"It is notorious that each species is adapted to the climate of its own home"

"..Species in a state of nature are limited in their ranges by the competition of other organic beings quite as much, or more than, by adaptation to particular climates"
        Charles Darwin, The Origin of Species

These two lines, from the same page of Darwin's most important work, demonstrate the uncertainty that has surrounded, and still surrounds, our understanding of the factors that limit the geographic ranges of species.  Indeed, the question of what determines the distribution of species is an enduring issue in ecology.  The answers to questions regarding the determinants of species distributions are important for understanding the origin of geographic patterns of biological diversity, and for planning for biodiversity conservation as human activities reduce available habitat, decrease opportunities for successful dispersal, and alter climate to a degree that is physiologically significant.

The Niche:  A Useful and Unifying Concept

The idea of the ecological niche has become a powerful and unifying concept, bringing together ecology, evolution, biogeography, and physiology. I am interested in using this concept to help drive questions that will lead us to understand how evolutionary processes and changing environment have impacted the diversity of species and their geographic distribution.  The niche can be thought of as a characteristic of species themselves, and can be operationalized for use in both ecological and evolutionary analysis. I use the niche concept to structure evolutionary and ecological models that can help us to identify key processes that affect the distribution and diversity of species. Since my initial efforts to model tree species distributions during the Holocene (see Pearman et al 2008, Ecology Letters), I have become increasingly interested in how the niche changes over time.  These changes can, in some cases, be associated with evolutionary processes, as when patterns of niche variation are associated with the evolutionary distances among species in a phylogeny, and when particular morphological and physiological features are associated with the environments inhabited by species, subspecies or varieties.  I am seeking to establish collaborations with both young and established researchers to expand our understanding of niche evolution, its causes and its consequences for all aspects of diversity. The following sections explain how the niche can be integrated into a number of research areas.

Macroecology and Biogeography

Darwin was very aware that species varied in both location and extent of distribution.  One of my research interests focuses on the patterns of species distributions, the processes that influence these patterns, and the evolutionary consequences of these processes.  I am interested in addressing these questions at multiple scales, from global and continental scales down to the scale of a sample plot in an ecological field study. I am especially interested in how we can use information on species distributions and the factors that affect them to understand the potential impacts of global change on populations, species and biotic communities.  Global change, in this case, includes historical changes that occurred over long periods, thousands or hundreds of thousands of years, as well as ongoing changes that occur over tens to hundreds of years. My collaborators and I currently use a variety of approaches to the creation of statistical models to study these patterns and develop explanatory hypotheses that can account for variation in species distributions.  These approaches include ordinary linear regression, statistical ordination, mixed-effect models in a Bayesian context, and species distribution models. Understanding the evolutionary consequences of changing climate and consequent limitation of gene flow has led us to species distribution modeling that incorporates information on the geographic distribution of genetic variation. 

Relevant questions: 
1. Why are particular lineages of species more diverse in some areas of the planet than in others?
2. How have major fluctuations in earth's climate influenced the diversification and distribution of species?

Phylogenetic Comparative Analysis

Earth has experienced repeated cycles of accumulation and loss of species and the ecological diversity they represent. Study of the processes that impact diversity in lineages of species is increasingly facilitated by the expansion of databases of DNA sequences, species traits, and the geographical occurrence of species as represented in biodiversity collections of museums and herbaria.  These resources allow us to address questions focused on how changes in particular species characteristics have influenced subsequent diversification in other traits.  Recent advances in computational algorithms allow comparison of evolutionary models that represent distinct types of processes that affect the mode and tempo of diversification of both species and their traits. Comparative approaches that evaluate the degree to which competing models find support in the data can lead us to understanding of the origin of the current diversity of species. I am seeking students and collaborators to develop phylogenetic analyses of species traits and niches to understand how species may also display trends toward niche evolution. Rapid niche evolution may be the only alternative to extinction in species that have limited powers of dispersal or for which dispersal opportunities are limited by anthropogenic habitat destruction. 

Relevant questions:
3. How does the advent of particular characteristics (key innovations) impact the evolution of species diversity and its geographic distribution? Which characteristics are these?
4. In what kinds of niche characters is evolution relatively rapid, thus constituting the primary components of changing ecological variability?

Phylogeography and Intraspecific Ecological Variation

Phylogeography has traditionally addressed the identification of the structure of genetic diversity as it has been influenced by large-scale climatic and geological changes. These studies primarily use genetic markers that are nearly selectively neutral and have been impacted primarily by the changing demography of groups of populations within species.  In contrast, the genetic variability that can allow some populations to exist successfully in unique environments can be directly adaptive, or indirectly adaptive by facilitating adaptive plasticity.  Widespread plant species with a large number of distinct varieties offer an opportunity to investigate the distribution of adaptive variation both geographically and across the genome.  I am currently developing research on Eriogonum umbellatum (Polygonaceae), which has 40 described varieties distributed across western North America.  Previous work in my group and others suggests that the genes responsible for adaptation likely differ among varieties, some of which may represent cryptic species.  I am currently developing genomic resources to address evolution of the climatic and soil niches of this species.  I am seeking motivated graduate students to expand this work.

Relevant questions:
5. How does a species maintain a very broad niche, and what are the relative roles of generalist genotypes versus locally adapted variants? Is there substantial geneflow among varieties and species cohesion, or is the species in the process of speciation?  Are some varieties actually cryptic species?

6. To what degree are the mechanisms of adaptation to environment shared by the populations within varieties of highly variable and widely distributed species?  Do populations and varieties differ in how adaptation is distributed across the genome?

7. What is the history of the formation of intraspecific variation? Are adaptive variants at loci that are under selection ancestral and relatively old, or have they arisen repeatedly and independently?  Is there a mix of ancestral and recent adaptive variation?