Point of View
Syst. Biol. 61(4):703–708, 2012
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DOI:10.1093/sysbio/sys005
Advance Access publication on January 10, 2012
Searching for Ancestral Areas and Artifactual Centers of Origin in Biogeography:
with Comment on East–West Patterns Across Southern Australia
PAULINE Y. L ADIGES ∗ , M ICHAEL J. B AYLY ,
AND
G ARETH N ELSON
School of Botany, The University of Melbourne, Victoria 3010, Australia
∗ Correspondence to be sent to: E-mail: [email protected].
Received 14 September 2011; reviews returned 29 November 2011; accepted 3 January 2012
Associate Editor: Adrian Paterson
Stemming from acceptance of continental drift theory
and development of cladistic systematics in the 1960s,
interest in historical biogeography was revived. The
field and methodology have since been reviewed several times (e.g., Nelson and Platnick 1981; Myers and
Giller 1988; Morrone and Carpenter 1994; Humphries
and Parenti 1999; Crisci et al. 2003; Ebach and Tangney
2007; Santos 2007; Parenti and Ebach 2009). In essence,
early methods focussed on discovering relationships of
areas based on phylogenies of taxa endemic to those
areas and biogeographic patterns presented as summary area cladograms. Comparisons across different
groups (clades) of organisms revealing congruent patterns were interpreted as evidence of simple vicariance,
which could be compared with other data, such as geological breakup of Gondwana, whereas incongruence of
patterns was interpreted as requiring a complex explanation or stochastic long-distance dispersal (Nelson and
Platnick 1981). A plethora of methods to determine area
cladograms arose to deal with widespread taxa (representing range expansions or a failure to differentiate),
missing areas (extinctions), and other complications.
But by the 1990s, there was no unifying accepted biogeographic method (Nelson and Ladiges 2001), and
different methods applied to the same data gave different results, indicating that at least some of the results
are artifacts of the method.
What is the current trend in methodology? Most
recent studies follow a similar procedure: reconstruct
taxon relationships based on molecular data, estimate
clade divergences using a molecular dating method
(sometimes employing few or distant fossils or only
secondary calibration points), and employ a method of
ancestral area reconstruction to determine areas at the
internal nodes of the phylogenetic tree. The Progression
Rule (acknowledged or implied) is often invoked to explain the results with interpretation constrained by the
estimated ages of divergences.
T HE P ROGRESSION R ULE
The Progression Rule is based on the ideas of Hennig
(1966) and Brundin (1966) and assumes that the most
primitive or basal taxa are likely to occur at the center
of origin, indicating the ancestral area of the group. Progression from the basal nodes to the most derived is interpreted as the direction of dispersal of the group. To
illustrate the argumentation, Hennig (1966, figure 40,
p. 136) presented a phylogeny of 5 subspecies of flies in
Mimegralla albimana, which occur in areas 1–5 in Southeast Asia from west to east to Tonga, although the relationships of the subspecies he described do not match
the exact pattern in his Figure 40, which showed the tree
1(2(3(4,5))). Other patterns interpreted using the progression rule (e.g., Ross 1974, p. 214–219) include situations where particular areas occur more than once, such
as in the hypothetical area relationship D(D(C(B(B,A))))
where areas D and B are “repeated,” as discussed by
Parenti (2007, p. 63–71).
G EOGRAPHIC PARALOGY AND C ENTERS OF O RIGIN
We have argued previously that the Progression Rule
is invoked where there is geographic paralogy, with
artifactual results (Nelson and Ladiges 1996; Nelson
and Ladiges 2001). In molecular systematics, “paralogy” refers to misleading comparison between duplicated genes that have independent histories (Fitch 1970).
Geographic paralogy is analogous to this use and is
evident where there are partly or wholly overlapping
distributions for different taxa—duplicated geographies
of taxa that have had independent histories (Nelson and
Ladiges 1996; Parenti and Ebach 2009). For example, in
redFigure 1, the only information of area relationships in
the tree is A(B + C), geographically informative nodes
being 6 and 7; nodes 1–5 we see as paralogous and
thus uninformative. In this example, the area relationship A(B + C) can be interpreted as a result of a vicariance event that affected 2 clades (nodes 4 and 5) and
which occurred after speciation events at older nodes 1–
3. Crisci (2001) also provides an example (his figure 2,
p. 161) illustrating paralogy and sympatry of taxa. Similarly, in the example D(D(C(B(B,A)))) above, nodes
leading to the repeated areas D and B are paralagous.
Ancestral area analyses (e.g., Ronquist 1997; Ree et al.
2005) that attempt to push optimization down to the
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and Givnish (2002) for Banksia (see below). Among elements of the fauna, hypotheses of the origin of frog genera have suggested repeated invasions in the opposite
direction, from east to west (e.g., Main et al. 1958;
Littlejohn 1981; see discussion by Morgan et al. 2007).
Recently, Unmack et al. (2011) have argued for multiple movements (invasions) of pygmy perches across
southern Australia prior to Mid Miocene aridity because eastern and western lineages are not “reciprocally
monophyletic.”
FIGURE 1. Hypothetical example of geographic paralogy: taxa I–
VIII in areas A–C. Nodes 6 and 7 are informative of the area relations
A(B,C); lower paralogous nodes are uninformative.
lowest nodes on a tree, without regard for paralogous
nodes, “find” centers of origin and conclude dispersal. Indeed, a center of origin seems always to be a
product of paralogous comparison (Ebach 1999). Croizat
et al. (1974) rejected the Darwinian center of origin
and its corollary of dispersal as a conceptual model of
general applicability in historical biogeography, being
inconsistent with allopatric speciation (subdivision of
ancentral biotas). In our view, by recognizing and eliminating geographic paralogy, relationships of areas can
be discovered without recourse to optimization of areas
on internal and basal nodes to “discover” ancestral areas/centers of origin. This is not to dismiss the reality
of dispersal as range expansion. Without other information, area relationships summarized as area cladograms
can be variously interpreted. In the hypothetical example in Figure 1, the area relationships may be interpreted
as dispersal from A to B + C (progression rule) or vicariance and fragmentation of area ABC; in the latter case,
without dispersal from a center of origin, the first split
of the widespread ancestor isolated a lineage in area A
before the later split between the taxa in area B + C. Vicariance may be argued from congruence of geographic
patterns (generalized patterns) and dispersal may be
recognized with reference to sympatry or incongruent
patterns (see Croizat et al. 1974 for examples).
Geographic paralogy is illustrated in the following by
examples of taxa that include endemics in eastern and
southwest Australia, the southwest being a recognized
biodiversity hotspot (Hopper and Gioia 2004).
E AST– WEST PATTERNS A CROSS S OUTHERN
C ONTINENTAL A USTRALIA
Diels (1906) considered the southwest of Western
Australia to be a center of origin of the autochthonous
element of the Australian flora, from which taxa dispersed to the east; some more recent authors have
held similar views for particular plant taxa, for example, Thompson (1976) for Tremandraceae and Mast
Eucalyptus Subgenus Eucalyptus (Family Myrtaceae)
A significant component of the Australian flora with
east–west connections is Eucalyptus subgenus Eucalyptus
(commonly called the “monocalypts”), with 111 species.
Ladiges et al. (2010) recently published a phylogeny
based on morphological and molecular data (Fig. 2).
Eucalyptus subgenus Eucalyptus shows early lineages in
the southwest and an east–west divergence at a higher
node. The pattern of early lineages diverging in the
southwest is not interpreted as evidence of the region
being a center of origin, first because the lower nodes
are paralogous and second because the sister taxon
E. loeziana (subgenus Idiogenes) occurs in the east, which
for a center of origin model would require dispersal
from east to west and back to the east.
Ladiges et al. (2010) argued that the group was
present in the Early Eocene across western and eastern Australia when conditions were warm and wet
(Fig. 3). This time frame is supported by macrofossils
of the sister group Eucalyptus subgenus Symphyomyrtus dated at 52.2 Ma from Patagonia, South America
(Gandolfo et al. 2011), Paleocene–Eocene Eucalyptus
macrofossils from Queensland, Australia (Rozefelds
1996), as well as Late Paleocene fossil pollen from the
Lake Eyre Basin in central Australia (Martin 1994).
The supposition that the eucalypts date back to at
least the early Paleogene is supported also by molecular dating of family Myrtaceae (Sytsma et al. 2004).
Figure 2 shows the earliest lineages of Eucalyptus subgenus Eucalyptus to include tall wet forest species, such
as E. marginata (Jarrah) and taxa that occur today on
lower nutrient soils on weathered lateritic plateaux.
Later lineages include mallee species associated with
granitic and quartzite hills and sands. The divergence
of the western taxa from the monophyletic eastern
group may date back to the earliest phase of marine
inundation during the Late Eocene and major climatic
change at the Eocene/Oligocene boundary. This is consistent with a molecular dating analysis of the eucalypt
group by Crisp et al. (2004); their chronogram suggests
divergence times between southwest and southeast
temperate clades of Eucalyptus subgenus Eucalyptus as
approximately 30–40 Ma. The oldest of the eastern lineages survive today in Queensland in a climate as warm
as that of the Eocene, whereas more derived clades occur
in the cooler southeast.
Of the 111 species in Eucalyptus subgenus Eucalyptus,
there is one that occurs both to the west and to the east
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POINT OF VIEW
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FIGURE 2. Phylogeny of Eucalyptus subgenus Eucalyptus based on sequences of the internal and external transcribed spacers of nuclear
ribosomal DNA, and morphology (based on figure 14.6, p. 280, Ladiges et al. 2010); insert map shows present-day distribution of the subgenus.
Nodes of the tree are numbered, with bootstrap values shown below branches. Geographic distribution of lineages are indicated as W =
southwest Western Australia and E = eastern Australia. Eucalyptus diversifolia is tolerant of calcareous soils and occurs on both sides of the
Nullarbor Plain and was interpreted as having a range expansion from west to east into South Australia and far west Victoria (Wright and
Ladiges 1997; Ladiges et al. 2010).
of the Nullarbor Plain. Eucalyptus diversifolia (Fig. 2) is
a western species tolerant of coastal limestone soils that
has evidently expanded its range eastwards, probably
during periods of low sea level (Wright and Ladiges
1997).
Phebalium Group (Rutaceae) and Tremandra Group
(Former Tremandraceae)
Family Rutaceae is a relatively old family, which has
been estimated using molecular dating as Late Cretaceous (Muellner et al. 2007). The Phebalium group sensu
Wilson (1998) in the family Rutaceae comprises 9 genera
and approximately 97 species of woody shrubs, many
of which are localized endemics found in temperate
Australia. Molecular phylogenetic analysis by Mole
et al. (2004) revealed 3 clades with southwest–southeast
(W–E) connections: their clade E, including genera
Rhadinothamnus, Chorilaena, and Nematolepis, shows the
pattern of W(W,E); in clades D and F, Asterolasia and Phebalium, respectively, show simple W–E divergences.
Tremandra, Platytheca, and Tetratheca (previously family Tremandraceae) form a dry-adapted clade (ca. 49
species) that diverged from rainforest taxa within Elaeocarpaceae during the Paleocene, diversifying in the
Oligocene and Miocene (Crayn et al. 2006). The clade
is distributed in southern temperate Australia, with
Tremandra and Platytheca endemic to the southwest
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SYSTEMATIC BIOLOGY
FIGURE 3. Vicariance scenario for Eucalyptus subgenus Eucalyptus.
(a) Ancestrally widespread distribution based on present-day range
but hypothesized to have been further inland in the past; the sister
group E. cloeziana (Eucalyptus subgenus Idiogenes) shown in the east.
(b) Earliest divergences in Eucalyptus subgenus Eucalyptus in southwest, tall wet forest habitats, increasingly weathered soils and drying.
(c) Marine incursion splits west from east. (d) Eastern clades differentiate; range contraction toward the coast with increased aridity inland.
and Tetratheca, the largest genus occurring in both the
southwest and the east, with rainforest relatives in eastern Australia. Relationships of the dry-adapted taxa and
clades show a pattern similar to that for the Phebalium
group of W(W(W(W(W(W,E))))) (molecular phylogeney
in Butcher et al. 2007, figure 3, p. 133 congruent with the
morphological phylogeny in Downing et al. 2008, figure
13, p. 219).
The area relationships for the Phebalium and Tremandra groups, which have earliest lineages in the southwest and later divergence between west and east, are
congruent with those of Eucalyptus, providing support
for the notion of a general shared history of widespread
ancestral distribution and vicariance. The recognition of
geographic paralogy in these examples and the congruence of the area relationships allows us to reject the hypothesis of Thompson (1976) that the southwest was
the center of origin for the Tremandra group and the
claim of Butcher et al. (2007) that “it is apparent that
Tetratheca [the widespread genus in the group] underwent an early transcontinental radiation, and following
the Eocene separation of the eastern and western populations, there was substantial diversification of lineages
in the western region” (p. 135).
Banksia (Proteaceae) Reinterpreted
The tribe Banksieae (Proteaceae) provides an example of east–west connections that is open to reinterpretation. The group includes 2 subtribes: Musgraveinae,
including 4 eastern species in 2 rainforest genera
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FIGURE 4. (a) Area relationships of subtribes Musgraveinae and
Banksiinae based on figure 5b of Mast and Givnish (2002, p. 1318).
Ancestral areas were derived from DIVA analysis. (b) Alternative interpretation of internal nodes, recognizing geographic paralogy and
the repeated E–W relationship in clades C and P. C = Cryptostomata
clade, P = Phanerostomata clade, W = southwest Western Australia,
and E = eastern Australia.
Musgravea and Austromuellera and Banksiinae, including
sclerophyllous Banksia (91 taxa) and, nested within it,
Dryandra (93 taxa). Mast and Givnish (2002) presented
a molecular phylogeny of the tribe, aiming “to determine the area of origin” for the lineage Banksia +
Dryandra. They used DIVA (Ronquist 1997) to determine
ancestral distributions (Fig. 4a, based on their figure 5,
p. 1318) and concluded that it “identifies the origin of
subtribe Banksiinae as having occurred in southwestern
Australia” (p. 1316). They also commented, however,
that “The origin of subtribe Banksiinae is . . . incongruent with the distribution of the earliest fossils in New
South Wales” (p. 1321). The earliest known fossils are
southeast Banksieaephyllum taylorii of the Late Paleocene,
whereas southwest fossils attributed to Banksiinae are
Middle or Late Eocene; Musgraveinae from the southeast are dated Late Middle Eocene. Mast and Givnish
(2002) concluded that “perhaps there is a problem with
our choice of a vicariance framework” (p. 1321).
Their problem stems from optimization of ancestral
nodes and application of the Progression Rule, which
led to an artifactual result and explanation that the
southwest is the center of origin for Banksia, which
later dispersed (twice, clades C and P, Fig. 4a) to
the east. A simpler explanation, not in conflict with
the fossil record, is similar to that for Eucalyptus subgenus Eucalyptus. The 2 main lineages within Banksia
(Fig. 4b) were ancestrally widespread and experienced a
vicariant event.
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POINT OF VIEW
C ONGRUENCE OF PATTERNS IN S PACE AND T IME
Crisp and Cook (2007) considered competing explanations of multiple dispersal or vicariance and analyzed
various plant groups across southern Australia. Their
approach was to use molecular dating of phylogenies to
compare various plant groups, arguing that any congruent temporal pattern among them would be evidence
of vicariance—similar to the argument based on finding
general area relationships. Crisp and Cook (2007) identified a congruent signal of divergence at a mean age
of approximately 16–14 Ma (confidence limits ranging
back to 30 Ma), indicative of vicariance driven by marine
inundation, the uplift of the Nullarbor limestone plain,
and periods of cooling. Their results also suggested earlier divergences for one of the major clades of Banksia
(ca. 27.5 Ma) and Allocasuarina (21.7 Ma) (range to 40
Ma), corresponding with earlier marine transgressions
and periods of cooling.
With regard to fauna, Morgan et al. (2007) also rejected dispersal (in this case from east to west) for anuran frogs and concluded from their molecular dating
analyses that southwest endemics are ancient and east–
west splits are contemporaneous with splits in elements
of the flora (e.g., Banksia). In contrast, for Australian
pygmy perches that have a disjunct distribution from
the southwest to the east, Unmack et al. (2011) claim
to have a clear example of multiple movements. Their
dated phylogeny shows the area pattern W(E(W,E) and
they state as “inescapable . . . that a minimum of two
migrations/range expansions occurred . . . by two independent lineages.” An alternative interpretation is that
their area tree shows geographic paralogy; the relictual
and sympatric distributions of some of the taxa are suggestive of extinction, for example, in the earlier of the
2 eastern lineages (mean age 21.3 myr) that today includes only one narrow range species (Nannoperca variegata) sympatric with species from the other lineage.
The results of the dating studies of Crisp and Cook
(2007) and Morgan et al. (2007) are in a broad agreement
with conclusions based on patterns of area relationships
described earlier, and the 2 approaches are similar in
using congruence as the test of a vicariance history.
Morgan et al. (2007) recommended that further studies
of other taxa focus on divergence dates and that “exploring the pattern and timing of divergences will be an important step towards increasing our understanding of
the biogeography of southern Australia. This would be
an effective strategy even in the absence of accurate absolute calibration points . . .” (p. 383). However, seeking
congruence among dates of taxon divergences and also
with geological events poses a particular methodological challenge, which Crisp and Cook (2007) attempted
to address by their statistical analysis. Over what period
of time does a vicariant event occur? How variable are
taxa in their evolutionary response to a vicariant event?
How long do separated components of biota remain
undifferentiated if the separated regions remain environmentally similar? How much variation in estimated
divergence times is an accepted tolerance range?
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C ONCLUSIONS
In searching for ancestral areas, artifactual results
and the finding of centers of origin can stem from not
recognizing geographic paralogy. Confidence in interpreting biogeography, based on spatial patterns and/or
temporal patterns, relies on congruence among different
clades.
Comparative studies support the conclusion that the
southwest of Australia, with its high level of endemicity, is not a center of origin but a region that contains
elements that represent old differentiation. Analysis of
examples of extant flora and fauna that are distributed
across the continent reveal lineages that first diverged in
the southwest during the Paleogene, as Australia rifted
from Antarctica and as environments differentiated earlier than in the more humid eastern part of Australia.
The southwest later became isolated by marine inundations and periods of climatic cooling—vicariant processes that led to the isolation and differentiation of
clades from west to east.
F UNDING
This work was supported by Australian Research
Council [ARC LP0455375, LP0776737]; Maud Gibson
Trust, who have supported our work on eucalypts and
Rutaceae.
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