Cactus evolution info

http://www2.labs.agilent.com/botany/cacti_etc/html/evolution.html

 A BRIEF SYNOPSIS OF EVOLUTION IN THE CACTUS FAMILY

    As in nearly all fields of science, it seems we DON'T know more
than we DO know, but if you will forgive the brevity of discussion
for each of these enormous topics, I'll try to give a summary of what
we do know about cactus evolution in general.  Readers are advised
to examine pertinent references such as Gibson and Nobel's (1986)
^Cactus Primer^ and Benson's (1982) Cacti of the United States and
Canada, as well as other resources such as  Cullmann, Goetz &
Groener, "The Encyclopedia of Cacti", all of which  have some
discussion of cactus evolution.  None of these sources are
complete with respect to a "total assessment" of current data, but
Gibson & Nobel do an admirable job of covering a very complicated
topic.  Given the above caveat, I'll try to give some "highlights"
of cactus evolution, with respect to the systematic (phylogenetic)
study of the family, and relevant biology when appropriate.

    FOSSIL CACTI - Since the major processes of fossilization
generally  require sedimentation of mineral materials over
vegetative/floral materials, you can realize very quickly that with
even a hypothetical [xerophytic] ancestor to the cacti (perhaps
Pereskia-like in appearance), the environmental factors that favor
fossil preservation (save for amber entombment) would have been an
unlikely combination  of cacti and ample water for sedimentation.
Thus the fossil record for cacti is poor, or in fact absent.  In
1944, a presumed fossil cactus ^Eopuntia douglasii^ was described
from Eocene deposits in Utah.  There was much controversy over
whether this compression fossil was indeed a cactus, and after
significant bantering about the literature, most people do not
believe this "fossil" is indeed a cactus.  For literature citations
see Benson (1982), pages 76-79.  Except for relatively recent
pack rat middens (containing a variety of cactus seed) our knowledge
of cacti from any fossil remains continues to be depauperate at
best.  Scientists today must rely on the morphology of present day
cacti, or other sources of data (biochemical, chromosomal, or in my
case nucleic acids [DNA]).  Through the study of these sources of
evolutionary information, we can make some general conclusions
about cactus evolution.

CACTUS RELATIVES:

    From some~ very  recent studies of DNA variation and from
vascular anatomy, the closest angiosperm family to the cacti is the
Portulacaceae, with the genera Portulaca, Talinum, and Anacampseros
the likely "cousins" (among others).  The long standing previous
assumptions that the Aizoaceae (including the mesembs) was the
sister family to the cacti (due to the floral hypanthium) was proved
fallacious in independent tests of phylogeny for the order
(Caryophyllales).  We are ~continuing to amass various sorts of data
which continue to corroborate this hypothesis.

EVOLUTION WITHIN THE CACTI

   ASSUMING that from some common ancestor between the
Portulacaceae and the "proto-cacti", a xerophytic lineage arose
which was capable of radiating and speciating in the "New World",
most likely after the split up of Gondwanaland.  The "exact" center
of origin for  the cacti has been disputed for many years.  two
prevailing "centers" of likely origin persist:  The first is within
the Caribbean Islands (proposed by Buxbaum); and the other is in
north [western] South America;  I tend to support the latter
hypothesis, and hopefully my studies of chloroplast DNA variation
will help provide data to answer this question.  Regardless of the
geographic center of origin, the result we see today is a very
diverse and widespread New World family of angiosperms, having
approximately 1,500-2,200 species and about 100 genera
[conservatively].  It is the second largest family of angiosperms
restricted to the New World (the first being the Bromeliaceae), and
has geographic limits extending from central Canada to Patagonia,
and virtually every habitat in-between.

Divergence of the Major Groups

    Students of of the cacti can generally recognize three major
lineages within the family,  based upon gross morphology, flower
and fruit structures, as well as other characters.  These three
groups, treated previously as tribes, are now considered to be
subfamilies, and have been shown to be monophyletic (of one common
ancestral lineage).  The three subfamilies are briefly discussed
below:

SUBFAMILY PRERESKIOIDEAE

    The smallest subfamily of the Cactaceae has 18 species; all
having persistent leaves and large shiny black seeds.  The two
genera, ^Pereskia^ (16 spp) and ^Maihuenia^ (2 spp.) are found
predominantly in South America.  Pereskia plants are usually
shrubby, tree-like, or in one species (P. aculeata) form vines.  The
flowers can be very showy (i.e P. grandifolia, P. bleo) or can be
diminutive (i.e. P. humboldtii, P. weberiana, P. diaz-romeroana).
The genus Maihuenia is found only in relatively high elevation
habitats of Andean Argentina and Chile; these are low growing mat or
cushion-forming plants, and are strikingly different than their
sister genus Pereskia in vegetative form.  Maihuenia was originally
placed next to (or in some cases within) the genus Opuntia.  Both of
these genera of the subfamily Pereskioideae are on an independent
evolutionary lineage from the other cacti, and have not been as
"successful" as the other two subfamilies in terms of speciation and
geographic range.  They are likely a relictual group, PERHAPS
similar (but not the same) as the "proto-Cactaceae" ancestor, and
have differentiated into at least four or five geographically
isolated groups within South America and the Caribbean.

SUBFAMILY OPUNTIOIDEAE

    The monophyly of this subfamily can be clearly (and painfully)
demonstrated with the uniquely derived feature of the opuntioid
areole... the presence of small bristles called glochids are unique
derived feature of the opuntioids, obvious to anyone~who~has~touched
one of these plants.  Additionally, the seeds of members of this
family (originally described somewhat inaccurately as arillate) have
a funicular envelope and accessory tissues (which become stone-like
in many taxa) which are shared by virtually all members of the
subfamily.  There are about 300 species or so (more or less) within
this subfamily.  Conservatively, there are five genera [Opuntia with
>200 species; Pereskiopsis with ca. 9 spp; Quiabentia with 2-4 spp.;
and Tacinga with ca. 2 spp.]  The taxonomy of this subfamily is
perhaps the worst in the family, and is in desperate need of study.
"Splitters" will fragment the genus Opuntia into as many as 20
genera, while "lumpers" will take a broader concept of the genus and
transfer the splitting to an infrageneric level.  Regardless of its~
generic level classification, this subfamily is clearly the winner
when it comes to occupying the most geographic range for may major
group of cacti----> coast to coast (E-W) in both N. America and S.
America, and from central Canada to Patagonia...  It's also
successfully invaded (with the help of.....???) southern Africa and
Australia where it's become a noxious plant pest, and it's been
cultivated in the Mediterranean region for perhaps nearly 500 years
(probably brought back by good ol' Chris Columbus).  Except for
horticultural interest in the other cacti, this subfamily represents
a major economic use for the cacti, especially the "prickly pear"
group of Opuntia, which provides Nopales and Tunas for human
consumption.  The major growth forms are the cholla-type,
cylindrical stemmed forms (chollas with short-lived ephemeral
leaves; Pereskiopsis and Quiabentia with cylindrical stems and
persistent succulent leaves; Tacinga with cylindrical stems and
hummingbird flowers (!), and Pterocactus (9 spp) from Argentina with
unusually winged seeds and a geophyte growth habit.  Some very
interesting research on Opuntioid evolution is being done by
Wolfgang Stuppy (University of Kaiserslautern, Germany) a Ph.D.
student (who's actually writing his thesis on Euphorbiaceae!) nearly
completed, using the seed internal anatomy as a useful source of
phylogenetic morphological information.  We need as much information
as we can get about this group......Any prospective students want to
go to grad school???

SUBFAMILY CACTOIDEAE

    Wow!  About 86% of the species diversity of the family is found
in this subfamily..Probably 99% or more of the cactus cultivated in
hobbyists collections are from this subfamily as well.  With over
1,000 species, showing extremes of morphological diversity where do
I start?  Basically, this group shows many of the really confusing
problems of parallel evolution within the cacti.  Witness, for
example, the morphological similarities of the north American "ball"
cacti (Mammillaria, Coryphantha, etc. of Tribe Cacteae) and the
similarly structured members of the Notocacteae (Parodia, Notocactus
s.str., Frailea) or the Tall, columnar members of the Tribe
Pachycereeae (saguaro, Lophocereus, Stenocereus, Pachycereus,
Myrtillocactus, Polaskia) from Mexico/Central America/USA and the
Andean members of Tribes Browngieae and Trichocereeae which have
virtually the same vegetative structures.  It is no wonder that
Britton and Rose, using a purely "pigeon-hole" concept of taxonomy
grouped evolutionarily unrelated cacti based upon superficial
resemblance.  Today most cactologists recognize about eight to ten
(presumably) evolutionarily independent lineages within this
subfamily which are called TRIBES.  It would take many many pages of
text to explain the presumed hypotheses of relationships between the
tribes of this subfamily, and for many of these the relationships
are nothing more than educated speculation.  I am attempting to
clarify some of these relationships using DNA techniques, but we are
several months (years?) away from resolving the problems
conclusively. There also is the question of which of certain
controversial genera should be placed within what tribe.  There are
countless unanswered questions still remaining about this subfamily
as well.  Many of these would warrant their own discussions!!!
    Basically there are about 4 tribes which we feel evolved in
North America (Tribes Cacteae [largest], Pachycereeae, Echinocereeae
[incl. Leptocereeae], and Hylocereeae).  These have morphological
"counterparts" in South America:  Tribes Notocacteae, Trichocereeae,
Browningieae, Cereeae, and Rhipsalideae.  There~are~parallel
evolution scenarios hypothesized for barrel cacti, columnar cacti,
and epiphytic cacti, each occurring in North and South America.
This creates many more phylogenetic and biogeographic hypotheses
which are in need of study.  The degrees of specialization,
biogeographic affinities, and changes in floral/pollinator
syndromes preclude further elaboration here; it would take at least
a seminar or two to get through the basics of cactus evolution, let
alone discuss adequately, how recent data is answering many of these
questions.

SUMMARY

    I hope I've given a digestible synopsis of major trends in
cactus evolution, -- if you're still awake, I'll refer you the
previously mentioned references for more reading.  Please feel free
to discuss various issues - I'll TRY to jump in when I can.  The
preceding summary was done as part of my displacement behavior which
is keeping me from grading ca. 60  term papers, each about 20 pages.
(Arrrgh!)  Cactus evolution is fascinating, and its result is the
manifestation of natural selection and speciation processes which
has given us a marvelous array of xerophytes which we cherish and
admire in our collections.  Please respect the evolutionary process
and practice conservation whenever possible; don't buy field
collected plants, and propagate as much as possible.

I'd appreciate knowing if this was digestible and/or useful (I don't
want to babble-on incessantly if no one can use the information!).

All Best Regards,

(Dr.) Robert S . Wallace     Internet: rwallace@iastate.edu
Associate Professor of Botany
Department of Botany
Iowa State University
Ames, Iowa 50011 USA