Points of View
Syst. Biol. 61(3):522–532, 2012
c The Author(s) 2012. Published by Oxford University Press, on behalf of the Society of Systematic Biologists. All rights reserved.
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DOI:10.1093/sysbio/syr121
Advance Access publication on January 2, 2012
Are Asteraceae 1.5 Billion Years Old? A Reply to Heads
U LF S WENSON 1, * , S TEPHAN N YLINDER 2 ,
AND
S TEVEN J. W AGSTAFF3
1 Department
of Phanerogamic Botany, Swedish Museum of Natural History, PO Box 50007, 104 05 Stockholm, Sweden;
of Plant and Environmental Sciences, University of Gothenburg, Box 461, 405 30 Gothenburg, Sweden; and
3 Allan Herbarium, Landcare Research, PO Box 40, Lincoln 7640, New Zealand;
∗ Correspondence to be sent to: Department of Phanerogamic Botany, Swedish Museum of Natural History, PO Box 50007, 104 05 Stockholm, Sweden;
E-mail: [email protected].
2 Department
Received 6 July 2011; reviews returned 16 August 2011; accepted 16 September 2011
Associate Editor: Adrian Paterson
In a recent issue of this journal, Heads (2011) reiterated
his critique of commonly used methods for calibrating
molecular phylogenies, especially the use of island ages
in divergence time estimates (Heads 2005a, 2009). His
critique essentially rests in the notions that geological
ages of islands need not correspond to cladogenetic
events in the same area and that fossil ages given as minimums “are quietly and mysteriously transformed into
maximum dates” (Heads 2011, p. 204). He calls this use
of geological data manipulation and transmogrification
(change in form or appearance) and asserts that various
authors do so to reject hypotheses of earlier Mesozoic
(65–250 Ma) vicariance. Instead, Heads (2011, p. 214) suggests that phylogenies are better calibrated with just tectonic events because “this approach combines the best
of molecular biology with hard-rock geology and avoids
the many problems of fossil calibration.” In other words,
if a molecular phylogenetic analysis recovers sisters in,
for example, New Zealand and South America, it is better to calibrate the phylogeny on the tectonic event that
separated these landmasses. This reasoning, however,
is circular because a priori assumption of vicariance is
not tested by the use of sister relationships that are a
priori calibrated as vicariance. Despite this shortcoming, Heads (2011) criticizes many authors and their work
on various organisms across the world. We suggest that
he fails to demonstrate that his tectonic calibration approach is scientifically sound. One of many examples of
alleged transmogrification that Heads (2011) refers to is
the trans-Pacific Asteraceae genus Abrotanella. Here we
use a testable hypothesis of vicariance in Abrotanella, a
genus that is widely distributed in the southern hemisphere, to challenge the idea of a link between lineage
divergence and southern hemisphere plate movements.
Historical biogeography has developed during the
past two decades from being a narrative to a more rigorous hypothesis-testing approach with sound scientific
conclusions (Crisp et al. 2011). Modern studies of divergence times frequently use a relaxed molecular clock
model in a Bayesian framework implemented in the
software BEAST (Drummond and Rambaut 2007). One
advantage of BEAST is that it allows modeling of calibration uncertainties, such as hard or soft minimum
bounds (in contrast to a point calibration) (Renner 2005;
Ho and Phillips 2009). Another advantage is that 95%
posterior density intervals are given as output data and
estimates are therefore moving beyond the simplistic use
of a minimum age constraint. One drawback, however,
is that appropriate clock models are not available for
morphological and/or indel characters. However,
groups supported by such data can be constrained a priori. Thus, assumptions of vicariance cannot be rejected
when molecular divergence time estimates overlap tectonic events that could have caused lineage splitting
(Crisp et al. 2011).
The genus Abrotanella comprises 21 alpine species
(Swenson 1995, 1996; Heads 1999; Wagstaff et al. 2006)
distributed in southeast Australia, New Guinea, New
Zealand, South America, Tasmania, and the subantarctic
Auckland and Campbell islands. It is a group of small
cushion-forming plants, nearly always growing above
the timberline, which barely expose their often small and
sessile capitula above the cushions but frequently form
continuous communities in association with members
from other plant families. Abrotanella has traditionally
been placed in the Asteraceae tribe Senecioneae (Bremer
1994; Swenson and Bremer 1997a), but recent molecular
studies have demonstrated that it has an isolated position in the family and is likely sister to the Senecioneae
(Pelser et al. 2007). Biogeographic analyses using a
morphology-based phylogeny and an area cladogram
approach, compared with geohistorical events,
suggested that the history of Abrotanella involved
several cases of long-distance dispersal (Swenson and
Bremer 1997b). One such inferred event involves dispersal from South America to New Zealand (Stewart
Island) giving rise to the sisters Abrotanella submarginata
in the former area and A. muscosa in the latter. Both
species have fruits with twin hairs and an apical crown
that easily adheres to various materials (Swenson 1995),
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which is at odds with the statement that the genus lacks
structures that could facilitate dispersal (Heads 1999,
2009), adopted by other panbiogeographers (McCarthy
2003). In addition, Swenson and Bremer (1997a, 1997b)
suggest that the New Zealand lineages of Abrotanella are
young and their evolution is consistent with the active
uplift of the Southern Alps that began some 3–5 Ma (Fleming 1979; Stevens 1980; Campbell and Hutching 2007).
High mountains and alpine vegetation did not exist in
New Zealand before the Pliocene (Wardle 1978).
The morphology-based analyses of Abrotanella provided the background for a molecular analysis of divergence times in the genus using nrDNA and cpDNA
sequence data (Wagstaff et al. 2006). Divergence times
were estimated by penalized likelihood, which accommodates rate heterogeneity across lineages as implemented in the program r8s (Sanderson 2002, 2006).
The phylogeny was point-calibrated with a 38-myr-old
Asteraceae fossil (Graham 1996) on the split between
the subfamily Barnadesioideae (Dasyphyllum) and the
remaining members of the family (DeVore and Stuessy
1995). Findings of the molecular study supported the
earlier morphological analysis in that several longdistance dispersals, colonization, and radiation are required to reconcile the evidence. Wagstaff et al. (2006)
further suggested that Abrotanella originated ca. 19.4 Ma
followed by a split at 4.2 Ma in an ancestor whose two
descendent lineages colonized Australasia and South
America. The molecular analysis further nests, with
strong support, A. muscosa from New Zealand in a clade
of South American taxa. The sister relationship with
A. submarginata is supported by a maximum likelihood
analysis, but the internal branches are so short that exact relationships are uncertain. In any event, the two
species share several morphological features that group
them as sisters (Swenson and Bremer 1997a). Suffice to
say, regardless of whether morphological or molecular
data are used, the historical biogeography of Abrotanella
is consistent with recent long-distance dispersal and
establishment across the Pacific.
In contrast, panbiogeographers view the history of
Abrotanella differently, suggesting that its distribution
correlates with tectonic features and processes of continental rifting. Heads (1999, 2005a, 2009), as well as
McCarthy (2003), interprets Abrotanella, especially the
sister relationship between A. muscosa and A. submarginata, as clearly the result of vicariance, that is,
an old Mesozoic derivative since these occur in New
Zealand and South America, respectively. Heads frames
it in 1999 (p. 415), “The ranges of A. submarginata
and A. muscosa occupy what are currently outlying islands but were once part of much larger Gondwanic
and Pacific terranes which, during the history of the
angiosperms, have fractured, rifted, sundered and accreted.” Heads (2011, p. 213) states that Wagstaff et
al. (2006) transmogrified the fossil-calibrated dates in
that Abrotanella initially diverged during (not before,
as preferred by Heads) the early Miocene with a second species radiation about (not before, as preferred
by Heads) 3 Ma. Nonetheless, we believe that this
semantic difference should not predispose the evidence
of long-distance dispersal and a recent species radiation
to Mesozoic vicariance. We demonstrate here that
calibration using tectonic events, as suggested by Heads
(2011), generates an unrealistic hypothesis where the
evolution of Asteraceae must have taken place contemporary with or earlier than the Cambrian explosion
(Butterfield 2007). This would be hundreds of million
years earlier than the origin of the asterids (Martı́nezMillán 2010), a large group of some 80,000 angiosperm
species where Asteraceae and Abrotanella are embedded
and furthermore even predates the origin of the land
plants (Zimmer et al. 2007).
M ATERIALS AND M ETHODS
Data
Wagstaff et al. (2006) used a data set of nuclear
(ITS) and chloroplast (trnK/matK) sequences of
Abrotanella, available and downloaded from TreeBase:
http://purl.org/phylo/treebase/phylows/study/TB2:
S1343 (last accessed January 2012). The matrix was
realigned in MAFFT 6.0.2 (Katoh et al. 2009) using the
“linsi” command with subsequent minor manual corrections. The sister relationship between A. muscosa and
A. submarginata was constrained because the molecular
branches are too short to resolve the relationship with
any support, both species are nested in a clade of South
American taxa, and morphology supports their sister
relationship (Swenson and Bremer 1997a; Wagstaff et al.
2006). The accession from New Guinea (A. nivigena 1.27)
was renamed A. papuana since Wagstaff et al. (2006)
demonstrated its systematic rank, that is, a species endemic to New Guinean alpine bogs, not possible to unite
with the Australian species A. nivigena that grows in
alpine herbfields and rock crevices as suggested by
Swenson (1995).
Calibration
We used two different calibration methods: one based
on fossil evidence as criticized by Heads and the other
on tectonic events as advocated by Heads. Transmogrification of minimum fossil age to maximum fossil age in
calibration of molecular phylogenies is a great concern
of Heads (2005a, 2009, 2011). Indeed, correct calibration is essential to assure rigorous testable divergence
estimates (Graur and Martin 2004; Near and Sanderson
2004; Ho and Phillips 2009; Crisp et al. 2011). A fossil that shares synapomorphies defining a crown group
can be placed on the stem below and given a minimum
age (Renner 2005; Ho and Phillips 2009). Wagstaff et
al. (2006) calibrated their phylogeny by using fossilized
pollen estimated to 38 Ma (DeVore and Stuessy 1995;
Graham 1996), fossils that may not have met all
recommendations for accurate calibration as proposed
by Gandolfo et al. (2008). On the other hand, Wagstaff
et al. remarked that their divergence estimates “should
be viewed as preliminary and await the discovery
of more detailed fossil evidence to establish multiple
calibration points” (p. 104). A well-preserved, almost
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FIGURE 1. Priors used for the 47.5 Ma fossil-based calibration in the dating analyses of Abrotanella (Asteraceae): (a) exponential minimum
distribution, (b) lognormal minimum distribution (mean value in real space), (c) normal distribution, and (d) hard minimum bound. Solid gray
lines give mean values and dashed lines show the 95% sampling interval. Million years before present (Ma) are along the x-axis where the upper
scale shows a narrow (I) and the lower a wide (II) sampling distribution. The upper 97.5% sampling intervals for wide distributions are set to
approximately twice the same value as for a narrow distribution.
10 myr older fossil, unequivocally assigned to deep
roots in Asteraceae, but excluding Barnadesioideae, was
recently discovered in northwestern Patagonia, South
America (Barreda et al. 2010). The fossil includes an
entire capitulum with several series of phyllaries, ligulate or lipped florets, pappus, tricolporate and echinate pollen and bilayered sexine, and was recovered in
Middle Eocene deposits dated to 47.5 Ma. This character combination excludes the fossil from the subfamily
Asteroideae that is the core sample in our study. We use
it to calibrate the Asteraceae phylogeny by placing it
on the stem above Dasyphyllum, a member of Barnadesioideae (Bremer 1994; Stuessy et al. 2009).
Several calibration points are possible in Abrotanella
on the basis of tectonic events. There are at least two
strong cases, one split leading to radiation of taxa in
Australasia and in South America (labeled V1) and another between the sister pair A. muscosa (New Zealand)
and A. submarginata (South America) (labeled V2). Since
Heads (1999, 2005a, 2009) proposes that these splits represent vicariance, we use both in separate analyses as
tectonic calibration priors, both corresponding to when
Zealandia rifted from Antarctica 80–84 Ma (McLoughlin 2001; Veevers 2001; Trewick et al. 2007; Neall and
Trewick 2008).
In order to explore the alleged effect of transmogrification of minimum age to maximum age, we
applied four different prior distributions (exponential
minimum, lognormal minimum, normal distribution,
and hard minimum bound) in the fossil-calibrated
analysis (Fig. 1). Ho and Phillips (2009) described these
distributions and advocated a hard minimum bound
(age) following a lognormal distribution for fossil-based
calibrations. We used these priors in our analyses to
produce posterior distributions of trees and to calculate
individual maximum clade credibility trees. Three
out of four distributions use a hard minimum bound,
that is, they do not allow transmogrification of the
calibrated node (Fig. 1a,b,d). However, to explore the
effect of a soft minimum bound, we also applied a
normal distribution that allows ages to be younger
as well as older (Fig. 1c). Except for the hard minimum bound without limits for older ages (Fig. 1d),
each distribution was implemented with two sets of
uncertainties surrounding the fossil: a narrow uncertainty with a mean and/or standard deviation using
the default value one (labeled I) and a wide uncertainty where the upper 97.5% sampling interval was
defined as twice the age of the narrow (labeled II). The
narrow and wide uncertainties are reported in Table 1
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TABLE 1. Posterior divergence time estimates of the Asteraceae root node and the split between Abrotanella muscosa and A. submarginata
based on fossil calibration (47.5 Ma) and four prior distributions (Fig. 1)
Asteraceae root node (Ma)
Median (95% HPD)
Distribution
Exponential minimum
Lognormal minimum
Normal distribution
Hard minimum bound
Narrow (I)
48.4 (47.5–50.3)
48.0 (47.5–50.3)
47.4 (45.4–49.3)
60.1 (47.5–137.0)
Wide (II)
55.2 (47.5–71.1)
52.6 (47.9–67.3)
30.6 (18.1–56.3)
—
A. muscosa/A.submarginata (Ma)
Median (95% HPD)
Narrow (I)
2.3 (1.1–3.9)
2.3 (1.2–3.9)
2.3 (1.0–3.8)
3.0 (1.2–7.6)
Wide (II)
2.6 (1.2–4.5)
2.6 (1.2–4.6)
1.6 (0.6–3.2)
—
Note: Roman numerals refer to narrow (I) and wide (II) modeled uncertainty, respectively.
and Fig. 2b,c for the root node and the sister pair A. muscosa and A. submarginata. We applied the normal distribution prior (Fig. 1c) for the tectonic calibration because
rifting is a long-term geological process with no exact
date.
Phylogeny and Molecular Dating
The occurrence of rate variation across lineages
poses a potential stumbling block not only for estimating divergence times but also for phylogenetic inference (Felsenstein 1978). Nonetheless, two common
approaches are used to account for rate variation:
rate smoothing penalized likelihood (Sanderson 2002)
and relaxed clock approaches (Drummond et al.
2006); the latter of which can infer tree topologies. A Bayesian relaxed clock approach across all
lineages and all analyses identified a rate heterogeneity coefficient between 0.5 and 1.0 (mean 0.73)
for ITS and 0.15 and 0.9 (mean 0.5) for matK
data, which indicate deviations from clock-like rates
(Drummond and Rambaut 2007).
Each gene partition was tested for the best substitution model using jModelTest (Posada 2008) with default
settings based on the Bayesian information criterion
(Posada and Buckley 2004), averaging over all included parameters in order to select against favoring of
parameter-rich models, as are often suggested by the
Akaike information criterion (Akaike 1974). The model
test resulted in the selection of TIM2 for each of the two
data partitions. Aligned matrices were prepared as output files for analysis in BEAUti 1.6.1 (part of the BEAST
package) and analyzed in BEAST 1.6.1 (Drummond and
Rambaut 2007). A Yule prior (Yule 1924) was set for
the tree model together with separate relaxed lognormal
clock models on substitution rates for each locus. The
Markov chain Monte Carlo chains were set to run
for 15 million generations, logging parameters every
5000 generations. Chain mixing and convergences were
checked in Tracer v1.5 (Rambaut and Drummond 2007)
with all parameters showing estimated sample sizes
values of >200, and maximum clade credibility trees
were calculated in TreeAnnotator 1.6.1 (Drummond and
Rambaut 2007). The summary trees with 95% highest
posterior density (HPD) intervals of divergence time
estimates were prepared in FigTree v1.3.1 (Rambaut
2009).
R ESULTS AND D ISCUSSION
Potential sources of error associated with estimating
divergence times are widely acknowledged and have
been the subject of several recent reviews (see Sanderson 2002; Near and Sanderson 2004; Heads 2005a; Gandolfo et al. 2008; Ho and Phillips 2009). Foremost
among these are recovering a well-supported hypothesis of relationships, accounting for rate variation across
lineages, integrating current understanding of the fossil record and geological history, defining realistic probability distributions around this prior knowledge and
selecting the appropriate nodes to use as calibration
points. We concede these are complex issues and provide a cautious synthesis of the results.
Topology
Our phylogenetic estimate recovers a maximum clade
credibility tree similar to that reported by Wagstaff et al.
(2006). Abrotanella is monophyletic, strongly supported,
and the lineage is found on a long branch (Fig. 2). At
the base, the Tasmanian endemic A. forsteroides is sister to the remaining species that in the next split form
two clades, one confined to South America (except A.
muscosa) and one to Australasia. Within each of these
clades, the branches are short and few relationships are
strongly supported. Our results support the findings of
Wagstaff et al. (2006) that the species from New Guinea
(A. papuana) is distantly related to and not conspecific with A. nivigena from southeast Australia. Both
species are nested in a clade from New Zealand. The
topologies calibrated on fossils and tectonics are identical except for the switched order of A. emarginata and
A. linearifolia. However, if the stem of the family is calibrated on a 47.5-myr-old fossil simultaneously as the
derived sister relationship between A. muscosa and A.
submarginata is calibrated on a tectonic event of 80–
84 Ma, the analysis reverses the tree topology and
places Abrotanella at the root of the family. This relationship is contrary to all present knowledge of Asteraceae
evolution (Funk et al. 2009).
Variation in the Rate of Nucleotide Substitution Across
Lineages
It has been widely demonstrated that nucleotide
substitution rates are seldom clock like (Ayala 1997).
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FIGURE 2. Maximum clade credibility tree and divergence time estimates of Abrotanella (Asteraceae) using a 47.5-myr-old fossil for calibration. (a) Chronogram with 95% HPD intervals shown as transparent bars with median ages (Ma) using a narrow lognormal distribution. Bold
branches have PP > 95%. (b) Divergence time estimates for the root node and (c) for the sister pair A. muscosa and A. submarginata, using four
distribution priors, each (except d) with a narrow and a wide uncertainty (I and II), as reported in Fig. 1a–d.
In the earlier study of Abrotanella (Wagstaff et al.
2006), a likelihood ratio test was used to assess
sequence data for departures from clock-like behavior, and the test determined that there was
significant rate variation across lineages. The sequences
of the annual Senecio vulgaris appeared to be evolving at a faster rate than those of the tree Dasyphyllum dicanthoides. However, within Abrotanella, the
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POINTS OF VIEW
substitution rate was almost clock like and the mean
nucleotide substitution rate calculated across the tree
corresponded quite closely to the rate calculated for
Dendroseris and Robinsonia (Asteraceae), a rate derived
by using the recent formation of the Juan Fern ández
Islands as a calibration point (Sang et al. 1994, 1995).
We found nothing in this study that would change this
view.
Heads (2011, p. 213) questioned the use of comparing molecular rate changes in the Juan Fern ández genera with Abrotanella because he believes that the genera
are much older than the islands. He then addressed that
Lactoris, a formerly widespread paleoherb with a fossil
record of some 90 myr (Macphail et al. 1999; Gamerro
and Barreda 2008), is restricted to the archipelago, implicitly favoring vicariance. The Juan Fern ández Islands
consist of two main islands, Robinson Crusoe Island
(Masatierra) and Alejandro Selkirk Island (Masafuera),
where potassium–argon dates are ca. 4 Ma for the former and ca. 2 Ma for the latter (Stuessy et al. 1984). With
the fact that one species of Abrotanella (A. linearifolia) occurs in southern South America and Alejandro Selkirk
Island, while Lactoris is endemic for Robinson Crusoe Island, it is improbable to reconcile their distribution with
vicariance. First, the origins of these two lineages differ
by almost 100 myr. Second, the Juan Fernández Islands
are of oceanic origin. Third, only very fast rates in the
deep past with a dramatic decrease of rate changes at
some point in the evolution could yield such an overestimation of deep divergence, but a mechanism for such
wildly fluctuating substitution rates is not known.
Fossil Calibration and Time Estimates
The main differences between the present and earlier
divergence time estimate of Wagstaff et al. (2006) is that
more complete and well-preserved fossils of Asteraceae
have been recently described (Barreda et al. 2010) and
that rigorous Bayesian approaches have been developed
allowing incorporation of error associated with divergence time estimates. We used a calibration point that
was almost 10 myr older and ran the analyses using the
relaxed molecular clock implemented in the software
BEAST (instead of penalized likelihood). Four different
prior distributions were used to describe possible uncertainty of the fossil age (instead of point calibration),
and we reported 95% HPD intervals surrounding our
divergence estimates. In general, the median age of the
Asteraceae root node for all distributions (except hard
minimum bound), using a narrow standard deviation
for the prior distribution, is estimated at 47.4–48.4 Ma
and 30.6–55.2 Ma when a wide uncertainty is used
(Fig. 2b and Table 1). A hard minimum bound recovers
a median of 60.1 Ma but the HPD extends back to 137.0
Ma. Our result differs from Wagstaff et al. (2006) in that
Asteraceae is ∼10 myr older, which is expected given
that an older fossil calibration was used, but the estimated ages are not transmogrified into maximum ages.
A normal prior distribution, using a wide standard deviation, estimates the mean age of the family to 30.6 Ma,
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which is clearly much younger than the fossil age and,
hence, makes it unsuitable for fossil calibration (Ho and
Phillips 2009).
Wagstaff et al. (2006) estimated that the lineage leading to Abrotanella diverged from the stem of Asteroideae
at 19.4 (17.1–21.9) Ma, an age that here is estimated to
38.4 Ma. This node, however, has relatively low support (PP 0.47) and the 95% HPD interval calculated
by TreeAnnotator must be viewed as a rough estimate
since it falls below the chosen 0.5 posterior probability
(PP) limit for reliable node estimates. The first split in
Abrotanella was estimated by Wagstaff et al. (2006) to 4.2
(2.9–6.1) Ma and now to 8.5 (4.9–12.6) Ma. The difference
between these two estimates, despite the use of a 10 myr
older fossil, is small and does not support a hypothesis of vicariance between Tasmania and South America
and/or New Zealand. To infer a Mesozoic vicariance between the derived sister pair in New Zealand (A. muscosa) and South America (A. submarginata), as suggested
by Heads (1999, 2005a, 2009), additional illogical ad
hoc hypotheses are needed. Our analyses estimate this
event to 1.6–3.0 (0.6–7.6) Ma considering all the distribution priors that were used. This result is inconsistent
with geophysical evidence on the Gondwana breakup
involving Australia, New Zealand, and South America (Barker and Burrell 1977; Hallam 1994; McLoughlin
2001; Veevers 2001; Neall and Trewick 2008).
Tectonic Calibration and Time Estimates
If we accept that tectonic calibration is applicable as
proposed by Heads (2011), what would the implication
be for Abrotanella and Asteraceae? First of all, both splits
are not contemporaneous and cannot simultaneously
be explained as vicariance because one is topologically
older than the other. Second, the use of a normal distributional prior, applicable to the radiation in South
America and Australasia (V1 in Fig. 3), estimates an age
of the Asteraceae root node to ca. 511 (288–775) Ma and
27.6 (12.2–45.5) Ma for the split between A. muscosa and
A. submarginata (Table 2). In other words, accounting
for the 95% HPD, this estimate does not push back the
crown node of sister taxa long enough into the Mesozoic era to make a vicariance event plausible between
80 and 84 Ma. In fact, long-distance dispersal between
South America and Zealandia is still the most plausible scenario since the HPD interval does not overlap the
breakup time frame between Zealandia and Antarctica (Coleman 1980; Hallam 1994; McLoughlin 2001;
Veevers 2001). To make that scenario possible, it is necessary, just as Heads (2011) suggests, to calibrate the tectonic event corresponding to the split between A. muscosa and A. submarginata (Fig. 3, V2). Such calibration
does yield an age estimate of 84.6 (78.4–90.3) Ma and
would be consistent with a putative vicariance event
resulting from the split between South America and
New Zealand, but aside from circularity, this approach
pushes back the origin of Asteraceae to the Proterozoic
eon, about 1.5 billion years ago.
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FIGURE 3. Maximum clade credibility tree and divergence time estimates of Abrotanella (Asteraceae) using the tectonic event when Zealandia
separated from Antarctica 80–84 Ma, an event applicable to two nodes (V1 and V2) in the phylogeny. V1 (green) corresponds to a hypothesized
vicariance between two subclades in Abrotanella and V2 (blue) between A. muscosa (New Zealand) and A. submarginata (South America); 95%
HPD intervals are shown as transparent green (V1) and blue bars (V2). Median ages (Ma) are reported above (V1) and below (V2) branches.
Bold branches have PPs >95%.
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TABLE 2. Posterior divergence time estimates of radiation in Abrotanella (Asteraceae) calibrated on the breakup event between Zealandia
and Antarctica at 80–84 Ma, using a normal distribution and two possible nodes, labeled V1 and V2 (Fig. 3)
Node
Asteraceae root node
A. muscosa/A. submarginata
Radiation in South America
Radiation in Australasia
Vicariance event V1
Median (95% HPD)
511 (288–775)
27.6 (12.2–45.5)
45.2 (25.9–66.5)
46.7 (27.5–68.4)
Feasibility of Tectonic Calibration
With the above example in mind, it is important to
state that we do not dismiss tectonic or geological calibration in general. Instead, we agree with Renner (2005)
and Ho and Phillips (2009) that tectonic calibrations
may provide important validations of divergence time
estimates, but the events must be selected and discussed
carefully in every case.
We acknowledge that tectonic events can be a useful means to calibrate divergence times especially when
fossils are not known for the group of interest, as is
the case in many young insular or alpine lineages such
as Abrotanella. For example, Sang et al. (1994) used
the emergence of the Juan Fernández Islands to estimate the diversification of the endemic genus Dendroseris. The nucleotide substitution rates estimated for
ITS by Sang et al. (1994) were independently corroborated by our estimates of the substitution rate for
Abrotanella that were based upon a fossil calibration
(Wagstaff et al. 2006). Baldwin and Sanderson (1998)
used the onset of summer-dry paleoclimates that accompanied mountain-building in Western North America some 15 Ma as a conservative maximum age
calibration point to date the origin, dispersal, and divergence of the silver sword alliance in the Hawaiian Islands. Baldwin and Wagner (2010) summarized
evidence suggesting that the oldest Hawaiian lineages
mostly date back to the oldest high elevation island,
Kaua‘i.
Tectonic calibrations can also be used as a means to
corroborate fossil calibrations. For instance, Wagstaff
et al. (2010) used a cross-validation procedure to assess the reliability of both fossils and the emergence of
Lord Howe Island as calibration points to corroborate
the age and origin of Dracophyllum in continental Australia, New Caledonia, and New Zealand. Wagstaff et
al. (2011) used both fossils and the emergence of the
Chatham Islands to estimate the origin of the subantarctic island endemic genus Pleurophyllum. Assuming that
the divergence of two endemic Chatham Island sister
species could not be older than the emergence of the
archipelago, they applied a uniform prior of between 1
and 3 Ma to account for uncertainty associated with this
tectonic event.
A number of studies have addressed the impact of
drowning on insular biotas, and such scenarios and
dates could be used as tectonic calibrations if properly validated. New Caledonia has traditionally been
Vicariance event V2
Median (95% HPD)
1456 (770–2360)
84.6 (78.4–90.3)
122 (85–182)
123 (63–210)
considered a continental island with a biota that dates
back to the separation from Australia some 80 Ma.
Grandcolas et al. (2008) summarized geological and
biological evidence that the archipelago was completely
submerged for an extensive period during the Paleocene
and Eocene, reemerging during the Oligocene, some 37
Ma. It has been suggested that the entire biota colonized
New Caledonia after this drowning. One early attempt
to use the emergence of New Caledonia as a calibration
point was made by Pratt et al. (2008) in their divergence
time estimates of the Australasian weta (family Anostostomatidae) but without firm conclusion. Support for
reemergence at 37 Ma comes from a study of geckos in
which Nielsen et al. (2011) used fossil taxa and the emergence of New Caledonia as independent calibrations.
They concluded that omission or inclusion of the emergence of New Caledonia yielded similar time estimates.
Similarly, Nattier et al. (2011) investigated cricket biogeography in Australasia and the Pacific to determine
whether New Caledonia was colonized before or after
the postulated 37 Ma reemergence. Fossils are poorly
conserved in this group of crickets and, hence, different
combinations of tectonic calibrations, excluding New
Caledonia, were used. Their results suggest that the
lineages in New Caledonia are younger than 37 Ma.
Divergence time estimates among plants also support
this geological scenario. For instance, Woo et al. (2011)
studied Gesneriaceae in the southwest Pacific, used a
seamount of the Lord Howe Ridge with an age of 23
Ma for calibration, and found that the family dispersed
to New Caledonia long after the reemergence. Fossilcalibrated phylogenies provide additional support in
that Sapotaceae colonized and diversified in New Caledonia multiple times, the oldest occurring between 38
and 24 Ma (Bartish et al. 2011).
Tectonic events are associated with vicariance or
diversification. However, the reasoning would be circular if one was attempting to date these relationships
using the very tectonic event that was implicated. Ho
and Phillips (2009) recommend using a normal probability distribution as a prior in conjunction with tectonic calibrations. Unlike the exponential probability
used with fossil calibrations, a normal distribution has
both a soft upper and lower bound, which models the
uncertainty associated with tectonic events more realistically. In spite of the large error associated with tectonic events, they can provide independent verification
of fossil calibrations and convincing evidence that either
supports or rejects predictions of vicariance. But tectonic
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SYSTEMATIC BIOLOGY
calibration is also associated with the uncertainty of
selecting the appropriate node in a phylogenetic tree.
Our example illustrates the difficulty of whether V1 or
V2 could be applicable for tectonic calibration.
We suggest that future studies will continue to incorporate multiple calibration points to refine phylogenetic estimates of divergence times and they will assess
the conflict among them. We agree with Heads (2011)
that using the emergence of islands to calibrate molecular clocks suffers from some of the same difficulties as
when using the fossil record. There is uncertainty associated with the timing of tectonic events and when a
lineage arrives on an island after formation. It is also
conceivable that fragmented populations could persist
for long periods of time on adjoining islands providing
a source of new founders in which case the divergence
times would be underestimated. But diversification of
an insular group strongly suggests presence, like the examples for New Caledonia above, where the geological
age could be used as a calibration point.
Transmogrification
We are not the first to criticize the paradigm of panbiogeography (Seberg 1986; Cox 1998; Wallis and Trewick
2001; Goswami and Upchurch 2010; Murienne 2010)
that Heads advocates in his paper and elsewhere (2001,
2005a, 2005b, 2008a, 2008b). But the contention (Heads
2011, p. 213) that Wagstaff et al. (2006) transmogrified
the fossil-calibrated divergence times of Abrotanella by
stating during or about instead of before to subsequently
reject hypotheses of earlier Mesozoic (65–250 Ma) vicariance must be examined in the light of the current
dating analyses. We argue that the most reliable evolutionary hypotheses are based upon independent confirmation of the results. The phylogenetic relationships
of Abrotanella, regardless of whether parsimony, maximum likelihood or Bayesian analyses of both morphological and molecular data are used, recover overall the
same topology that is reported here (Figs. 2 and 3) and
elsewhere (Swenson 1995; Swenson and Bremer 1997a,
1997b; Wagstaff et al. 2006). This result suggests robustness of both the previous and the current results. As
stated above, three out of four distribution priors (Fig. 1)
use a hard minimum bound and do not allow transmogrification of the calibrated node (Fig. 2b and Table 1).
But the relevant point of transmogrification is the estimated time on the node leading to the alleged vicariance between A. muscosa and A. submarginata (Fig. 2c).
Since there is significant rate variation across lineages,
an incomplete fossil record, and the timing of breakup
between landmasses is imprecise, it is appropriate to incorporate credibility intervals into the divergence time
estimates (Drummond et al. 2006). Thus, Wagstaff et al.
(2006) semantically used during and about, and despite
the current calibration with a fossil that is 10 myr older,
the estimated split from that study and this corroborate
each other. In fact, depending on the distribution priors used, a time span between 0.6 and 7.6 Ma is best
described as during or about not before.
C ONCLUSIONS
Heads (2009, 2011) has provided insightful reviews
of common fallacies associated with using fossil- and
tectonic-based calibrations when deriving molecularbased estimates of divergence times. We offer a rebuttal
to his criticisms using the widely distributed southern
hemisphere genus Abrotanella as an example. Heads
(1999) interprets the origin and diversification of the
genus from the perspective of vicariance biogeography
and terrane tectonics, whereas we employ recently discovered fossil evidence and rigorous Bayesian analysis to reach an alternative conclusion. We suggest that
calibrating molecular phylogenies a priori on sister relationships as if they represent alleged vicariance events,
as advocated by Heads (1999, 2005a, 2009, 2011) is inappropriate. As demonstrated here, different alleged vicariance events in Abrotanella can be used to calibrate
the very same phylogeny: the calibration point placed
at node V1 estimates the origin of Asteraceae as ca.
511 Ma. This early date precedes the split between seed
plants and mosses (Zimmer et al. 2007). The calibration point at V2 suggests that the family originated at
a time when the biosphere was nearly exclusively populated by microscopic marine organisms (Butterfield
2007; Strother et al. 2011) some 1.5 billion years ago. The
tectonic calibration advocated by Heads (1999, 2005a,
2009, 2011), obfuscate the available evidence to support
a philosophy of continental drift and Mesozoic vicariance that we strongly refute.
F UNDING
This work was supported by the Swedish Museum of
Natural History to U.S. and the Ministry of Science and
Innovation through the Defining New Zealand’s Land
Biota Nationally Significant Database to S.J.W.
A CKNOWLEDGMENTS
We thank Adrian Paterson, Philip Garnock-Jones,
Steve Trewick, Stefan Bengtson, and three anonymous reviewers for constructive comments on
this manuscript. The conciseness and clarity of this
manuscript benefited from careful editing of Christine
Bezar and the Editor-in-Chief of Systematic Biology,
Ron DeBry.
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