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 Research
in our lab focuses on investigating the mechanisms of adaptive divergence
between populations and species and the consequences of divergence for
patterns of distribution and abundance. We combine observations of natural
populations and experimental manipulations in the field and in the lab
with tools from quantitative genetics and physiological ecology. Much
of our research uses species of the genus Mimulus
("monkeyflowers") because of their
remarkable ecological diversity, history of study, modern genomic resources,
and ease of propagation. Read more about current projects below.
Evolutionary ecology of geographic distributions
Range limits
Every species occupies a limited geographic area, but the ecological
and evolutionary factors that give rise to range limits remain poorly
understood .
Range boundaries are a particularly interesting place to study the process
of adaptation because they present an evolutionary conundrum. If organisms
are maladapted to environmental conditions beyond their ranges, why
don't they evolve by natural selection and expand their ranges through
time? To answer this question, we study closely related species of monkeyflower,
Mimulus cardinalis and M. lewisii, with contrasting
altitudinal and latitudinal ranges. We combine a variety of approaches,
including demographic modeling of central and marginal population dynamics,
growth chamber studies to examine physiological responses to limiting
environmental variables, and reciprocal transplants to experimentally
evolve populations beyond range boundaries. We are developing a new
ecological genetics project to study the effects of gene flow between
central and marginal populations on adaptive divergence and range limits.
Range size and rarity
Although every species has a limited distribution, some species have
much more limited ranges than others. We are extending our research
on geographic range limits to investigate the evolutionary ecology of
range size and rarity in Mimulus, a genus where species vary
in range size and abundance by orders of magnitude. We are using a comparative
phylogenetic framework to address questions such as what mating system,
life history, or physiological traits are associated with small range
size? We are interested in using empirical approaches to address questions
such as do rare species have narrow environmental tolerances, and if
so, what constrains the evolution of broader tolerance? Are widespread
species highly plastic, specialized on a common environment, or composed
of many locally adapted populations?
Ecological speciation and niche evolution
Questions about range limits are intimately related to the geography
of speciation and the process of ecological divergence between sister
taxa. Recently diverged sister taxa often have non-overlapping (allopatric)
or abutting (parapatric) geographic distributions. The evolution of
ecological differences in different geographic regions can contribute
to reproductive isolation by reducing opportunities for interbreeding.
We are generally interested in studying the mechanisms of adaptation
to contrasting habitats between populations and sister species of Mimulus
and in quantifying axes of niche divergence across the genus.
Functional biology and community dynamics of desert annuals
Species coexistence
In
collaboration with Larry
Venable, Travis
Huxman and Peter
Chesson, we are investigating how species differences in physiology
and morphology promote coexistence. A
long-term study begun by Larry in the early 1980s has  demonstrated
that species of desert winter annuals show very different population
dynamic responses to inter-annual variation in rainfall. These demographic
differences in response to rainfall create temporally decoupled
population dynamics that contribute to species coexistence via the
storage effect. Our current project focuses on physiological and
morphological traits that mediate responsiveness to rainfall to
investigate how functional differences underlie species coexistence.
Our studies combine coexistence theory, demographic observations,
measurements of leaf-level photosynthetic characteristics and whole-plant
biomass allocation patterns, and experimental manipulations of the
amount and timing of rainfall. We have found that species exhibit
a strong tradeoff between growth capacity and low-resource tolerance,
and species position along this trade-off can predict the demographic
responses to rainfall variation that promote local biodiversity.
Community genetics
Tradeoffs are often invoked to explain
both interspecific and intraspecific patterns, from resource partitioning
and species coexistence to the evolution of life history strategies.
Ecologists have hypothesized that many of the same tradeoffs that
shape life histories also affect interspecific interactions, mechanisms
of coexistence, and community structure. Such tradeoffs are often
assumed to be pervasive, underlying constraints. If so, they should
be reflected in measurable within-species genetic structure and
constrain evolution. However, tradeoffs described at different scales
may not be wholly transitive. For example, a within-species tradeoff
might be set by biophysical constraints in terms of what can be
built with a given amount of resources (construction constraint).
Yet an interspecific community tradeoff might be set by assembly
rules of what can coexist (assembly constraint). With Sarah
Kimball, we are investigating
the growth capacity/low-resource tolerance tradeoff within multiple
desert annual species to determine whether the key functional traits
that trade off across species are similarly constrained within each
species. By examining this key tradeoff within and between populations
and at the phenotypic and genetic levels, we will determine how
tradeoffs scale from phenotypic and genetic variation within species
to interspecific diversity and community structure.
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