This document discusses the biogeographical patterns of plants in the Neotropics. It summarizes that while plate tectonics was often thought to explain plant distributions, molecular studies have shown that long-distance dispersal has played a much larger role than previously believed. Many plant families thought to fit a plate tectonic model based on age are actually too young based on molecular clocks. Overall, long-distance dispersal, rather than plate tectonics, provides the best explanation for the majority of plant family distributions in the Neotropics and connections to other regions. In some old lineages adapted to dispersal, plate tectonics may still explain patterns, but these appear to be exceptions.

1. REVIEW ARTICLE
Biogeographical patterns of plants in the Neotropics –
dispersal rather than plate tectonics is most explanatory
MAARTEN J. M. CHRISTENHUSZ FLS1,2
* and MARK W. CHASE FLS2
1
Botany Unit, Finnish Museum of Natural History, University of Helsinki, Helsinki, Finland
2
Jodrell Laboratory, Royal Botanic Gardens, Kew, Surrey TW9 3DS, UK
Received 16 January 2012; revised 23 July 2012; accepted for publication 7 August 2012
The phylogenetic relationships of some Neotropical plant groups have proved to be different from expectation
assuming plate tectonics as the underlying model, and these unanticipated relationships (and their timing) have
required further work to explain how they came into existence. Well-known Neotropical families, such as
Bromeliaceae and Cactaceae, have one to a few species in Africa, but these can be explained by recent long-distance
dispersals; the estimated ages of the Transatlantic crown and stem clades of these families are of relatively recent
origin, so long-distance dispersal is the only possible explanation and plate tectonic explanations are not viable.
Other families with a crown age appropriate to be explained by plate tectonics did not seem to have distributions
indicating an involvement of long-distance dispersal, but the advent of molecular systematics and molecular clocks
has shown this to be otherwise. Families exhibiting this pattern include Fabaceae and Lauraceae (and many others),
and in spite of the unlikely dispersal qualities of their generally large seeds, they exhibit many instances of
long-distance dispersal occurring throughout their evolutionary history; in fact, long-distance dispersal is the most
probable explanation to account for the current distributions of the great majority of land plant families. One factor
that sometimes is overlooked is the boreotropics hypothesis, which may explain more recent connections between
American, African and Asian floristic components. However, in some old lineages with small propagules that appear
well adapted to long-distance dispersal, such as Marattiaceae and Orchidaceae, we find a pattern suggesting that
tectonics is the primary factor, with clades clearly restricted to one tropical region. These patterns appear to have
little to do with dispersability and it is probable that their conforming to expected (tectonic) biogeographical patterns
has to do with their specialized life-history strategies that have acted to make intercontinental establishment
unlikely even though dispersal almost certainly occurs. Molecular studies have identified additional, unanticipated
clades, and these fit into the relictual category with what may be an overlay of old long-distance dispersal events.
Examples at the ordinal level are Crossosomatales and Huerteales, and we expect future studies to identify more
of these relict and unexpected clades at many taxonomic levels. Overall, molecular systematic studies and molecular
clocks have demonstrated that there are many more connections between the Neotropics and Palaeotropics, both
Asia and Africa, than previously would have been thought under a plate tectonic model. © 2012 The Linnean
Society of London, Botanical Journal of the Linnean Society, 2013, 171, 277–286.
ADDITIONAL KEYWORDS: Amaryllidaceae – boreotropics hypothesis – Caricaceae – Crossosomatales –
Huerteales – islands – long-distance dispersal – Marattiaceae – Nicotiana – Orchidaceae – Petenaeaceae – plate
tectonics – relictual distributions – Rhipsalis – Strelitziaceae – vicariance.
INTRODUCTION
The Neotropical flora has many plant groups that
appear to be unique to the area. South America was
part of Gondwana, and after break up of Gondwana
(c. 155–65 Mya; Jokat et al., 2003) it was physically
isolated until the formation of the Isthmus of Panama
(c. 18–5 Mya; Knowlton & Weigt, 1998) connected
it to North America, although exchange of plants
and animals from the Northern Hemisphere is now
known to have occurred long before this date (see
*Corresponding author. E-mail:
maarten.christenhusz@helsinki.fi
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© 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 171, 277–286 277

2. many examples below). The origins, relationships and
distribution of Neotropical plant families and genera
have therefore always elicited great interest.
In historical biogeography, there are two estab-
lished theories to explain biotic distributions: disper-
sal and vicariance. In the dispersalist view, organisms
spread from a centre of origin and by chance cross
natural barriers (such as oceans, deserts, mountain
ranges) that are unlikely to be frequently traversed.
Thus, they manage to become established beyond the
range of their former predators, diseases, pollinators,
seed dispersers, symbionts, etc. After dispersal and
establishment, evolution of the source and founder
populations takes place independently, usually result-
ing in speciation. Centres of origin as a concept has
been widely criticized as ad hoc, and the earlier close
association of this concept with dispersalist views led
some authors to reject dispersal as equally ad hoc
(e.g. Nelson & Platnick, 1981). However, dispersal
hypotheses can be rigorously tested independently,
and do not depend on assuming a centre of origin for
a taxon.
The vicariance hypothesis assumes that a species
was already dispersed over a wide area and subse-
quently becomes fragmented as geographical or biotic
barriers develop. In explaining general biogeographi-
cal patterns, the major vicariant factor is continental
drift (plate tectonics). Even though these two schools
of thought have been viewed as competing and views
have been strongly polarized among biogeographers
in the past (de Queiroz, 2005; Encyclopaedia Britan-
nica, 2012), most distributions can be explained by a
combination of dispersal and vicariant events. Prior
to the advent of molecular systematics, most biogeo-
graphers thought that vicariance must be the under-
lying mechanism accounting for most of the diversity
observed today, with dispersal operating much more
sporadically and more recently (Lomolino, Riddle &
Brown, 2006).
Dating of molecular phylogenetic trees, particularly
with multiple fossil constraints and Bayesian ap-
proaches (Bell, Soltis & Soltis, 2005, 2010), and
increased knowledge of species distribution patterns
allows us to assess dispersability of Neotropical plant
families. Here we review the historical biogeography
of many plant groups and show that the biogeography
of plants is not always as straightforward as has
previously been assumed.
VICARIANCE THROUGH PLATE TECTONICS
Plate tectonics was frequently considered to be the
primary agent responsible for the distribution of
widespread plant families (e.g. Raven & Axelrod,
1975), especially families with a fossil record old
enough to suggest a Gondwanan origin. However,
there are few good examples that show geographical
persistence of groups over sufficiently long periods
of time to explain continental separation of clades
through plate tectonics. This would have to involve
old plant lineages with limited dispersibility. The best
examples are probably relictual plant groups such as
Thyrsopteridaceae and Lactoridaceae, known from
fossils from various parts of South America, where
they became extinct. Currently these two families are
represented by a single species each on the recent
volcanic Juan Fernández Islands (Stuessy, Crawford
& Marticorena, 1990), but their earlier fossil distri-
butions and those of their relatives appear to fit the
vicariant hypothesis.
Another likely relict distribution can be found in
Strelitziaceae, a family with a crown group that may
be sufficiently old to be Gondwanan (stem and crown
groups dated at 78 and 59 Mya, respectively; Janssen
& Bremer, 2004), with Phenakospermum guianense
(Rich.) Miq. in northern South America, Ravenala
madagascariensis Sonn. (and perhaps an additional
undescribed species) in Madagascar, and the slightly
more diverse genus Strelitzia Aiton in southern
Africa. Fossils reported for the related families Heli-
coniaceae and Musaceae (Zingiberales) from the
Arctic in the Tertiary (Boyd, 1992) increase the com-
plexity of this hypothesis but do not rule Gondwana
out as an explanation for the relict distribution of
Strelitziaceae. It is possible this was once a more
widespread group that became separated by drift of
the continents and then suffered extensive extinction
in most areas while its relatives in other families
were more widely distributed.
Nevertheless, there are plant families in the South-
ern Hemisphere that were once considered to be dis-
tributed via plate tectonics but do not have sufficient
age for their current distribution to be explained by
breakup of Gondwana, such as Berberidopsidaceae,
Nothofagaceae, Restionaceae and Proteaceae. The
stem group of Nothofagaceae, for instance, is certainly
old enough to have been widespread before the break
up of Gondwana (up to 95 Mya), but Cook & Crisp
(2005) estimated the crown group to be too recent, and
the sequence of break up of continental areas does not
fit relationships of the clades in Nothofagus Blume.
Other groups, such as Araucariaceae, cycads,
Gnetaceae and several fern families, have crown
group nodes that are certainly old enough, but extant
genera and species have been shown to have only
recently radiated (e.g. Pryer et al., 2004; Nagalingum
et al., 2011), and general patterns observed in these
families do not fit well in a tectonic framework (i.e.
clades within these families do not reflect the known
sequence of Gondwanan disintegration). Although not
satisfying from a plate tectonic perspective, most
groups that have been purported to exhibit vicariant
278 M. J. M. CHRISTENHUSZ and M. W. CHASE
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3. patterns have turned out to be too young by molecular
clock estimates to be around at the appropriate time,
and when the phylogenetic patterns have been clari-
fied the expected area relationships have not been
realized (as in the case of Nothofagus above). Disper-
sal is stochastic in nature and related to factors such
as niche conservation and dispersal capabilities
(Crisp et al., 2009), although some authors have cited
dispersal as unpredictable, which, if true, would
justify the critique of Nelson & Platnick (1981). If
dispersal is stochastic in nature, then it can be mod-
elled (as in the unified neutral theory of biodiversity
and biogeography; Hubbell, 2001), and dispersal is
then clearly in the ascendency as the preferred expla-
nation for most groups of plants in the study of
historical biogeography.
LONG-DISTANCE DISPERSAL
In the case of intercontinental distributions, disper-
salist biogeography considers long-distance dispersal
across oceans to South America (mainly from Africa
and, to a lesser extent, from Australasia simply
because of the greater distances involved) to be com-
monplace, something that has been disputed often in
the past, due to the lack of evidence for dispersal
vectors for the seeds or spores in question. Plants
with these types of distributions have often been
assumed to be species with great dispersability,
usually by wind, water or migratory birds, and often
are plants that are opportunistic and highly competi-
tive. However, these sorts of assumptions do not take
into consideration the long periods of evolutionary
time involved, and this evens out what appear to be
different capacities for dispersal. Dispersal events
may be rare, but given enough time, then most groups
of plants demonstrate the capacity to travel over
great distances, regardless of their seed or spore
characteristics (Dick, Abdul-Salim & Bermingham,
2003), probably through non-standard means of dis-
persal (e.g. by methods of transport for which they
appear not to be adapted; Richardson et al., 2001;
Higgins, Nathan & Cain, 2003; Pennington, Richard-
son & Lavin, 2006).
Coastal species being dispersed over long distances
by sea currents is the most easily believed scenario,
and many cases of seeds of such plants remaining
viable after prolonged periods in seawater exist.
Darwin (1856) immersed various seeds in seawater to
determine their viability, and the fact that coconuts
occasionally wash ashore in Europe (e.g. Cadée &
Rühland, 2010) also shows that distance is no
problem for these sea-dispersers. However, it is
becoming clear that such over-water dispersal is not
limited to species with seeds resistant to the effects of
seawater. Even massive seeds, such as those of Sym-
phonia globulifera L.f. (Clusiaceae sensu Dick et al.,
2003; APG III, 2009), seem to move as easily as those
that clearly have characteristics that suggest they
can be dispersed effectively.
The difference between truly long-distance disper-
sal and stepping-stone dispersal (or short- to medium-
distance dispersal) is only a matter of distance, one
factor in the probability of dispersal. Usually, disper-
sal over longer distances has been considered to be
influenced by the degree of vagility of the dissem-
inules (e.g. size, resistance, shape, viability and
physical characteristics) and time. Because long-
distance dispersal is potentially continuously occur-
ring, it is implied that it could have occurred recently,
as can be observed by the dispersal to new or recent
distant oceanic islands in the Caribbean, Mascarenes
and Polynesia. However, one has to be careful about
assuming probabilities of long-distance dispersal
simply based on current distribution and vagility of
disseminules.
The almost exclusively New World family Cacta-
ceae occurs from Canada to Chile, but, as is often the
case, is most diverse in the tropical areas between
these two. They have their greatest diversity in
deserts and seasonally dry tropical forests/woodlands
(e.g. Brazilian caatinga) and in the Old World their
niche is (mostly) filled by species of Aloë L. (Xan-
thorrhoeaceae) and succulent species of Euphorbia
L. (Euphorbiaceae). Fruits of Cactaceae are small
bird-distributed berries, and a number of species have
become worldwide invasives, indicating the degree to
which they could occupy suitable habitats anywhere.
Only one species, Rhipsalis baccata (J.S.Muell.)
Stearn, an epiphyte in the canopy of rainforests (but
also found on city trees) that occurs throughout tropi-
cal South America, is also native in Africa, Madagas-
car, Sri Lanka and the Mascarenes. It has been
generally assumed that R. baccata in Africa is the
result of long-distance dispersal, probably by birds,
but Maxwell (http://www.rhipsalis.com/maxwell.htm)
and others find this unlikely (because questions
remain as to why only this species has made the leap
and which, if any, bird frequents coastal rainforests
and flies across the Atlantic) and thus stated that it
must be old and its distribution due to vicariance.
This hypothesis, however, would make Rhipsalis
Gaertn. an ancient genus (older than the break-up of
Gondwana), but recent molecular analyses have
placed it in the crown group, subfamily Cactoideae,
which is dated to c. 21 Mya (Arakaki et al., 2011) and
therefore decidedly not Gondwanan; of course, Rhip-
salis is undoubtedly much younger than the crown
age of Cactoideae. Also there are many older groups of
cacti that are not in Africa. Additionally, berries of
Rhipsalis are sticky so they must have hitched a ride
from its centre of diversity in coastal eastern Brazil to
BIOGEOGRAPHY OF NEOTROPICAL PLANTS 279
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4. West Africa. Such rare single occurrences must be
anticipated in groups that are otherwise poorly
adapted for long-distance dispersal, although it
appears to have happened only for a single species of
Cactaceae (which then radiated with polyploidy in the
Indian Ocean). If viewed in the context of the stochas-
tic nature of dispersal, then a wet-forest species is the
most likely to establish after long-distance dispersal
not because it can more easily disperse (due to its
seed structure), but rather because its ecology makes
it better able to tolerate such disturbance. The rela-
tively recent dispersal of another species, Pitcairnia
feliciana (A.Chev.) Harms & Mildbr. (Bromeliaceae),
is a similar such instance and is estimated to have
occurred about 9 Mya (Givnish et al., 2008).
Another taxon traditionally associated with the
Neotropics is the papaya family Caricaceae. The stem
lineage of Caricaceae split from its Afromadagascan/
Indian sister family Moringaceae around 58–60 Mya
(Wikström, Savolainen & Chase, 2001). In Africa,
Caricaceae include only Cylicomorpha Urb., with two
species of disjunct distribution: one, C. solmsii (Urb.)
Urb., in West Africa and another, C. parviflora Urb.,
in East Africa. In contrast, there are four Neotropical
genera: Carica L., Jacaratia A.DC., Jarilla Rusby and
Vasconcellea A.St.-Hil., the last having the greatest
diversity with c. 20 species. The African species of
Cylicomorpha are sister to the rest of the family
(Kyndt et al., 2005), which, together with the knowl-
edge that Moringaceae are sister to Caricaceae, indi-
cates that the family probably evolved in Africa. A
much more recent long-distance dispersal from Africa
to America led to a Neotropical radiation. Based
purely on their distribution and without a molecular
clock, one could argue for a tectonic (vicariant) expla-
nation, but this has obviously not been the case.
These examples show that long-distance dispersal is
not unidirectional and species-richness is not neces-
sarily an indication of taxon origin.
A different example in which plate tectonics was
assumed to have played a role is the genus Nicotiana
L., which has the greatest number of species in the
Americas (49 species) but is also represented by the
allotetraploid N. section Suaveolentes in Australia (25
species) and Africa (one species in Namibia). Molecu-
lar studies have demonstrated that the Old World
species are allotetraploids approximately 10 Myr old
and derived from parental lineages now restricted
to the Americas (Chase et al., 2003; Clarkson et al.,
2004). The seeds of Nicotiana are small, but not
wind-dispersed, and the age of section Suaveolentes
definitely rules out other explanations; it seems clear
that long distances across oceans are being covered
by taxa without any clear adaptations for travelling
such distances. Explanations such as the boreotropics
hypothesis cannot be invoked in these cases. Other
groups also demonstrate dispersal over great dis-
tances in the Pacific and Indian Oceans, such as
Melastomataceae (Renner & Meyer, 2001; Renner,
Clausing & Meyer, 2001).
There are many other groups for which vicariance
through plate tectonics was assumed to play the
major role in explaining the distribution patterns, but
long-distance dispersal is actually more important in
explaining their current distribution. Good examples
are found in Araceae (Cabrera et al., 2008), Arecaceae
(Couvreur, Forest & Baker, 2011), Ebenaceae (Duang-
jai et al., 2006), Ericales (Sytsma et al., 2006),
Euphorbiaceae (Wurdack et al., 2004), Lauraceae
(Chanderbali, van der Werff & Renner, 2001), Sapin-
daceae (Buerki et al., 2011) and Phyllanthaceae
(Wurdack, Hoffmann & Chase, 2005), all families
with relatively large seeds, but in which all major
clades have representatives on several major land-
masses and are thought to have dispersed across the
oceans several times during their evolutionary
history. These sorts of examples are becoming so
frequent that it is rare to observe families in which
major clades isolated by continents occur.
BOREOTROPICS HYPOTHESIS
Long-distance movement from Africa and Eurasia to
the Neotropics (including South America) does not in
all cases have to involve dispersal across water. An
excellent alternative is reflected in the boreotropics
hypothesis of Wolfe (1975), Tiffney (1985a, b) and
Tiffney & Manchester (2001). This was based on fossil
taxa that were shared by North America and Europe
during the Eocene, but also had relatives in Central
America/Caribbean and the Asian tropics. With
greater distances between Europe and North America
and decreasing global temperatures in the early Oli-
gocene, this tropical connection was lost, and the taxa
involved were pushed further south, including into
South America. Thus, relationships of these lineages
in South America are not directly linked to African
taxa, but rather to species further north in the Ameri-
cas and southern Europe; they also date from much
more recent times (30–40 Mya rather than > 65 Mya).
Such patterns of relationships have been demon-
strated for several families, in particular Fabaceae
(Lavin & Luckow, 1993), based on cladistic analysis of
morphological data. Later, molecular data (Wojcie-
chowski, Lavin & Sanderson, 2004), to which rates
and age estimates were added by Lavin et al. (2004),
supported this hypothesis for legumes (Lavin, Her-
endeen & Wojciechowski, 2005).
Another example in which a tectonic explanation
was expected but another set of relationships
emerged with molecular analyses was Amaryllidoi-
deae (Amaryllidaceae; Meerow et al., 1999). In this
280 M. J. M. CHRISTENHUSZ and M. W. CHASE
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5. analysis, South American genera were not sister to
the South African clades, but rather to those from
further north in the Americas and then to Eurasian
genera. Given later information about molecular
clocks and ages of clades in the monocots, this also
supports a boreotropics explanation for the tropical
American and Andean taxa in Amaryllidoideae. Other
examples of a boreotropics explanation include genera
of Annonaceae (Scharaschkin & Doyle, 2005; Erkens,
Maas & Couvreur, 2009; Couvreur et al., 2011;
Erkens, Chatrou & Couvreur, 2012), Lauraceae
(Chanderbali et al., 2001) and Malpighiaceae (Davis
et al., 2002, 2004).
SURPRISING VICARIANT PATTERNS
OF RELATIONSHIPS
Two families for which geographical patterns reflect-
ing tectonic movements predominate are Maratti-
aceae and Orchidaceae, both of which have diaspores
that could potentially be dispersed effectively over
great distances. As for angiosperms, it is clear that
some ferns do disperse over great distances and some
have done so only recently (Rouhan et al., 2012).
Others are clearly doing something different. Marat-
tiaceae are an ancient fern family with a fossil history
dating back to the Carboniferous (Christenhusz,
2007). The extant genera are of somewhat more
recent descent, but they are restricted to either the
Palaeotropics (Angiopteris Hoffm., Christensenia
Maxon and Ptisana Murdock) or the Neotropics
(Danaea Sm., Eupodium J.Sm. and Marattia Sw.).
Although their spores are relatively large among
ferns, dispersability in these genera does not appear
to be a limiting factor: Neotropical Danaea has occu-
pied all the wet Caribbean islands and reached Cocos
Island in the Pacific (Christenhusz, 2010), Marattia
has reached as far as Hawaii, with its closest relative
in Jamaica (Murdock, 2008), Palaeotropical Ptisana is
represented on Ascension Island in the mid-Atlantic,
New Zealand and New Caledonia, and Angiopteris is
widespread in the Indian and Pacific Oceans, with
outliers in Madagascar, the Seychelles, Polynesia and
Pitcairn Island. Despite this ability to establish on
isolated oceanic islands, Danaea and Marattia are not
found in the Palaeotropics and Angiopteris and
Ptisana are absent from the Neotropics. It has been
shown that when brought as plants by humans Angi-
opteris can successfully invade Neotropical areas
(Christenhusz & Toivonen, 2008), but they are not
able to do this on their own accord.
In Orchidaceae, seeds are nearly always wind-
dispersed, but there are only a few cosmopolitan
genera, e.g. only Bulbophyllum Thouars (Smidt et al.,
2011) and Polystachya Hook. (Russell et al., 2010b). In
the case of Bulbophyllum, there are major clades
confined to continental areas (Gravendeel et al., 2004;
Fischer et al., 2007; Smidt et al., 2011), so, although
the genus as a whole is cosmopolitan, its internal
structure reflects more limited dispersability. In Polys-
tachya, one clade has managed recently to migrate
from Africa to both the New World and the Asian
tropics (Russell et al., 2010a, 2011), a situation again of
a wet-forest species, much akin to the situation with
Rhipsalis (see above), becoming successfully estab-
lished after long-distance dispersal. Despite these
examples of long-distance dispersal, major clades
in Orchidaceae are confined to single continents
(Cameron et al., 1999; Górniak, Paun & Chase, 2010).
For example, at the subtribal level (sensu Chase
et al., 2003) Catasetinae, Laeliinae, Maxillariinae,
Oncidiinae, Pleurothallidinae, Stanhopeinae and
Zygopetalinae are all exclusively New World, whereas
Coelogyninae, Collabiinae, Dendrobiinae, Eriinae and
Podochilinae are Asian except for one species of Calan-
the R.Br. (Collabiinae). Angraecinae and Aerangidae
are all African/Madagascan except for one group of
genera that has recently reached the New World
tropics (Carlsward, Whitten & Williams, 2003). At the
subfamily and tribal levels, such restricted distribu-
tions are much rarer, but at these taxonomic levels
older tectonic explanations become feasible; the
orchids diverged from their sister clade about 110 Mya,
and all five subfamilies (the orchid crown group)
existed before the end of the Cretaceous (Chase, 2001;
Ramírez et al., 2007; Gustafsson, Verola & Antonelli,
2010), so we could expect that the oldest clades would
be likely to have evolved at a time when the continents
were amalgamated and thus these groups have
broader distributions dissected by plate tectonics. This
is in fact the pattern that would have been expected in
other old families, such as Lauraceae, but has not been
observed.
For groups in which the phylogenetic patterns do
not reflect what we expect through long-distance dis-
persal, even though they are capable of it (e.g. groups
are restricted to continents), we have to look at
factors of their life-history strategies that would limit
their successful establishment elsewhere; their dis-
persal is taken for granted, given the successful long-
distance dispersals that characterize families more
poorly adapted to transoceanic migration, such as
Ebenaceae, Fabaceae and Lauraceae. In the cases of
both Orchidaceae and Marattiaceae it is possible that
mycorrhizal relationships are behind their inability
to establish following intercontinental dispersal,
although this would also be expected to prevent their
artificial introduction and subsequent invasiveness.
Perhaps in these cases, their mycorrhizal associates
have also been introduced when the mother plants
were transplanted.
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6. Another possible explanation is that old herbaceous
families (palaeoherbs) do not exhibit the same pro-
pensity for establishment as woody families once dis-
persal occurs. We examined this topic by looking at
other old tropical herbaceous families and found evi-
dence to the contrary. Araceae (Cabrera et al., 2008)
and Aristolochiaceae (particularly Aristolochia L.;
Ohi-Toma et al., 2006) exhibit relationships indicating
frequent long-distance dispersal, just as in the tropi-
cal woody families. The situation in Piperaceae (Jara-
millo & Manos, 2001; Wanke et al., 2006) is less clear,
but the sampling of species is highly skewed and not
particularly suitable to address this question. It may
also be that the particular ecological preferences of
orchid and marattioid fern species are at least partly
responsible for their apparent lack of successful dis-
persal and establishment and this possibility should
be examined.
SURPRISING BIOGEOGRAPHICAL
PATTERNS OF UNEXPECTED LINEAGES
Families that exhibit widely disjunct distributions
could fall into one of the above categories (although
we admit that they are not really distinct), depending
on their age. For example, Coriariaceae have about
30 species in the single genus Coriaria Niss. ex L.,
disjunctly distributed in Central and South America,
the Mediterranean, East Asia, some Pacific Islands
and New Zealand. To achieve such a distribution
must have required long-distance dispersal, but that
they were related was obvious from the start of their
recognition as a genus. Lardizabalaceae, a family
disjunctly distributed in eastern Asia and southern
South America (Qin, 1997; Christenhusz, 2012) is
another such potential case. There are fossils known
from the Oligocene of Europe and the Late Cretaceous
of North America (Page, 1970; Mai, 1980), but the age
of the South American lineage is probably not old
enough to explain its occurrence there by vicariance.
Other cases have only come to light since the wide-
spread use of molecular systematics to look at rela-
tionships within and between families. For example,
the APG III classification (APG III, 2009) contains
two orders that no one had recognized before the
advent of molecular systematics: Crossosomatales
and Huerteales.
Crossosomatales are composed of seven families
(Aphloiaceae, Crossosomataceae, Geissolomataceae,
Guamatelaceae, Stachyuraceae, Staphyleaceae,
Strasburgeriaceae), all of which are monogeneric
except for Crossosomataceae (four genera) and Stras-
burgeriaceae (two genera). Most genera are also
monospecific, so this order is really composed of an
odd set of taxa, formerly included by previous authors
in other families such as Flacourtiaceae [Aphloia
(DC.) Benn.], Ochnaceae (Strasburgeria Baill.),
Rosaceae (Guamatela Donn.Sm.) and Saxifragaceae
(Ixerba A.Cunn.) from a diverse set of orders. Only
seed anatomy seems to be a clear synapomorphy for
the order (Matthews & Endress, 2005). Guamatela, in
particular, was only recently recognized as a member
of Crossosomatales (Oh & Potter, 2006), but as far as
it has been studied it appears to fit reasonably well
there, although it is not morphologically similar
enough to be accommodated into one of the pre-
existing families of the order. Crossosomatales are
split into two major clades, a Southern and a North-
ern Hemisphere clade. Wikström et al. (2001) dated
the age of stem Crossosomatales at 91 Ma, but Magal-
lón & Castillo (2009) came to an earlier age of
106 Ma. This is clearly old enough for plate tectonics
to have played a role in their current distribution,
and the crown groups were estimated to have evolved
about 30 Myr later, so there is probably an overlay of
some long-distance dispersal as well. For example,
Staphyleaceae are now found throughout the North
Temperate zone as well as into tropical Southeast
Asia and the Americas, and this radiation occurred
too recently to be an effect of tectonics; however, a
primary influence of plate tectonics seems most
obvious, and their now largely disjunct distribution
appears to us likely to have been caused by a mixture
of vicariance and long-distance dispersal, some of it
occurring within the last 20 Myr.
Huerteales were only described as an order with
three families by Worberg et al. (2009). The order
was subsequently accepted in APG III (2009) and
then a fourth family was added (Petenaeaceae; Chris-
tenhusz et al., 2010). Similar to Crossosomatales,
only characters of seed anatomy that link these four
small families [Dipentodontaceae (two genera), Ger-
rardinaceae (one genus), Petenaeaceae (one genus)
and Tapisciaceae (two genera)] have been identified
(Christenhusz et al., 2010). As for Crossosomatales,
most of these genera were previously placed in other
families: Celastraceae (Perrottetia Kunth), Elaeocar-
paceae (Petenaea Lundell), Flacourtiaceae (Gerrar-
dina Oliv.) and Staphyleaceae (Huertea Ruiz & Pav.
and Tapiscia Oliv.), and ordinal placements were like-
wise diverse. Petenaeaceae were found to be related
to the recently described South African family Ger-
rardinaceae (Christenhusz et al., 2010). The distribu-
tion of a Central American species distantly related to
an African genus follows the pattern in other families
of Huerteales, e.g. Tapisciaceae, with Tapiscia
restricted to central China, whereas Huertea is from
Neotropical mountains. Unlike Crossosomatales,
there is no clear split into northern and southern
clades for Huerteales. The age of the overall clade is
more recent than Crossosomatales (Wikström et al.,
282 M. J. M. CHRISTENHUSZ and M. W. CHASE
© 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 171, 277–286

7. 2001), and thus relationships in Huerteales seem less
likely to be relictual and more likely to have resulted
from dispersal rather than vicariance, although a
boreotropics hypothesis with extinction in the higher
latitudes is also a possible scenario; probably, a com-
bination of the two is responsible for the overall
pattern.
CONCLUSIONS
We can now state without hesitation that the flora
of the Neotropics, and of the Americas in general,
has many more shared relationships with the Palaeo-
tropics than ever before imagined (Pennington &
Dick, 2004; Pennington, Cronk & Richardson, 2004;
Renner, 2004; Sanmartín & Ronquist, 2004; San-
martín, Wanntorp & Winkworth, 2007). Most families
of land plants have more easily dispersed across the
oceans than many ever thought possible, especially
given the lack of vagility of their propagules and their
apparent inability to survive long periods in seawater.
Phytogeographical distributions appear to be mostly
explainable through dispersal, although in a few rare
cases an underlying effect of vicariance through plate
tectonics can be observed. Many Neotropical connec-
tions are with plants that have a decidedly temperate
nature (e.g. Amaryllidaceae), so it is also possible that
some dispersal took place via land routes (e.g. the
boreotropical route). Plants appear to have gone back
and forth between the continents frequently and can
successfully become established on a new continent
when the ancestral ecology is preserved in their new
home (e.g. when metacommunity processes are at
work; Lavin et al., 2004; Crisp et al., 2009). Given the
timescales involved, rare events begin to loom large,
and in the results of molecular phylogenetic analyses
combined with molecular clocks we now find evidence
for relationships of which we were previously not
cognizant due to the lack of obvious morphological
synapomorphies for the taxa involved.
ACKNOWLEDGEMENTS
We thank Alex Antonelli, Colin Hughes and Toby
Pennington for organizing the symposium at the
International Botanical Conference in Melbourne.
Constructive reviews by Chuck Davis and Matt Lavin
made us rethink some of our ideas and improved the
bibliography of our paper. We are grateful for their
comments.
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