I'm 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 has involved the application
of population genetics in a phylogeographic, phylogenetic or
ecological context to identify lineages and explore the
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
Quercus, 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.
Secondarily 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.
Research & Work Opportunities
Students interested in
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 digital publically-accessible database
of the herbarium collections.
Studies of Quercus
biogeography and hybridization in Mexico and the S.W. United
The 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.
As a 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.
Of 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.
Given 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.
Poster 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
Closer 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.
Poster Presentation 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."
Biogeography and conservation
genetics of Guaiacum (Zygophyllaceae) in Western Mexico
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.
To date 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.
Systematics and Phylogeography
of Ostrya (Betulaceae) in North America
Ostrya is a small genus of understory
tree in North America and Asia commonly known as hop-hornbean.
In North America the genus ranges from southern Canada to Honduras.
While the group shows much morphological variation most of the
variation has been included within a single species, O. virginiana
which has a distribution covering the entire range of the genus.
Various segregations have been proposed, particularly in Mexico,
but the characters for recognizing species segregation have been
inconsistent and thus have not been maintained. Some recognition
of species segregation has also been variously proposed and supported
for the taxa in the SW United States. I am working on a comprehensive
systematic revision of the genus across its North American range
to determine the number of distinct species, the phylogenetic
relationship among them, and reconstruct the continental patterns
My initial work with the
genus is focusing on determining the distinctions of the taxa
in the southwest US. In this region the genus Ostrya has
a very restricted distribution to the Chisos Mountains in SW
Texas, the Guadalupe and adjacent Sacramento Mountains in W Texas
and S New Mexico, and the drainage of the Colorado River in N
Arizona and S Utah. One to three species have been recognized.
The most segregated view could recognize O. chisosensis
restricted to the Chisos Mountains, O. baileyi in the
Guadalupe Mts. and O. knowltonii in the Colorado River
drainage. The most inclusive includes all taxa within O.
knowltonii. Along with students I have been performing field
studies, herbarium investigations of morphological variation,
ecological niche analysis, and completing a detailed DNA barcoding
study to determine the circumscription of species across the
region. Future work will involve the examination of species
circumscription in other regions of North America and the development
of chloroplast microsatellite markers to serve as a marker for
Studies in Western
North American Viola
In Viola I have
been working on two projects. One is a biosystematic study of
the Viola adunca complex in the San Juan Mountains of
SW Colorado. In the local area V. adunca appears to be
speciating along elevational and ecological gradients into discrete
ecotypes. This work is being performed with undergraduate students
(see Senior Thesis Viola ecotypes
for more info) and has focused on studies of plant phenology,
pollinator behavior, and population genetics using microsatellites.
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, just published this past year as V. calcicola
R.A. McCauley & H.E. Ballard, is restricted to cracks of
limestone along the southeastern extent of the range.
Systematics and Conservation
of the Endemic Hawaiian Arborescent Amaranthaceae (Charpentiera
This work in collaboration
with Dr. J. Christopher Havran at Campbell University is investigating
the phylogeny of these unique genera in the Hawaiian Islands.
Our studies have initially been focused on the development of
a molecular phylogeny for the two groups to understand the dynamics
of species relatedness and origin.
Systematics of the
genus Froelichia (Amaranthaceae)
The 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 taxonomic description of new species, to biogeography,
population ecology, and phylogenetic reconstructions and am currently
working on a taxonomic monograph of the group.