Download Cactus Biogeography

Cladistics of Early Diverging Cactus Lineages
Biogeography: GSI Supplement
The Cactus Family is primarily a New World group with centers of diversity in
North and South America. The geographical origin of the Cactaceae is difficult to infer
because there is insufficient knowledge of its closest relatives. Although recent
phylogenetic evidence places Cactaceae within a paraphyletic Portulacaceae, it is
uncertain which taxa are sister to Cactaceae.
Hypotheses of geographic origin of the Cactaceae are based on which lineages are
thought to be basal in the group. The Caribbean and central South America have been
suggested as possible places of origin because Pereskia and certain opuntioid and cactoid
lineages, considered “ancestral” by some researchers, are located there. Others have
argued for an Andean origin due to alternative ideas of ancestral lineages within
Cactaceae. Northwestern South America has also been suggested because the family was
thought to have originated during the late Cretaceous. This placement of the Cactus
Family’s geographical origin as far away from Africa as possible might explain its poor
representation in Africa. However, recent data shows limited sequence divergence
between cactus subclades suggesting that the Cactaceae originated after the Cretaceous
and is therefore a younger group.
In the Cactus Cladistics lab exercise, morphological and molecular characters
were used to build cladograms of the Cactaceae, and these characters were then
interpreted in the context of the resulting cladogram. New characters can be interpreted
in the same way by mapping them onto the existing cladogram. In this exercise, the
Cactaceae cladogram will be used to study the biogeographic patterns of the early
diverging cactus lineages. Biogeographic areas have been coded as different character
states and using the program MacClade the characters are “mapped” onto the cladogram
to determine the character states of the internal nodes. Internal nodes on a cladogram
represent hypothetical ancestors and character mapping allows for the investigation of
character evolution.
A Cactaceae phylogeny with biogeographical characters mapped onto it is
provided in a MacClade file entitled “biogeography“. This phylogeny includes more taxa
than was used in the lab exercise because biogeographic information is lost when using
the pruned taxa list and cladogram from lab. The MacClade file includes cladograms
with biogeography coded in two different ways. The first cladogram (Tree 1) has
biogeography coded as broad areas (Tree 1: Widespread, Northern, and Southern), while
the second cladogram uses biogeography coded with areas that are more divided (Tree 2:
Caribbean, Andean, and southern South America). This demonstrates how character
coding can affect the interpretation of those characters on a cladogram. For instance,
Tree 1 clearly shows a distinct separation between Northern and Southern taxa at the base
of the Cactaceae cladogram implying an initial north-south geographic split in the group.
This separation is not as readily visible when biogeography is coded using more divided
character states as shown in Tree 2.
In the lab exercise we learned that Pereskia made up the early diverging lineages
within Cactaceae forming a paraphyletic group. Tree 2 provides the following additional
information within each geographic area. The Pereskia clade that is sister to the rest of
the family consists of taxa found in northern South America, Central America, and the
Caribbean (the Northern clade). Within the Northern clade, there is a Caribbean-centered
group that contains P. bleo, P. quisqueyana, P. marcanoi, P. portulacifolia, and P.
zinniiflora. Pereskia aureiflora is also a member of this clade but is found in southern
Brazil and may represent a dispersal event to this region from the north. The remaining
Pereskia taxa are found in southern South America (Southern clade). P. horrida, P. diazromeroana, and P. weberiana are found in the dry Inter-Andean valley regions of Peru
and Bolivia (Andean clade). This clade is sister to P. aculeata, which is the only
geographically widespread and ecologically diverse Pereskia species.
Maihuenia is restricted to Chile and Argentina, and is sister to the Cactoideae.
The cladogram (Tree 1) implies that both the Cactoideae and Opuntiodeae originated in
the southern half of South America (Southern clade). This region’s geologic history
suggests that the diversification of these groups likely occurred under a fluctuating
arid/semi-arid climate regime during the early uplift in the central Andean region (~20-25
mya). The combination of climatic and geologic changes in this region has been viewed
as the cause for the diversification of many plant groups. Movement northward to
Mexico and the US occurred early within particular lineages (e.g. Cacteae represented by
Echinocactus).
Fig. 1: Cladogram with geography coded broadly. 1 = Cactoideae, 2 = Opuntiodeae.
Fig. 2: Cladogram with more finely divided geographic areas added. 1 = Cactoideae, 2 =
Opuntiodeae.
Geographic distribution of basal cacti. Species ranges correspond to shaded area, except
for the geographically widespread taxa: Pereskia aculeata (a circle), the Opuntioideae (a
triangle), and all Cactoideae minus Blossfeldia (a triangle).