs in emphasizing the expansion of geographic
distributions over time; we see ``niches'' of
taxa expanding over time, perhaps as species
proliferate within and among climate zones. The
general scarcity of robust spatial data on where
individual taxa originate has hindered direct tests
of these dynamics.
The OTT dynamic documented here suggests
that the LDG is shaped by the interaction of two
different kinds of processes: those that drive the
higher origination rates in the tropics and those that
determine the geographic range limits of individual
taxa, which makes it difficult to untangle causal
mechanisms. We still know little about why taxa
preferentially originate at lower latitudes; of the
many proposed hypotheses (46, 59­61), empirical
tests have yielded mixed results for some (61­66)
whereas others remain untested. Similarly, the
controls on the geographic range limits of taxa
are poorly understood, although theoretical and
empirical studies are beginning to address this issue
(67, 68). Progress is clearly needed on both fronts,
particularly if the source-sink macroevolutionary
and biogeographic dynamic outlined here is a
general feature of diversity gradients (for example,
along bathymetric, elevational, and longitudinal
gradients) (56, 57).
The OTT model also has implications for
present-day biodiversity, beyond providing a
framework for modeling biotic responses to future
climate changes. If the tropics are the engine of
globalbiodiversity,assuggestedbyouranalyses(see
also table S1), then major losses of tropical taxa will
have a global effect by suppressing the primary
source of evolutionary novelty for all latitudes. A
tropical diversity crisis would thus not only affect
tropical biotas but also have profound long-term
evolutionary consequences for biotas at higher
latitudes.
References and Notes
1. H. Hillebrand, Am. Nat. 163, 192 (2004).
2. L. R. Leighton, Evol. Ecol. 19, 563 (2005).
3. J. A. Crame, Divers. Distrib. 7, 175 (2001).
4. M. L. Rosenzweig, Species Diversity in Space and Time
(Cambridge Univ. Press, Cambridge, 1995).
5. F. Cecca, Palaeobiogeography of Marine Fossil Invertebrates--
Concepts and Methods (Taylor and Francis, London, 2002).
6. J. Z. Shen, G. R. Shi, Palaeogeogr. Palaeoclimatol.
Palaeoecol. 208, 235 (2004).
7. F. Cecca, B. Vrielynck, T. Lavoyer, H. Gaget, J. Biogeogr.
32, 535 (2005).
8. D. Jablonski, Paleobiology 31 (suppl.), 192 (2005).
9. J. R. G. Turner, Basic Appl. Ecol. 5, 435 (2004).
10. B. A. Hawkins, Trends Ecol. Evol. 16, 470 (2001).
11. R. Lewin, Science 244, 527 (1989).
12. B. A. Hawkins et al., Ecology 84, 3105 (2003).
13. M. R. Willig, D. M. Kaufman, R. D. Stevens, Annu. Rev.
Ecol. Syst. 34, 273 (2003).
14. D. J. Currie et al., Ecol. Lett. 7, 1121 (2004).
15. M. A. Rex, J. A. Crame, C. T. Stuart, A. Clarke, Ecology 86,
2288 (2005).
16. S. L. Chown, K. J. Gaston, Trends Ecol. Evol. 15, 311 (2000).
17. K. J. Gaston, Nature 405, 220 (2000).
18. G. L. Stebbins, Flowering Plants: Evolution above the
Species Level (Belknap, Cambridge, MA, 1974).
19. A. R. Wallace, Tropical Nature and Other Essays
(Macmillan, London, 1878).
20. A. G. Fischer, Evol. Int. J. Org. Evol. 14, 64 (1960).
21. N. C. Stenseth, Oikos 43, 417 (1984).
22. S. Nee, E. C. Holmes, R. M. May, P. H. Harvey, Philos.
Trans. R. Soc. London Ser. B 344, 77 (1994).
23. E. Paradis, J. Theor. Biol. 229, 19 (2004).
24. J. A. Todd et al., Proc. R. Soc. London Ser. B 269, 571
(2002).
RESEARCH ARTICLES
www.sciencemag.org SCIENCE VOL 314 6 OCTOBER 2006 105
onMay23,2007www.sciencemag.orgDownloadedfrom
25. M. Cardillo, Proc. R. Soc. London Ser. B 266, 1221
(1999).
26. M. Cardillo, C. D. L. Orme, I. P. F. Owens, Ecology 86,
2278 (2005).
27. S. T. Jackson, J. W. Williams, Annu. Rev. Earth Planet. Sci.
32, 495 (2004).
28. J. W. Valentine, D. Jablonski, in Species Diversity in
Ecological Eommunities, R. E. Ricklefs, D. Schluter, Eds.
(Univ. of Chicago Press, Chicago, IL, 1993), pp. 341­349.
29. J. J. Wiens, M. J. Donoghue, Trends Ecol. Evol. 19, 639
(2004).
30. A. D. Hecht, B. Agan, Syst. Zool. 21, 308 (1972).
31. J. A. Wiens, Funct. Ecol. 3, 385 (1989).
32. R. E. Ricklefs, Ecol. Lett. 7, 1 (2004).
33. P. J. Darlington, Evol. Int. J. Org. Evol. 13, 488 (1959).
34. K. J. Gaston, T. M. Blackburn, Proc. R. Soc. London Ser. B
263, 63 (1996).
35. E. E. Goldberg, K. Roy, R. Lande, D. Jablonski, Am. Nat.
165, 623 (2005).
36. W. S. Judd, R. W. Sanders, M. J. Donoghue, Harv. Pap.
Bot. 5, 1 (1994).
37. C. A. Campbell, J. W. Valentine, Paleobiology 3, 49
(1977).
38. K. Roy, D. Jablonski, J. W. Valentine, Proc. R. Soc. London
Ser. B 267, 293 (2000).
39. D. Jablonski, K. Roy, J. W. Valentine, R. M. Price,
P. S. Anderson, Science 300, 1133 (2003).
40. J. W. Valentine, D. Jablonski, S. M. Kidwell, K. Roy, Proc.
Natl. Acad. Sci. U.S.A. 103, 6599 (2006).
41. S. M. Kidwell, Science 307, 914 (2005).
42. E. M. Harper, in The Adequacy of the Fossil Record,
S. K. Donovan, C. R. C. Paul, Eds. (Wiley, Chichester, UK,
1998), pp. 243­267.
43. For example, an inventory of published material and
the holdings of the Leiden Natural History Museum for
the molluscan faunas of Indonesia, the richest and
most extensive paleontological sample of the entire
Indo-West Pacific core of marine molluscan diversity (69),
finds 1596 late Miocene and 4532 Pliocene specimens.
Extratropical samples for the same interval are far too
numerous to list, but five studies out of the many
dozens published involve considerably more than
150,000 late Miocene and Pliocene bivalve specimens
(70­74).
44. D. Jablonski, K. Roy, J. W. Valentine, in Macroecology;
Concepts and Consequences, T. M. Blackburn, K. J. Gaston,
Eds. (Blackwell, Oxford, 2003), pp. 368­390.
45. K. Roy, D. Jablonski, J. W. Valentine, in Frontiers of
Biogeography, M. V. Lomolino, L. R. Heany, Eds. (Sinauer,
Sunderland, MA, 2004), pp. 151­170.
46. D. Jablonski, Nature 364, 142 (1993).
47. J. B. C. Jackson, K. G. Johnson, Science 293, 2401
(2001).
48. P. A. Allison, D. E. G. Briggs, Geology 21, 65 (1993).
49. Materials and methods are available as supporting
material on Science Online.
50. J. A. Crame, Paleobiology 26, 188 (2000).
51. S. M. Stanley, W. O. Addicott, K. Chinzei, Geology 8, 422
(1980).
52. R. H. Ree, B. R. Moore, C. O. Webb, M. J. Donoghue, Evol.
Int. J. Org. Evol. 59, 2299 (2005).
53. J. A. Crame, Paleobiology 28, 184 (2002).
54. D. Schluter, The Ecology of Adaptive Radiation (Oxford
Univ. Press, Oxford, 2000).
55. R. E. Ricklefs, D. Schluter, in Species Diversity in
Ecological Communities, R. E. Ricklefs, D. Schluter, Eds.
(Univ. of Chicago Press, Chicago, IL, 1993), pp. 350­363.
56. J. C. Briggs, J. Biogeogr. 30, 1 (2003).
57. B. A. Hawkins, J. A. F. Diniz-Filho, S. A. Soeller, J. Biogeogr.
32, 1035 (2005).
58. D. D. McKenna, B. D. Farrell, Proc. Natl. Acad. Sci. U.S.A.
103, 10947 (2006).
59. K. Rohde, Oikos 65, 514 (1992).
60. D. Schemske, in Ecological and Evolutionary Perspectives
on the Origins of Tropical Diversity, R. Chazdon,
T. Whitmore, Eds. (Univ. of Chicago Press, Chicago, IL,
2002), pp. 163­173.
61. A. P. Allen, J. H. Brown, J. F. Gillooly, Science 297, 1545
(2002).
62. S. D. Wright, R. D. Gray, R. C. Gardner, Evol. Int. J. Org.
Evol. 57, 2893 (2003).
63. S. S. Pawar, Evol. Int. J. Org. Evol. 59, 234 (2005).
64. J. M. Brown, G. B. Pauly, Evol. Int. J. Org. Evol. 59, 238
(2005).
65. L. Bromham, M. Cardillo, J. Evol. Biol. 16, 200 (2003).
66. P. R. Martin, J. K. McKay, Evol. Int. J. Org. Evol. 58, 938
(2004).
67. T. J. Case, R. D. Holt, M. A. McPeek, T. H. Keitt, Oikos 108,
28 (2005).
68. C. Parmesan et al., Oikos 108, 58 (2005).
69. A. G. Beu, Scripta Geol. 130, 1 (2005).
70. E. Ferrero, B. Merlino, Boll. Malac. 28, 101 (1992).
71. E. Ferrero, B. Merlino, A. Provera, Boll. Malac. 33, 43 (1998).
72. G. Pavia, Boll. Soc. Paleontol. Ital. 14, 99 (1975).
73. S. L. B. Wood, thesis, Virginia Polytechnic Institute,
Blacksburg, VA (2006).
74. R. A. Cooper et al., Geology 34, 241 (2006).
75. We thank K. Amano, L. C. Anderson, A. G. Beu, J. S. Crampton,
E. V. Coan, T. A. Darragh, H. H. Dijkstra, E. M. Harper,
C. S. Hickman, the late K. Lamprell, P. A. Maxwell,
P. M. Mikkelsen, P. Middelfart, N. J. Morris, G. Paulay,
F. Scarabino, J. A. Schneider, J. D. Taylor, J. D. Todd,
T. R. Waller, A. Ware´n, and F. P. Wesselingh for generously
sharing their expertise on bivalve systematics, biogeography,
and paleontology; S. K. Donovan and
F. P. Wesselingh for access to Leiden Museum collections;
P. S. Anderson, G. Hunt, A. J. McGowan, R. M. Price,
T. A. Rothfus, R. J. Rundell, and H. J. Sims for assistance
in data acquisition and entry; participants in the Gradients
in Biodiversity and Speciation Working Group at the
National Center for Ecological Analysis and Synthesis for
insightful discussions; and S. M. Kidwell and three
anonymous reviewers for valuable comments. Supported by
a grant from NASA.
Supporting Online Material
www.sciencemag.org/cgi/content/full/314/5796/102/DC1
Materials and Methods
Figs. S1 to S3
Tables S1 and S2
References
5 June 2006; accepted 9 August 2006
10.1126/science.1130880
REPORTS
Molecular Loops in the
Galactic Center: Evidence for
Magnetic Flotation
Yasuo Fukui,1
* Hiroaki Yamamoto,1
Motosuji Fujishita,1
Natsuko Kudo,1
Kazufumi Torii,1
Satoshi Nozawa,2
Kunio Takahashi,3,4
Ryoji Matsumoto,5
Mami Machida,3
Akiko Kawamura,1
Yoshinori Yonekura,6
Norikazu Mizuno,1
Toshikazu Onishi,1
Akira Mizuno7
The central few hundred parsecs of the Milky Way host a massive black hole and exhibit very violent
gas motion and high temperatures in molecular gas. The origin of these properties has been a
mystery for the past four decades. Wide-field imaging of the 12CO (rotational quantum number
J 0 1 to 0) 2.6-millimeter spectrum has revealed huge loops of dense molecular gas with strong
velocity dispersions in the galactic center. We present a magnetic flotation model to explain that
the formation of the loops is due to magnetic buoyancy caused by the Parker instability. The model
has the potential to offer a coherent explanation for the origin of the violent motion and extensive
heating of the molecular gas in the galactic center.
T
he magnetic field in the central hundred
parsecs of the Milky Way is substantially
stronger than elsewhere in
the Galaxy, at least in the prominent nonthermal
features emitted from high-energy electrons
spiraling along magnetic field lines. The
magnetic field of these electrons is estimated
to be typically a milligauss (1, 2), although some
recent works suggest a weaker global magnetic
field in the galactic center (3). Magnetic fields
have the potential to affect the dynamics of
molecular gas and may control star formation
on a small scale and govern the motion of
molecular clouds on a large scale. An observational
link between the molecular gas and the
magnetic field in the galactic center has been
obtained through polarization measurements of
magnetically aligned dust grains at mid- to farinfrared
to submillimeter wavelengths (4, 5).
Here, we report millimeter-wave observations
of two molecular features that have a looplike
shape with a length of several hundred
parsecs and width of 30 pc within 1 kpc
1
Department of Astrophysics, Nagoya University, Chikusa-ku,
Nagoya 464-8602, Japan. 2
Department of Science, Ibaraki
University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan.
3
National Astronomical Observatory of Japan, 2-21-1 Osawa,
Mitaka, Tokyo 181-8588, Japan. 4
The Graduate University for
Advanced Studies, Hayama, Miura, Kanagawa, 240-0193,
Japan. 5
Department of Physics, Faculty of Science, Chiba
University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.
6
Department of Physical Science, Osaka Prefecture University,
1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan. 7
SolarTerrestrial
Environment Laboratory, Nagoya University, Chikusaku,
Nagoya, 464-8601, Japan.
*To whom correspondence should be addressed. E-mail:
fukui@a.phys.nagoya-u.ac.jp
6 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org106
onMay23,2007www.sciencemag.orgDownloadedfrom