Journal of Biogeography (J. Biogeogr.) (2008)
ORIGINAL
ARTICLE
Comparative phylogeography of
pitvipers suggests a consensus of ancient
Middle American highland biogeography
Todd A. Castoe1 à, Juan M. Daza1 , Eric N. Smith2, Mahmood M. Sasa3,
Ulrich Kuch4, Jonathan A. Campbell2, Paul T. Chippindale2 and
Christopher L. Parkinson1*
1
Department of Biology, University of Central
Florida, Orlando, FL, USA, 2Department of
Biology, University of Texas at Arlington,
Arlington, TX, USA, 3Instituto Clodomiro
Picado, Universidad de Costa Rica, San José,
Costa Rica and 4Biodiversity and Climate
Research Centre, Frankfurt am Main,
Germany
ABSTRACT
Aim We used inferences of phylogenetic relationships and divergence times for
three lineages of highland pitvipers to identify broad-scale historical events that
have shaped the evolutionary history of Middle American highland taxa, and to
test previous hypotheses of Neotropical speciation.
Location Middle America (Central America and Mexico).
Methods We used 2306 base pairs of mitochondrial gene sequences from 178
individuals to estimate the phylogeny and divergence times of New World
pitviper lineages, focusing on three genera (Atropoides, Bothriechis and
Cerrophidion) that are broadly co-distributed across Middle American highlands.
Results We found strong correspondence across three highland lineages for
temporally and geographically coincident divergences in the Miocene and
Pliocene, and further identified widespread within-species divergences across
multiple lineages that occurred in the early–middle Pleistocene.
*Correspondence: Christopher Parkinson,
Department of Biology, University of Central
Florida, 4000 Central Florida Blvd, Orlando,
FL 32816, USA.
E-mail: cparkins@mail.ucf.edu
These authors contributed equally.
àPresent address: Department of Biochemistry
and Molecular Genetics, University of Colorado
School of Medicine, Aurora, CO 80045, USA.
Main conclusions Available data suggest that there were at least three major
historical events in Middle America that had broad impacts on species divergence
and lineage diversification among highland taxa. In addition, we find widespread
within-species genetic structure that may be attributable to the climatic changes
that affected gene flow among highland taxa during the middle–late Pleistocene.
Keywords
Atropoides, Bothriechis, Cerrophidion, Mexico, Miocene, montane forests,
Neotropics, Pleistocene, speciation, Viperidae.
Phylogenetic inferences coupled with robust estimates of
divergence times can provide tremendous insight into the
patterns and underlying causes of the historical diversification
of lineages. Despite the power of such inferences, however, it
is difficult to determine to what extent any single biogeographical example may be broadly representative of the
patterns exhibited by diverse biotic components of a region
or ecosystem. By comparing and contrasting phylogeographical scenarios from co-distributed lineages, comparative phylogeography (Bermingham & Martin, 1998; Bermingham &
Moritz, 1998; Avise, 2000; Sullivan et al., 2000; Lapointe &
Rissler, 2005; Hickerson et al., 2006) provides further understanding by identifying biogeographical patterns and the extent
to which these apply to various taxa. If multiple lineages
appear to be subject to spatially and temporally congruent
patterns of divergence, a more powerful inference of the major
events that have had a broad impact on multiple lineages of
co-distributed species can be made (Rosen, 1978; Nelson &
Platnick, 1981). Deductions from comparative phylogeographical analyses are particularly important and enlightening for
areas with either vague geological or tectonic information, or
where little historical consensus is available (Arbogast &
Kenagy, 2001; Riddle & Hafner, 2006).
Middle America, the zone extending from central Mexico
through Panama (Fig. 1), is extremely biodiverse, and a large
component of this diversity is endemic (Savage, 1982; Campbell, 1999). Although this region spans c. 16 latitude, the
landmass is fairly small (c. 2.5 million km2), rendering its high
ª 2008 The Authors
Journal compilation ª 2008 Blackwell Publishing Ltd
www.blackwellpublishing.com/jbi
doi:10.1111/j.1365-2699.2008.01991.x
INTRODUCTION
1
T. A. Castoe et al.
150 km
Figure 1 Map of Middle America showing the main highland
regions and putative biogeographical barriers for highland taxa
(based on NASA Shuttle Radar Topography Mission).
endemicity most impressive (Campbell, 1999). The exaggerated topography, the interdigitation of diverse habitats, and
the dynamic tectonic and climatic history of the region have
contributed synergistically to its high endemicity and diversity
(Whitmore & Prance, 1987; Jackson et al., 1996; Campbell,
1999). Middle America has experienced a complex tectonic
and geological history, and lies at the active junction of four
major tectonic plates and several tectonic blocks (IturraldeVinent, 2006; Marshall, 2007). Deciphering the events that
have historically shaped present-day biological diversity is
complicated due to the continual physiographical reshaping of
the region since the Cretaceous. Despite substantial progress
over the past several decades, the details of much of the
tectonic history of Middle America remain fragmentary and
controversial (Coney, 1982; Iturralde-Vinent, 2006; Mann
et al., 2007).
A majority of biogeographical studies concerning Middle
America have been focused on understanding this region’s role
in biotic dispersal between North and South America, in many
cases neglecting endemic patterns of Middle American biodiversity. Accordingly, most studies have dealt with biogeographical patterns in the late Pliocene–Pleistocene relating to
the establishment of the final land connection with South
America (Stehli & Webb, 1985; Hafner, 1991; Webb, 1997),
and relatively few have investigated earlier patterns in the
Miocene and early Pliocene using contemporary phylogenetic
data and analyses (Bermingham & Martin, 1998; Parra-Olea
et al., 2004; Pennington & Dick, 2004; Crawford & Smith,
2005; Ribas et al., 2005; Barker, 2007; Crawford et al., 2007;
Heinicke et al., 2007).
Several early broad-scale studies on the biogeographical
history of Middle American fauna have shaped current
perceptions of the historical patterns and processes that had
an impact on the regional fauna (Dunn, 1931; Duellman, 1966;
Savage, 1966, 1982; Stuart, 1966). In particular, Savage (2002)
proposed a model for highland speciation in Middle America
in which highland species diversity was primarily the result of
climatic cycles beginning in the late Pliocene and extending
2
through the Pleistocene. Savage proposed that subsequent to
the dispersal of Nearctic lineages to Middle America in the
Miocene–Pliocene, speciation in the highlands occurred as a
combination of mountain uplift and fluctuations in climate
during Pleistocene glacial periods (see also Savage, 2002: 830).
However, these studies lacked: (i) recent geological and
tectonic insights into the region’s history, (ii) robust and
detailed phylogenetic estimates, and (iii) explicit estimates of
divergence times independent of the assumptions of a strict
molecular clock.
Pitvipers represent an ideal model system for investigating
historical patterns of Neotropical diversification. This large
group of venomous snakes has a relatively well known
phylogeny (Parkinson et al., 2002; Malhotra & Thorpe, 2004;
Castoe & Parkinson, 2006) and an extensive fossil record in the
USA (reviewed by Holman, 2000), and appears to have
dispersed into the New World as a single lineage from Asia
during the Miocene (Kraus et al., 1996; Parkinson, 1999;
Parkinson et al., 2002; see also Holman, 2000). Pitvipers are
also good models for comparative phylogeography because
several distinct and diverse lineages are broadly co-distributed,
and extrinsic temporal constraints for divergence time estimates are available. Furthermore, because relaxed clock
inferences of the relative divergence times within a single tree
are particularly robust to the assumptions of calibration points
(Thorne & Kishino, 2005), pitvipers are ideal for testing
hypotheses of coincident divergence among multiple lineages.
Several studies have examined biogeographical hypotheses
for Neotropical pitviper lineages (Crother et al., 1992; Zamudio & Greene, 1997; Parkinson et al., 2000; Wüster et al., 2002;
Gutberlet & Harvey, 2004; Werman, 2005), but have resulted
in little explicit consensus. Most of these studies provided brief
comments on biogeography (Kraus et al., 1996; Parkinson,
1999; Parkinson et al., 2002) or employed limited phylogenetic
or phylogeographical data with no explicit temporal component (Crother et al., 1992; Castoe et al., 2003; Werman, 2005),
or with temporal estimates derived from a strict molecular
clock (Zamudio & Greene, 1997; Wüster et al., 2002). In this
study, we compare historical biogeographical patterns simultaneously across three lineages of Neotropical pitvipers that are
broadly co-distributed across the highlands of Middle America. These include members of the genera Cerrophidion (the
montane pitvipers), Atropoides (the jumping pitvipers) and
Bothriechis (the palm pitvipers).
To test the highland speciation model proposed by Savage
(2002) and previous hypotheses of Middle American biogeography/phylogeography, we used a large molecular phylogenetic data set for pitvipers that includes a dense (including
intraspecific) sampling of members of the three genera of
interest. We added new DNA sequences from members of the
genera Atropoides and Cerrophidion to the data available for
Neotropical pitvipers. We also estimated lineage divergence
times based on multiple flexible approaches to provide a
robust and probabilistic temporal component, avoiding
assumptions of a strict molecular clock. We synthesize these
inferences to address four questions: (1) Is the Savage
Journal of Biogeography
ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
Comparative phylogeography of Middle American pitvipers
speciation model supported by highland pitviper phylogeography? This model predicts that Middle American highland
species diverged from one another primarily during the late
Pliocene and Pleistocene, when dramatic fluctuations in
temperature may have affected highland habitat connectivity.
(2) Is there evidence that temporal and geographical patterns
of divergence are shared among multiple co-distributed
highland lineages, and is there evidence of underlying geological or climatic causes? (3) Is there phylogeographical signal
apparent from highland pitvipers that can be used to formulate
an explicit model of Middle American highland speciation?
(4) What effects did glacial cycles (in the late Pliocene–
Pleistocene) have on lineage diversification in highland
pitvipers of Middle America?
METHODS
Taxon sampling and laboratory methods
Because our goals included inferences of biogeographical
patterns ranging from ancient (Miocene) to recent in multiple
pitviper lineages, we incorporated a large mitochondrial DNA
sequence data set (including 178 terminals) designed to
provide accurate phylogenetic and divergence time estimates
across this range of time. We combined mitochondrial DNA
sequences from several studies (Parkinson, 1999; Malhotra &
Thorpe, 2000, 2004; Parkinson et al., 2002; Castoe et al., 2003,
2005; Castoe & Parkinson, 2006) to include representatives of
Old World pitvipers, and extensive sampling of all major New
World lineages. The data set included sequences of four
mitochondrial gene fragments: portions of the 12S and 16S
rRNA genes and the protein coding genes NADH dehydrogenase subunit four (ND4) and cytochrome b (cyt b), for a total
of 2306 aligned nucleotide positions. This included sequences
for all four genes for a vast majority of species, and essentially
all major lineages, although some intraspecific samples
included only sequences of the two protein-coding genes
ND4 and cyt b (1386 bp; for details see Appendix S1 in
Supplementary Material).
We included all inter- and intraspecific sampling available
from previous studies for the three genera of interest:
Atropoides, Bothriechis and Cerrophidion. All taxonomic references in this study follow Campbell & Lamar (2004). We also
added new sequences for 19 samples of Atropoides and
Cerrophidion (Appendix S1). Laboratory methods for generating new sequences followed Parkinson et al. (2002), Castoe
et al. (2005) and Castoe & Parkinson (2006), as did sequencealignment methods.
Phylogenetic analysis
Aside from the relatively small number of new intraspecific
sampling added in this study, the data used here essentially
represent the combination of data sets from Castoe et al.
(2005) and Castoe & Parkinson (2006), with the exclusion of
some fine-scale sampling of Old World pitvipers. To infer
phylogeny in this study, we applied the partitioning scheme
and partition-specific models identified by Castoe & Parkinson
(2006). The Bayesian Markov chain Monte Carlo (BMCMC)
estimate of the phylogeny was inferred using MrBayes ver. 3.1
(Huelsenbeck & Ronquist, 2001; Ronquist & Huelsenbeck,
2003) with default priors. As per the defaults, two parallel
BMCMC runs were executed simultaneously and each was run
for 5 · 106 generations. Parameters among partitions were
unlinked, as was the rate of evolution (using the ratepr = variable command). Based on diagnostics described by Castoe &
Parkinson (2006), both runs appeared stationary prior to 106
generations, and we conservatively excluded the first 1.5 · 106
generations of each run as burn-in. All post-burn-in estimates
(sampled every 1000 generations) were combined, and
phylogeny and parameter estimates were summarized from
this combined posterior distribution. We also tested the
alternative phylogenetic placement of Bothriechis lateralis as
the sister lineage to Bothriechis bicolor (Crother et al., 1992)
using the Shimodaira–Hasegawa (SH) test (Shimodaira &
Hasegawa, 1999) and the approximately unbiased (AU) test
(Shimodaira, 2002) implemented in the program consel
(Shimodaira & Hasegawa, 2001).
Divergence time estimation
We used two relaxed clock methods to estimate divergence
times across the pitviper phylogeny, the penalized likelihood
(PL) method implemented in r8s (Sanderson, 1997, 2002,
2003) and the Bayesian relaxed clock method implemented in
the program multidistribute (Thorne et al., 1998; Thorne &
Kishino, 2002). For the PL estimate, we estimated divergence
times using r8s and then obtained confidence intervals on
these dates using bootstrapped versions of the data set. To
estimate PL confidence intervals, 1000 bootstrap replicates
were generated using the program bootseq (Felsenstein,
2005). Branch lengths for each replicate data set were estimated
using the GTR+G+I model in paup ver. 4.10b (Swofford,
2001). Trees (and branch lengths) from the bootstrapped data
sets were run in r8s and confidence intervals were summarized
from this distribution using the Perl scripts provided at http://
www.bergianska.se/index_kontaktaoss_torsten.html.
For the Bayesian inference of divergence times in multidistribute, we partitioned the molecular data by gene (four
partitions) for all analyses. Using the program baseml (paml
package; Yang, 1997), model parameters were estimated using
the model F84+G for each partition. From this, branch lengths
and the variance–covariance matrix were calculated using the
program estbranches. Estimates of evolutionary rates and
divergence times were then estimated using the program
multidivtime. The priors used for analyses in multidivtime
included: rttm = 1.6, rttmsd = 0.2, rtrate = 0.3, rtratesd = 0.3,
brownmean = 0.5, brownsd = 0.5 and bigtime = 3.0. The
remaining priors used in multidivtime analyses were set to the
program’s default. Because the performance of divergence timeestimation approaches used here rely heavily on accurate
branch-length estimation, divergence estimates are extremely
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3
T. A. Castoe et al.
sensitive to short internodes that may have estimation variance
that includes negative values of length or time. To avoid this
potential problem, only subsets of the entire phylogenetic data
set were used for r8s and multidistribute analyses. For
both analyses, the topology was pruned to include
only phylogenetically distinct lineages, thereby excluding
lineages or samples that were associated with extremely small
(near zero) branch lengths (as per the suggestions of both
programs).
Calibration points
Because branch lengths represent the product of evolutionary
rate and time, calibration points are necessary to separate these
two underlying parameters and obtain an estimate of divergence times (Thorne & Kishino, 2005). We used four
calibration points as minimum constraints to obtain date
estimates for the pitviper phylogeny. In both the PL and
Bayesian divergence analyses, we constrained the minimum
ages of two temperate North American lineages based on fossil
data: the origin of Sistrurus at 9 Ma (Parmley & Holman,
2007) and the origin of Agkistrodon piscivorus at 4.7 Ma
(Holman, 2000). Because the PL method requires the age of
one node to be fixed, for PL we fixed the age of the divergence
between the two North American rattlesnake species Crotalus
ruber and Crotalus atrox at 3.2 Ma (Castoe et al., 2007). The
divergence between these two species is thought to have
occurred due to a well dated Pliocene marine incursion of the
(a)
Sea of Cortez, and is generally well corroborated across other
taxa (for discussion see Castoe et al., 2007). In the PL analyses,
we also constrained the split between New World and Old
World pitvipers as a minimum age at 16 Ma based on two
sources of evidence: the oldest fossil of a viper found in the
New World (Holman, 1977, 2000), and the end of the thermal
optimum in the Miocene (Böhme, 2003; see also Burbrink &
Lawson, 2007). For the Bayesian estimates of dates, the split
between Old and New World pitvipers was used as the
prior rttm; based on the evidence mentioned above, the rttm
prior was set to 16 Myr and the standard deviation for that
prior (rttmsd) to ± 4 Myr. The C. atrox/ruber split was also
added as a constraint to the Bayesian estimation, set as 2.9–
3.5 Myr.
RESULTS
Phylogenetic estimate
Our estimate of pitviper phylogeny is extremely similar to
those of recent studies (Wüster et al., 2002; Castoe et al., 2005;
Castoe & Parkinson, 2006), which was expected because the
majority of the data and analytical approaches are the same. To
maintain focus on groups of interest, we show summarized
relationships among New World genera (Fig. 2) as well as
detailed results only for genera of interest (Figs 2–4). We
found strong support for the monophyly of all New World
pitvipers (posterior probability, PP = 100), as well as a clade
(b)
Figure 2 (a) Summary of Bayesian phylogenetic estimates of relationships among New World pitviper genera and relationships among
species of the genus Bothriechis. All nodes shown received Bayesian posterior probabilities of 100% unless annotated on the tree.
(b) Geographical distribution of Middle American highland species of the genus Bothriechis based on Campbell & Lamar (2004).
4
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Comparative phylogeography of Middle American pitvipers
(a)
(b)
Figure 3 (a) Bayesian phylogenetic estimate of relationships among members of the genus Atropoides. All nodes shown received Bayesian
posterior probabilities of 100% unless annotated on the tree. Roman numerals to the right of taxon names indicate individuals used for
divergence dating, and correspond to those in Fig. 5. (b) Geographical distribution of Atropoides species. Shaded areas represent the known
distribution for each species based on Campbell & Lamar (2004); dots correspond to the geographical origin of samples used for the
molecular analyses.
representing the temperate genera Crotalus, Sistrurus and
Agkistrodon (PP = 96; Fig. 2). Intergeneric relationships
among Neotropical lineages match those determined by Castoe
& Parkinson (2006), and include a large South American
group (Bothrops, Bothriopsis and Bothrocophias) strongly
supported as the sister clade of the Middle American
Porthidium group (Atropoides, Cerrophidion and Porthidium).
As in previous studies, Bothriechis was inferred to be the sister
group to this Middle and South American assemblage (Fig. 2).
Monophyly of each genus of interest was inferred, with
strong support for Cerrophidion and Bothriechis (PP = 100),
and weaker support for Atropoides (PP = 57; Fig. 3). Relationships among all nominal species and major lineages within
each of these genera were well resolved, with strong support in
most cases. The new sequences of Atropoides and Cerrophidion
added in this study illuminate substantial genetic structure
within species. In Atropoides (Fig. 3), all species except A.
picadoi and Atropoides sp. appear to contain substantial genetic
diversity below the species level. We found substantial genetic
structure within Cerrophidion godmani, consisting of at least
four distinct and divergent clades (C1–C4; Fig. 4) that
correspond to four main geographical components of the
range of this species (Fig. 4b). Like Castoe & Parkinson (2006),
we found strong support (Fig. 2a) for B. lateralis forming the
sister group to the northern Central American highland
Bothriechis species, counter to the estimate that B. lateralis is
the sister lineage to B. bicolor (Crother et al., 1992; see also
Taggart et al., 2001). The sister-lineage relationship between B.
lateralis and B. bicolor was also rejected by SH tests (P < 0.001)
and AU tests (P < 0.001). These results provide strong
evidence in support of the topology with B. lateralis as the
sister group to all northern Central American highland
Bothriechis species (as in Fig. 2a).
Divergence times
Estimates of divergence times were generally similar between
the two divergence dating methods used (Table 1). The most
notable contrast between the two sets of estimates was a
substantial difference in confidence intervals, with the PL
intervals being narrower and symmetrically distributed around
the mean, whereas the Bayesian estimates had broader
confidence intervals that were asymmetrical and skewed
towards more ancient divergence times. This contrast between
Bayesian and PL estimates has been noted elsewhere, and some
have suggested that the current method of obtaining bootstrap-based intervals in PL can produce confidence interval
distributions that are improperly uniform and overly narrow
(Burbrink & Pyron, 2008). Thus, the credible intervals of
Bayesian estimates are thought to be more accurate in their
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T. A. Castoe et al.
(a)
(b)
Figure 4 (a) Bayesian phylogenetic estimate of relationships among members of the genus Cerrophidion included in this study. All
nodes shown received Bayesian posterior probabilities of 100% unless annotated on the tree. Roman numerals to the right of taxon
names indicate individuals used for divergence dating, and correspond to those in Fig. 5. (b) Geographical distribution of Cerrophidion
species in Middle America. Shaded areas represent the distribution for each species based on Campbell & Lamar (2004); dots correspond to
the geographical origin of samples used for the molecular analyses. Major phylogeographical lineages within Cerrophidion godmani are
labelled C1–C4 (a,b), and are indicated by polygons on the map (b).
BI
PL
Node
Mean
Lower
Upper
Mean
Lower
Upper
Origin of New World pitvipers
Origin of Bothropoid group
Origin of Atropoides
Origin of Bothriechis
Origin of Cerrophidion
(1) Nicaragua
Atropoides
Bothriechis
Cerrophidion
(2) Motagua–Polochic
Atropoides
Bothriechis
Cerrophidion
(3) Tehuantepec
Atropoides
Bothriechis
Cerrophidion
16.08
12.82
9.95
14.1
9.43
14.33
10.67
8.13
11.99
7.66
17.99
15.15
12.02
16.29
11.47
17.35
14.15
10.76
15.24
10.41
16.15
13.13
9.98
14.25
9.65
18.55
15.17
11.54
16.23
11.17
8.56
7.67
4.39
6.77
5.73
3.06
10.61
9.87
6.03
9.28
8.04
4.03
8.47
7.35
3.54
10.09
8.74
4.53
4.82
4.56
5.73
3.55
3.3
4.31
6.35
6.03
7.37
4.69
4.5
5.51
4.25
4.08
4.97
5.13
4.92
6.04
3.05
3.49
3.31
2.18
2.44
2.16
4.15
4.72
4.67
3.29
3.05
2.94
2.96
2.68
2.54
3.63
3.42
3.35
Table 1 Estimates of divergence times (Ma)
for major events in New World pitviper lineages.
Mean estimates of divergence times based on Bayesian inference (BI) and penalized likelihood
(PL) are given with corresponding upper and lower bounds of the 95% credibility (BI) or
confidence intervals (PL) for each estimate.
breadth and skew in contrast to PL bootstrap-based intervals.
Because of this potential bias in the PL estimates, we report
results based primarily on the Bayesian estimates and 95%
6
credible intervals, and comment on the PL estimates where
relevant. Direct comparisons of the results of both methods are
given in Table 1.
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Comparative phylogeography of Middle American pitvipers
Figure 5 Bayesian estimates of divergence times (Ma) for the pitviper phylogeny. The mean estimate is represented by the node; bars
represent 95% credibility intervals for divergence estimates; open circles represent calibration points described in the text. Numbers on
nodes (1–3) correspond to the biogeographical breaks for highland taxa: (1) Nicaragua Depression; (2) Motagua–Polochic Fault valleys;
(3) Isthmus of Tehuantepec. Roman numerals are used to cross-reference (with Figs 3 & 4) individuals per species used in divergence
estimation.
All intergeneric divergences within the New World pitvipers are estimated to have occurred during the Miocene, and
the New World lineage is estimated to have diverged from
Old World pitvipers in the early Miocene, between 14 and
18 Ma (Table 1; Fig. 5). The majority of cladogenetic events
that gave rise to the current genera and most of the species
occurred in the middle–late Miocene and early Pliocene. The
three genera we focus on here are inferred to have arisen
from the middle to late Miocene (Table 1; Fig. 5). All
nominal species of highland pitvipers appear to have diverged
prior to the late Pliocene, predominantly from late Miocene
to middle Pliocene (Fig. 5). Major divergences within highland pitviper species occurred over a broad period of time
(early Pliocene–Pleistocene; Fig. 5). Phylogroups within the
wide-ranging species C. godmani began to diverge in the late
Miocene (c. 5.7 Ma) and continued to do so through the
Pliocene and Pleistocene, before the divergence of many other
lineages of Neotropical pitviper species from their sister
groups (Fig. 5). Intraspecific phylogroups within Atropoides
diverged at the end of the Pliocene and the Pleistocene (2.1–
0.9 Ma; Fig. 5).
Three major phylogeographical divergence events that have
occurred in each of the three genera of interest show different
levels of temporal correspondence; these are labelled as 1–3 in
Fig. 6. For the first phylogeographical break at the Nicaraguan
Depression (labelled split 1; Fig. 6), Bothriechis and Atropoides
lineages show strong overlapping temporal divergence
(Table 1; Fig. 6) in the middle–late Miocene, whereas the
corresponding geographical split in Cerrophidion is substantially later, in the early–middle Pliocene (Figs 5 & 6; Table 1).
The posterior probability distributions of divergence times in
the first two genera broadly overlap, but show almost no
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7
T. A. Castoe et al.
Figure 6 Bayesian posterior densities for
divergence time estimates of the three highland genera across three major biogeographical breaks. Letters over the distributions
indicate the three genera (A. = Atropoides,
B. = Bothriechis, C. = Cerrophidion). Shaded
regions indicate period of glacial cycles in the
late Pliocene through the Pleistocene. Figures
on the right show the biogeographical break
and the potential approximate palaeogeographical reconstruction at that time; darker
shading indicates major highland masses in
palaeogeographical reconstructions. Palaeogeographical reconstruction in Lower Central
America based on Kirby & MacFadden (2005;
see discussion for other reconstructions).
overlap with that of Cerrophidion (Fig. 6), suggesting that
Atropoides and Bothriechis had undergone an essentially
coordinated divergence that was not shared with Cerrophidion.
For the second major divergence event, across the Motagua–
Polochic Fault, there is strong evidence for the shared
divergence between Atropoides and Bothriechis, also with
moderate evidence for this divergence being shared by
Cerrophidion (Table 1; Figs 5 & 6). Posterior probability
distributions of divergence times for all three genera do largely
overlap across the period of c. 4–5.5 Ma, providing evidence
that they experienced a mostly simultaneous divergence at the
Motagua–Polochic Fault in the late Miocene–early Pliocene
(Fig. 6).
The third major phylogeographical break, across the Isthmus of Tehuantepec, provides particularly strong evidence of a
shared simultaneous divergence across the three genera in the
middle Pliocene (Table 1; Figs 5 & 6). The posterior probability distributions of divergence time estimates are nearly
identical between Atropoides and Cerrophidion, which show a
divergence at the geographically defined Isthmus of Tehuantepec. Although Bothriechis does not occur north of the
geographical Isthmus, the divergence of Bothriechis rowleyi
(from Bothriechis aurifer) directly adjacent to the Isthmus
shows nearly perfect temporal correspondence with the breaks
in the other two genera (Fig. 6). Below we elaborate on
geological evidence suggesting that the break observed in
8
Bothriechis adjacent to the Isthmus of Tehuantepec may be
geologically tied to the events leading to divergence in the
other two genera in this region.
DISCUSSION
A consensus of ancient Middle American
highland speciation
Glacial climatic cycles during the late Pliocene–Pleistocene,
subsequent to the establishment of the late Pliocene land
connections between Middle and South America, have been
viewed as the predominant processes that have generated
substantial Middle American biodiversity, particularly for
highland taxa (Savage, 2002, and references therein). In
general, this also has been the dominant hypothesis for
explaining highland pitviper speciation – both Crother et al.
(1992) and Castoe et al. (2003) focused on the period from the
middle Pliocene and later, and on climatic fluctuations, as
having driven speciation in Bothriechis and Atropoides, respectively. Despite consensus in the identification of major
biogeographical boundaries that have shaped the region’s
biodiversity (Savage, 1982; Morrone, 2001), there has been
little quantitative insight as to when these barriers may have
led to diversification, and in what temporal order. This study
contributes three new important findings that reject previous
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Comparative phylogeography of Middle American pitvipers
hypotheses and clarify historical biogeographical patterns in
Middle American highland taxa.
First, estimates of pitviper phylogeny and divergence times
for Middle American highland lineages reject Savage’s model
of highland speciation in which late Pliocene and Pleistocene
climatic changes are major determinants of current species
diversity (Savage, 2002). Instead, our results suggest that
Miocene–Pliocene tectonic activity played a dominant role in
generating regional highland species biodiversity. This conclusion contrasts with the majority of previous suggestions by
taxon-specific studies on pitvipers (Crother et al., 1992; Castoe
et al., 2003; Werman, 2005); plants alone (Burnham &
Graham, 1999); and plants, insects and fish (Marshall &
Liebherr, 2000). This and other recent studies highlight the
significance of pre-Pliocene diversification in Middle America
(Smith et al., 2007; Wiens et al., 2007), together with ancient
faunal interchange between Middle and South America
(Bermingham & Martin, 1998; Barraclough & Vogler, 2002;
Wüster et al., 2002; Parra-Olea et al., 2004; Pennington &
Dick, 2004; Steppan et al., 2004; Crawford & Smith, 2005;
Concheiro-Pérez et al., 2007; Koepfli et al., 2007; Wahlberg &
Freitas, 2007).
Second, there is evidence for a congruent temporal pattern
of divergence across three different lineages of Middle
American highland pitvipers, corresponding to major geographical breaks among Middle American highland masses.
This, to our knowledge, is the first evidence of a clear pattern
of temporal and spatial congruence in divergence patterns
across multiple highland lineages of any taxon in Middle
America. This example therefore provides one of the first
explicit predictive models for speciation in this heavily studied
epicentre of biodiversity. These biogeographical break points
are obvious contemporary barriers for highland species and
have been the focus of previous biogeographical attention
(Savage, 1982, 1987; Campbell, 1999; Duellman, 1999; Sullivan
et al., 2000; Morrone, 2001), but no clear evidence or
consensus for when and how these regions have broadly
shaped biodiversity has previously emerged. It is also significant to bear in mind an important strength of our analyses –
regardless of the exact estimates of absolute divergence times,
our inference of relative temporal congruence among lineage
divergences is particularly robust because all estimates are
derived from a single, large, dated tree (Thorne & Kishino,
2005). Thus, the evidence in this study regarding the relative
correspondence of divergence times across multiple lineages of
pitvipers is robust and fairly independent of the accuracy of the
absolute estimates of divergence times.
Third, we do find evidence that climatic changes associated
with the onset of glacial cycles in the late Pliocene–Pleistocene
may have led to lineage diversification in Middle American
highland pitvipers, but only among populations within species.
This evidence is consistent with glacial climatic cycles
contributing to the fragmentation of once contiguous highland
habitats, leading to the subsequent divergence among populations of Atropoides and Cerrophidion. These inferences
provide new insight into corridors of highland habitat that
at one time facilitated gene flow that may have been
fragmented due to climatic changes in the late Pliocene and
Pleistocene.
Below, we first discuss evidence from this study for three
shared ancient (Miocene–Pliocene) divergences across Middle
American highland pitviper lineages, and the underlying
tectonic and biogeographical hypotheses surrounding these
divergences. Next we focus on the intraspecific sampling of
Atropoides and Cerrophidion and the evidence for late
Pliocene–Pleistocene effects on lineage diversification, and we
examine previous biogeographical hypotheses for Bothriechis
species.
Shared divergence (1): the Nicaraguan Depression
The lowland area known as the Nicaraguan Depression is the
geological result of a back-arc formation that has continued to
evolve for the past 10 Myr (Rogers et al., 2002; Marshall,
2007). This region separates two highland masses, the Chortis
block highlands (Honduras and Nicaragua) to the north, and
the Lower Central American highlands of Costa Rica and
Panama. Evidence suggests that a marine gap existed between
the Chortis and Lower Central American highlands during the
Miocene and the majority of the Pliocene (Coates & Obando,
1996; Iturralde-Vinent & MacPhee, 1999; Iturralde-Vinent,
2006). Alternatively, Kirby & MacFadden (2005) have suggested that a narrow landmass connected modern-day Honduras and Costa Rica during this time. The Nicaraguan
Depression has been identified as a major phylogeographical
break for many taxa, including frogs (Savage, 1987; Duellman,
1999), salamanders (Parra-Olea et al., 2004), snakes (Savage,
1982; Cadle, 1985), birds (Pérez-Emán, 2005), and plants,
insects and fish (Marshall & Liebherr, 2000; Halas et al., 2005).
Middle American highland pitvipers also provide strong
support for this region representing a major historical barrier
to gene flow. We found evidence for temporal congruence of
highland pitviper divergence across this break in two of the
three pitviper lineages. Bothriechis and Atropoides show broadly
overlapping divergence estimates across this break in the
middle–late Miocene, c. 7.7–8.6 Ma [Bayesian confidence
intervals (BCIs) = 5.7–9.9 and 6.8–10.6 respectively, Fig. 6].
Although estimates of these two genera appear to indicate a
fairly coincident divergence at the Nicaraguan Depression, the
third genus, Cerrophidion, appears to have diverged across this
region much later in the early–middle Pliocene, c. 4.4 Ma
(BCI = 3.1–6.0 Ma, Fig. 6). The posterior probability distribution of Cerrophidion divergence times shows very little
overlap with that of the other two genera (Fig. 6) and strongly
suggests a unique biogeographical scenario for Cerrophidion
divergence across this barrier.
The apparent lack of temporal correspondence of divergences between Cerrophidion and the other two genera may
indicate that Cerrophidion has different dispersal capabilities,
or that members of this genus may not have been distributed
across the Depression in the middle Miocene. Of the three
genera, Cerrophidion tends to inhabit the highest elevations
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9
T. A. Castoe et al.
(up to c. 2700 m; Campbell & Lamar, 2004), and it has been
suggested that high-elevation habitats may not have existed in
lower Central America until the Pliocene (Coates & Obando,
1996). The estimate of more recent cladogenesis within
Cerrophidion that is not observed in either Atropoides or
Bothriechis is intriguing, and suggests that dispersal and
vicariance of highland lineages across the Nicaraguan Depression has occurred multiple times in the Miocene–Pliocene. It is
notable that these estimates of divergence times are collectively
consistent with the model of Kirby & MacFadden (2005),
corroborating their suggestion of a narrow landmass across the
Nicaraguan Depression during the Miocene and Pliocene.
Shared divergence (2): the Motagua–Polochic Faults
The Motagua–Polochic Fault represents the contact zone
between the Maya and Chortis tectonic blocks (Marshall,
2007). The eastward motion of the Chortis block that has
continued since the Cretaceous is responsible for the generation of a majority of the mountain-building across southwestern Mexico and Nuclear Central America (Rogers et al.,
2002). Numerous studies have suggested that this physiographical barrier caused phylogeographical breaks in different
taxa (Humphries, 1982; Perdices et al., 2002, 2005; Halas et al.,
2005; Devitt, 2006; Concheiro-Pérez et al., 2007). For lowlandinhabiting snakes, Devitt (2006) estimated a cladogenetic event
in this region at 7.7 Ma, and Perdices et al. (2005) found that
freshwater eel-like synbranchid fishes diverged c. 11.2 Ma. In
contrast, our estimates suggest divergence of highland lineages of pitvipers later in the Miocene and/or early Pliocene
(Figs 5 & 6).
Our divergence time estimates show a geographically
congruent, nearly simultaneous diversification scenario in the
late Miocene, centred c. 4.1–5.0 Ma, for the three highland
lineages of pitvipers (Figs 5 & 6). The correspondence between
divergence times for Atropoides and Bothriechis is excellent (4.3
and 4.1 Ma, respectively), and it appears that Cerrophidion
may have diverged slightly earlier (5.0 Ma; Figs 5 & 6). This
result is consistent with the expectation that, because Cerrophidion is restricted to higher elevation habitats, gene flow may
have been severed slightly earlier in this group compared with
the other two lineages. It is interesting that there is fairly strong
evidence for simultaneous divergence across highland lineages
at this fault zone that contrasts substantially with more ancient
divergence estimates for lowland groups (Perdices et al., 2005;
Devitt, 2006). The extensive mountain-building and physiographical reshaping of the region makes historical inferences
difficult, but these results may indicate that this region has
contributed to the divergence of lineages with different habitat
requirements in markedly different ways over an extended
period of time.
Shared divergence (3): the Isthmus of Tehuantepec
Geographically, the Isthmus of Tehuantepec is the narrow
lowland region that separates the highlands of southern
10
Mexico (Sierra Madre Oriental and Sierra Madre del Sur)
from the Chiapan-Guatemalan highlands of Nuclear Central
America. This region is well known as a major biogeographical
node where historical events have formed a transition between
the Nearctic and the Neotropical biogeographical zones
(Halffter, 1987; Marshall & Liebherr, 2000; Morrone &
Márquez, 2001). Biogeographical studies on specific taxa have
found the Isthmus of Tehuantepec to be a phylogeographical
barrier for highland species (Chippindale et al., 1998; Sullivan
et al., 2000; León-Paniagua et al., 2007). More recent studies
on lowland species have revealed similar phylogeographical
structure separating lineages on both sides of the Isthmus
(Hasbún et al., 2005; Devitt, 2006; Mulcahy et al., 2006).
Tectonically, the Isthmus represents a visible marker for the
three-way junction of tectonic plates that have remained
extremely active in shaping the regional landscape since the
Cretaceous. It is thought that a highland corridor spanning the
Isthmus in the Miocene was subsequently destroyed due to
extreme tectonic activity relating to the subduction of the
Cocos Plate (Barrier et al., 1998; Manea & Manea, 2006).
Tectonic markers distributed both on the Isthmus of Tehuantepec and on surrounding upland areas show massive downdropping of the Chiapan-Guatemalan region with respect to
the areas to the north and west during the late Miocene–early
Pliocene associated with faulting occurring across the short axis
of the Isthmus, resulting in a significant reduction in elevation
and subsequent marine inundations (Barrier et al., 1998).
Atropoides and Cerrophidion each show clear phylogeographical breaks centred around the geographical Isthmus of
Tehuantepec, and estimates of divergence times between these
two genera show remarkable temporal congruence over this
boundary. Our results suggest that these two genera experienced a simultaneous divergence across this zone in the
Pliocene, c. 3.1–3.5 Ma (Fig. 6), consistent with geological
evidence for a tremendous tectonic event in which highlands at
the Isthmus were reduced to a submarine embayment over a
short period in the Pliocene (Barrier et al., 1998).
Unlike the other two genera, Bothriechis does not occur west
of the Isthmus of Tehuantepec, although one species, B. rowleyi,
is endemic to north-west Chiapas adjacent to the Isthmus
(Fig. 2). Bothriechis rowleyi is distributed only in the mountain
region of northern Chiapas, a recent geological formation
called the Modern Chiapas Volcanic Front (Manea & Manea,
2006). Around 3 Ma, the continued slab subduction of the
Cocos plate generated extensive orogenic changes not only at
the Isthmus proper, but also in surrounding regions that led to
the uplift of the Modern Chiapas Volcanic Front (Manea &
Manea, 2006). It is therefore reasonable to infer that the final
formation of the Chiapas highlands during the Pliocene,
associated with tectonic activity at the triple plate junction at
the Isthmus, led to the vicariance between the ancestors of
B. rowleyi and its sister species B. aurifer (Fig. 6). The temporal
congruence between this divergence in Bothriechis and that of
the other two genera at almost exactly 3 Ma is impressive, and
suggests that these vicariant events were nearly simultaneous
and possibly driven by the same tectonic activity surrounding
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Comparative phylogeography of Middle American pitvipers
the Isthmus. Although strongly supported by geological data,
this is the first evidence of which we are aware that
demonstrates a potential temporal (and tectonic) link between
evolutionary vicariance events at the Isthmus of Tehuantepec
and those in the neighbouring Chiapan highlands. Future
research to increase the resolution of biogeographical analysis
in the Isthmus region may provide tests of this hypothesis,
while further illuminating the complex role of this biogeographical node in shaping historical gene flow between the
Nearctic and Neotropical regions.
Intraspecific phylogeography of Atropoides and
Cerrophidion
New fine-scale sampling of Atropoides and Cerrophidion
highlights substantial genetic structuring (Figs 3 & 4) which
was estimated to have occurred during the Pliocene and
Pleistocene. Within Atropoides species, Pleistocene divergences
are estimated (1) within the Sierra Madre Occidental in
eastern-central Mexico (Atropoides nummifer), (2) across the
Nicaraguan Depression (Atropoides mexicanus), and (3) across
the Isthmus of Tehuantepec (Atropoides olmec). Similarly to
Atropoides, there is evidence that some among-population gene
flow in C. godmani may have been affected by glacial climatic
cycles in the Pleistocene. The divergence of phylogroups C3
and C4 (Fig. 4), representing the separation of the Northeastern from the Southwestern Guatemalan highlands, appears to
have occurred at the temporal boundary between the Pliocene
and Pleistocene (Fig. 5). Further divergences across highlands
in eastern Honduras (within C1), and among interior Guatemalan highlands (within C4) may also have been associated
with Pleistocene climatic change (Fig. 5).
New sampling within Atropoides and Cerrophidion also
provides insight into previous biogeographical and taxonomic
hypotheses. Castoe et al. (2003) hypothesized that a recent
corridor for gene flow extended across the Isthmus of
Tehuantepec to explain the close relationship between populations of A. olmec in Veracruz, Mexico and Baja Verapaz,
Guatemala; new Atropoides samples from Chiapas, Mexico, that
are associated with A. olmec further support this. Our new
sampling of C. godmani demonstrates an extensive amount of
ancient genetic structure, which has generally been suggested
previously (Castoe et al., 2003, 2005). Estimates of divergence
times also suggest that the species C. godmani began to diversify
prior to some major clades of Atropoides (all except A. picadoi)
and Bothriechis (all northern highland species; Fig. 5). The
question of whether major phylogeographical clades of
C. godmani may warrant recognition as distinct species is
currently being evaluated as part of our ongoing studies of the
biodiversity and historical biogeography of this region.
Alternative hypotheses for biogeography
of Bothriechis
Modern biogeographical hypotheses rely on the synthesis of
multiple layers of inference, including distribution patterns of
lineages, phylogeny and divergence time estimates, rendering
biogeographical inferences sensitive to all these underlying
estimates. The phylogeny of Bothriechis has been controversial
(Crother et al., 1992; Taggart et al., 2001; Castoe & Parkinson,
2006) largely because a previous study (Taggart et al., 2001)
suggested that conflicting phylogenetic estimates from morphology plus allozymes vs. mitochondrial gene sequences
indicated that mitochondrial introgression and/or incomplete
lineage sorting may confound mitochondrial gene phylogenies
of the group. Based on allozyme and morphological data,
Crother et al. (1992) suggested that B. lateralis was phylogenetically nested within northern Middle American highland
lineages (sister to B. aurifer), rather than the sister group to all
northern highland species, as in our phylogenetic placement
(Fig. 2). Based on our mitochondrial DNA data set, the former
hypothesis was strongly rejected by SH and AU tests
(P << 0.001) in favour of the relationships recovered in our
tree (Fig. 2). Our mitochondrial phylogeny of Bothriechis and
that of the combined data of Taggart et al. (2001: Fig. 6b) are
essentially the same; both place B. lateralis as the sister lineage
to the northern highland species. The conclusion of Taggart
et al. (2001), however, was that the mitochondrial tree was
incorrect because it differed from a tree estimated from
morphological and allozyme characters (from Crother et al.,
1992). In unpublished analyses, we have examined multiple
nuclear genes and sampled mitochondrial haplotypes extensively within species of Bothriechis, and all available data have
failed to reveal evidence of incomplete lineage sorting,
hybridization or mitochondrial vs. nuclear gene tree incongruence (C.L. Parkinson, T.A. Castoe and J.M. Daza, unpublished data). These data suggest that the nuclear gene tree (and
presumed organismal phylogeny) is consistent with our
mitochondrial phylogeny estimate.
Based essentially on geological evidence, Crother et al.
(1992) suggested that the diversification of Middle American
highland Bothriechis species spanned a broad time period, from
the Eocene–Oligocene boundary (middle Tertiary) through the
Pleistocene. In contrast, our estimates from relaxed molecular
clock models suggest that this group diversified over a more
contracted period, ranging from the middle Miocene through
late Pliocene (Fig. 5). Despite this contrast, both hypotheses
share most inferences of geographical boundaries and related
geological and tectonic events that historically underlie phylogenetic splits within Bothriechis. Because their phylogeny
estimate places B. lateralis within the northern highland species
group, however, Crother et al. (1992) suggested recent southward dispersal of the ancestor of B. lateralis from northern
Middle America to Costa Rica. In contrast, our biogeographical model essentially depicts a more simplistic northward
progression of cladogenesis that requires no inference of
dispersal and is more compatible with the patterns we
observed in Atropoides and Cerrophidion, suggesting vicariance
as the primary driving force underlying speciation. Unlike the
other two genera in this study, Bothriechis appears to have
diversified (into B. nigroviridis and B. lateralis) early within
Lower Central America during the middle–late Miocene. This
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11
T. A. Castoe et al.
divergence also is associated with a shift in altitudinal habitat,
as B. lateralis typically occupies lower elevations than does
B. nigroviridis (Campbell & Lamar, 2004). Despite this
uniqueness, our revised model for Bothriechis biogeography
yields exceptional temporal and phylogeographical correspondence between patterns in this group with those estimated for
Atropoides and Cerrophidion.
CONCLUSIONS
The species-level biodiversity of Middle American highland
pitvipers, as currently recognized, appears to have been
generated predominantly by tectonic events occurring during
the Miocene and Pliocene, independent of Pleistocene climatic
fluctuations. We do, however, find evidence that the onset of
glacial cycles may have had an impact on highland pitviper
lineage diversity, but only within species. Although future
taxonomic revisions (e.g. in C. godmani) may alter this general
conclusion, evidence that the current high taxonomic diversity
of pitvipers in the region owes its origins to events that predate the Pleistocene is both significant and impressive. We
have identified several major historical events, each of which
appears to have resulted in the simultaneous vicariance and
diversification of multiple highland lineages in Middle America. These findings suggest that Miocene and Pliocene events
may have broad predictive power across entire communities of
highland-distributed organisms. Inferences from highland
pitviper lineages show a strong underlying pattern of south
to north, Miocene–Pliocene vicariance across highland masses
that can be explicitly examined as a baseline hypothesis for
other taxa. This new evidence suggests the existence of an
underlying and unifying model of Middle American biogeography. It is therefore a strong motivation for future comparative phylogeographical work in the region, and suggests that a
cohesive hypothesis of the region’s history may eventually be
unveiled through the comparative phylogeography of its
biodiversity. The complex and controversial geological and
tectonic history of Middle America has posed a substantial
challenge for palaeogeography and biogeography. Further
comparative biogeographical research may thus present tremendous potential for both generating and testing hypotheses
leading to the formulation of a synthetic physical and biotic
inference of the region’s history and evolution.
ACKNOWLEDGEMENTS
We thank the many people who over the years have
contributed insight and suggestions that have added to this
study, including: B. Crother, T. Doan, J. Meik, B. Noonan,
J. Reece, W. Schargel and C. Spencer. Andrew Crawford is also
thanked for constructive advice and conversation, including
discussion of his unpublished work. Manuel Iturralde-Vinent
provided critical literature and insights regarding Caribbean
palaeogeography. Several researchers provided tissues under
their care and obtained during sponsored research, including
Laurie Vitt (University of Oklahoma, obtained through NSF
12
grants DEB-9200779 and DEB-9505518), Cesar Jaramillo
(Cı́rculo Herpetológico de Panamá), William Duellman, Linda
Trueb and Eli Greenbaum (University of Kansas), Robert
Murphy (Royal Ontario Museum), Marcio Martins (Universidad de São Paulo) and Gunther Köhler (Forschungsinstitut
Senckenberg). This research was supported by a UCF startup
package to C.L.P., a grant from Bioclon to E.N.S., an NSF
Collaborative Research grant to C.L.P., E.N.S. and J.A.C.
(DEB-0416000, 0416160), as well as NSF grants DEB-0613802
and DEB-9705277 to J.A.C. T.A.C. was also supported by an
NIH training grant (LM009451) while finalizing this project,
and U.K. by the LOEWE programme of the state government
of Hessen.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the
online oversion of this article:
Appendix S1 Sequences used in phylogenetic and divergence
time estimation, with Genbank numbers and voucher information. Sequences added specifically in this study are indicated
in bold.
Please note: Wiley-Blackwell is not responsible for the
content or functionality of any Supporting Information
supplied by the authors. Any queries (other than missing
material) should be directed to the corresponding author for
the article.
Journal of Biogeography
ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
15
T. A. Castoe et al.
BIOSKETCHES
Todd A. Castoe and Juan M. Daza performed this research collaboratively while they were doctoral students in the laboratory of
Christopher L. Parkinson at the University of Central Florida, building on their shared interests in snake genomics, molecular
evolution, phylogenetics, molecular dating and Neotropical biogeography. Todd Castoe is now a postdoctoral fellow and Associate
Director of the Consortium for Comparative Genomics at the University of Colorado School of Medicine.
Editor: David Hafner
16
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