interested in developing a broad understanding of
the ecological and organismal factors which contribute to the
creation and maintenance of plant biodiversity. Much of my work
involved the application of population genetics in a phylogeographic,
phylogenetic or ecological context to identify lineages and
evolutionary forces maintaining these lineages. Currently I am
involved in a variety of projects focusing on the evolution of
plant biodiversity through reciprocal hybridization in Mexican
the evolution and conservation of rare taxa, the evolution of
chromosomal races in widespread species, and ecotypic differentiation
controlled by elevational gradients in the Southern Rockies.
I am interested in traditional taxonomic studies focused on the
circumscription and identification of taxa and using the available data
to complete revisionary taxonomic treatments. I have been working
extensively on a monographic review of the genus Froelicha in
the Amaranthaceae and am currently involved in the production of other
taxonomic treatments and assisting with various flora projects.
Student Research & Work Opportunities
interested in plants are
encouraged to look into developing independent studies with me on the
flora of SW Colorado. These could be floristic-based studies or
evolutionary-based studies. Work (funded via Financial Aid's work study
program) is additionally available to work caring for plants in the
college greenhouse. Limited opportunities are available for
paid work in the college herbarium assisting in the development of a
publically-accessible database of the herbarium
of Quercus biogeography and hybridization in Mexico and the
S.W. United States
genus Quercus is well known for frequent hybridization among
closely related species and recurrent hybridization is an important
evolutionary process in the generation of species biodiversity. I
have been studying the effects of this recurrent hybridization in one
of the "hotspots" of oak diversity in the world. Of the approximately
500 species of Quercus Mexico shows a very high level of both
species diversity at 160 along with a high level of species
endimism. Much of this diversity is likely due to recurrent
hybridization and habitat changes during evolutionary history.
means of studying these processes I have been working with a natural
group of red oaks known as the Subsection Racemiflorae
consisting of four species (Q. conzattii Trel., Q. radiata
Trel., Q. tarahumara Spellenb., J.R. Bacon, & Breedlove, Q.
urbanii Trel.) which are united by the characteristic of producing
acorns on racemose inflorescences. These species form a natural group
of red oaks restricted to the western Sierra Madre and southern
Cordillera of Mexico. There is an apparent segregation of the species
based on edaphic factors and elevation with the more restricted species
(Q. radiata, Q. tarahmara, Q. urbanii) being
more or less specialists on highly sterile, often mineralized soils.
particular interest in the group is the bicentric distribution of both
Q. urbanii and Q. conzattii. Quercus urbanii is
disjunct by approximately 700 km from mountain slopes near the Balsas
Depression to the western Sierra Madre in Sinaloa and adjacent Durango.
Across this distribution it occurs mostly in small "islands" of
monospecific stands, however there are no quantifiable differences in
morphology between the northern and southern portions of the range and
there are no reports of it hybridizing with other oak species.
Extending this pattern of range disjunction is Q. conzattii
which is disjunct by approximately 850 km between its northern and
southern regions of distribution. While occurring in ecologically
similar habitats and often as the first black oak above xeric tropical
or desert scrub, its large disjunction has resulted in slight
morphological differences between the two portions of its range,
although almost all differences can be viewed as a continuum of
variation from south to north. An additional resulting characteristic
of its wide disjunction is the presence of a different composition of
sympatric species, particularly other Quercus.
the combination of broad and narrow range species, the clear range
disjunction, and occasional gene flow through hybridization present
within the subsection Racemiflorae it is thought that this
group can act as a good model system within the Mexican oaks for the
detection of historical patterns of gene flow and past species
distributions. I have collected and analyzed the genetic structure of
49 populations of the group throughout its range using a combination of
nuclear and chloroplast microsatellites. Results indicate recurent gene
flow via hybridization and genome sharing among at least three of the
species. Bayesian admixture analysis also suggests extensive
hybridization, likely with other species, particularly in the most
widespread Q. conzattii.
Presentation presented at the IVIII Botanical Congress, Melbourne
Australia: "Genetic Structure and Speciation in a
Lineage of Mexican Red Oaks: A Phylogeographic Study of Quercus Section Lobatae Subsection Racemiflorae."
to home I have been working to understand patterns of hybridization
among the oak species of the Four Corners region which is principally
dominated by Q. gambelii, Q. turbinella, and Q.
havardii. Initial work with Senior Thesis students during the
2009-2010 school year compared populations of pure and mixed species
using a combination of detailed morphometric analysis and Inter-simple
sequence repeat (ISSR) genetic markers. This work showed a high level
of gene exchange among the taxa, most evident in lower elevation zones,
while morphological characters were consistent and taxon-specific -
even in the light of high levels of gene sharing. Further work is hoped
to identify a set of specific genetic traits which identify specific
taxon groups. For more information on this project see the Senior
Seminar page under the heading teaching to the left.
presented at Evolution 2011, Norman, OK: "Influence of relictual species on
the morphology of a hybridizing oak complex: an analysis of the Quercus x undulata complex in the
Four Corners Region."
conservation genetics of Guaiacum (Zygophyllaceae) in Western
The genus Guaiacum or Lignum Vitae is a group of
six species native to the dry forests of the American tropics and
subtropics which have been used for a variety of purposes spanning from
medicinal to timber products for centuries. Today utilization of Guaiacum
is principally for its wood which is one of the heaviest currently in
trade with a mass of approximately 1303 kg/m3 and has a
self-lubricating quality making it useful for mechanical purposes. Past
overexploitation in conjunction with habitat loss and a slow rate of
regeneration has left most species of Guaiacum threatened and
are today listed on the IUCN Red List of Threatened Species and their
trade controlled under CITES. Two species of Guaiacum occur in
western Mexico G. coulteri, a shrub to small tree
endemic to the seasonally dry forests of the Pacific Coast of Mexico
ranging from Sonora to Oaxaca and G. unijugum, a small shrub endemic to the Cape
of Baja California.
my work with western Mexican Guaiacum has identified the
evolutionary relationships of the Baja California endemic and surveyed
the genetic diversity and structure of G. coulteri to assist in
conservation measures. The work with G. coulteri has
additionally uncovered potential instances of cryptic speciation within
the species range likely do to chromosomal rearrangements. I am
currently investigating the patterns of chromosome number across the
species range using flow cytometry.
in Western North American Viola
recently begun a series of new investigations into species distinctions
and speciation of Viola in
the Southern Rockies and SW US. I am initially focusing my
attention on ecotypic differentiation and potential ecological
speciation in the Viola adunca
complex in the San Juan Mountains of SW Colorado. This work is
being performed with undergraduate students (see Senior Thesis Viola ecotypes for more info) and
is currently funded by the Colorado Native Plant Society.
Additionally I have been working to describe a new species of
Viola endemic to
the Guadalupe Mountains of SE New Mexico and SW Texas. This new species, to be described as V. calcicola R.A. McCauley &
H.E. Ballard, is restricted to
cracks of limestone along the southeastern extent of the range.
of the genus Froelichia (Amaranthaceae)
genus Froelichia is a group of 16 species of small herbs and
shrubs native to a variety of habitats in the Western Hemisphere. I
have worked extensively with this group over the past ten years on
topics ranging from strict taxonomic description of new species, to
biogeography, population ecology, and phylogenetic reconstructions and
am currently working on a taxonomic monograph of the group. To
facilitate dispersion of this taxonomic information much of this is
being compiled into an electronic monograph.