Summary
of our laboratory’s research
Our
laboratory’s research is devoted to developing a robust
framework for the science of coevolutionary biology. The major
problem we are pursuing is how the process of coevolution organizes
the earth's biodiversity. Coevolution is reciprocal evolution
between interacting species driven by natural selection. It is
responsible for many of the major events in the history of life,
and it is a major force in the organization of biological communities.
The coevolutionary process shapes interacting networks of species
at local levels, regional levels, and global levels. It weaves
the threads of biodiversity into a rich and resilient fabric.
We now know that coevolution is a relentless
process, creating ever-changing geographic mosaics in how species
interact with one another. Populations of one species often become
adapted to local populations of other species.
We also now know that
these local coevolutionary changes can sometimes occur on time
scales of less than a hundred years, indicating that there is
often no real difference between ecological time and evolutionary
time. Despite this constant change, some coevolving interactions
persist for millions of years. The challenge, then, is to understand
how highly dynamic interactions coevolve and persist across landscapes
and millennia.
The
Geographic Mosaic Theory of Coevolution
The
geographic mosaic theory of coevolution argues that coevolving
interactions have three components that collectively drive ongoing
coevolutionary change:
Geographic selection mosaics:
Natural selection on interspecific interactions varies among
populations partly because there are geographic differences
in how fitness in one species depends upon the distribution
of genotypes in another species. That is, there is often
a genotype-by-genotype-by-environment interaction in fitnesses
of interacting species.
Coevolutionary hotspots: Interactions
are subject to reciprocal selection only within some local
communities. These coevolutionary hotspots are embedded
in a broader matrix of coevolutionary coldspots, where local
selection is non-reciprocal or where only one of the participants
occurs.
Trait remixing: The genetic structure
of coevolving species changes through new mutations, gene
flow across landscapes, random genetic drift, and extinction
of local populations. These processes contribute to the
shifting geographic mosaic of coevolution by continually
altering the spatial distributions of potentially coevolving
genes and traits.
Understanding how these components
of the coevolutionary process interact is becoming increasingly
important as climate change, fragmentation of environments,
and spread of invasive species are changing our biological
landscapes worldwide.
We
use a combination of ecological, populational, molecular, phylogeographic,
and mathematical approaches to probe the structure and dynamics
of coevolving interactions. We study a wide range of interactions
from antagonistic to mutualistic, using a wide range of taxa.
In recent years, we have used interactions as different as those
between pollinators and herbaceous plants, mycorrhizal fungi
and conifers, and bacteria and bacteriophage to explore how
coevolution proceeds in different forms of interaction. All
our work is directed toward understanding the links among microevolutionary
processes (evolutionary dynamics within local populations),
mesoevolutionary processes (geographic mosaics of evolving and
coevolving species) and macroevolutionary patterns (the patterns
observed among diversifying lineages).
The three major
focus areas
Our lab’s approach has been to divide
the study of coevolution into three major focus areas and then
to use a combination of ecological, genetic/molecular, and phylogenetic
techniques to analyze coevolution as a hierarchical process.
1) Almost all species are collections
of genetically distinct populations. Any theory of how coevolution
organizes biodiversity must begin with an empirical understanding
of this nearly universal property of species, and of how it
influences local adaptation and specialization in interacting
organisms.
2) Coevolving interactions are often locally transient and
geographically variable, yet many persist regionally for millions
of years. An important goal of coevolutionary biology is therefore
to understand how the temporal and geographic dynamics of
interspecific interactions shape coevolution.
3) Coevolving interactions commonly form networks of species
rather than simply pairs of species. The problem to solve
is whether coevolution shapes these networks in predictable
ways.
Most of the published papers
from our laboratory, including my three books on coevolution
(Thompson 1982, 1994, 2005), address these three central foci
from an increasing diversity of perspectives.
Details
of some recent and current studies
How do coevolving interactions
vary across broad geographic landscapes?
We are using the interactions between Greya moths and their
hostplants to evaluate this question. These moths are the closest
living relatives of yucca moths, whose interactions with yuccas
are one of the classic textbook cases of coevolution between
mutualists. Unlike yucca moths, Greya moths feed on small herbaceous
plants in the Saxifragaceae and Apiaceae, and the interactions
range geographically from antagonism to mutualism. We have a
molecular phylogeny of the moths and the plants. We also have
developed a large-scale molecular phylogeographic database for
Greya politella, the most widely distributed member of the genus,
and we have similar genetic data for the two major lineages
of hostplants used by Greya politella: Lithophragma and Heuchera.
These past and ongoing studies have provided us with a molecular
template for understanding how populations, traits, and ecological
outcomes with hostplants have evolved across western North America.
Current studies using these interactions are exploring two lines
of inquiry:
• How do moth and plant traits,
and the ecological outcomes associated with these traits,
vary over the thousands of kilometers north to south in western
North America in which the interaction occurs? Catherine
Fernandez, a research associate and overall coordinator
of the lab, and John Thompson
are leading this effort.
• How do local networks of related species of Greya moths
and related species of their hostplants vary geographically?
What is the comparative phylogeographic structure of these coevolving
networks? Kate Horjus
is devoting her doctoral dissertation work to addressing these
questions.
What is the geographic scale of local
adaptation in coevolving interactions?
We are using interactions between coastal pines and mycorrhizal
fungi to explore this question. These studies are taking advantage
of the observation that several species of pine are restricted
to a narrow band of coastal environments from Alaska to Baja
California, and these pines harbor specialized mycorrhizal fungi
in the genus Rhizopogon. That structure makes it easier than
in many interactions to interpret the geographic scale of local
coadaptation. Jason Hoeksema,
a postdoctoral associate in our lab, is leading this effort.
How
do geographic selection mosaics and gene flow shape the rate
and trajectory of coevolving interactions?
These studies are using laboratory microcosms of E. coli and
T7 phage to explore the dynamics of rapid coevolution. The experiments
have been designed to test specific predictions of the geographic
mosaic theory of coevolution. The work involves experimentally
induced coevolution followed by sequencing of the genes undergoing
selection. These studies are a collaboration between our laboratory
and Brendan
Bohannan’s laboratory at Stanford University. Samantha
Forde is the postdoctoral associate who is spearheading
this work.
Other Research Questions
In addition to these questions, our laboratory is continuing to
explore a wide range of related questions on how the genetic,
geographic, phylogenetic, and ecological structure of species
shapes the coevolutionary process. Our efforts include investigations
into how the evolution of plant polyploidy has shaped coevolution
between plants and animals, how outcomes of interactions vary
temporally on the time scale of decades, and how changes in species
ranges and the introduction of novel species may alter the dynamics
of coevolution.
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