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Biogeography of Pacific Northwest Mammals: The Geography of Genes

The Mammal Research Program at the Burke Museum has embarked on a major project to investigate the biogeographic history of our Pacific Northwest’s mammal fauna. We are using molecular biology techniques for assessing the genetic structure of contemporary populations, which also allows us to interpret conservation status of individual species. We hope to understand how mammal populations, geographic ranges of species, and habitat availability have responded to changes in the distribution of forests that resulted from Pleistocene climate cycles, glacial advances and retreats, and the associated geographic shifts of suitable habitat.

The contemporary distribution of mammals across the geographic landscape of the Pacific Northwest region of North America reflects both ongoing ecological processes and the dynamic geological and climatological history of the region. We have embarked on a new research program focused on reconstructing recent evolutionary relationships and comparing the genetic structure of natural populations of small mammals in this region. With the establishment of the Molecular Biology Lab at the Burke, we can use cutting-edge molecular techniques to investigate how the mammal fauna has responded to environmental change over the past several million years. With this perspective we can also assess the impact of contemporary anthropogenic changes such as deforestation and global climate change on the genetic signature that marks the biodiversity of mammals.

Phylogeography

Present potential vegetation of North America
Present potential vegetation of North America

By comparing the DNA sequences of individuals throughout the geographic range of a species, we can reconstruct gene genealogies, or "gene trees," which reflect evolutionary relationships among populations. These gene trees can then be mapped back onto the geographic distribution of the species. This approach, known as phylogeography ("phylogeny," or evolutionary relationships, plus "geography"), is a powerful tool for uncovering geographic patterns of genetic variation within natural populations. These patterns provide valuable insights into the historical processes that have shaped the biodiversity we see today. Reconstructing these patterns for a variety of small mammals from the Pacific Northwest will allow us to understand how the evolutionary history of this fauna has been shaped by the dynamic environmental history of our region.

Our Burke Mammalogy team currently consists of four Ph.D. students in the Biology Department’s graduate program, each of whom is contributing to various aspects of the overall Pacific Northwest Mammal Biogeography project. Sacha Vignieri anticipates completion of her Ph.D. on jumping mouse population biology in 2005. Josh Whorley will continue for another couple of years to complete his projects on ground squirrels, chipmunks, and other species. Chris Himes is currently working on water shrews and chipmunks. Corey Welch, newest member of the team, has begun to work on pocket gophers and moles. The project team initially included two members who helped to start our program: Brian Arbogast, who worked with us as a postdoc and then left to became an Assistant Professor at Humboldt State University in 2001, and Xiaoguang Zheng, who completed his Ph.D. in 2003 and took a position in biodiversity and conservation research with the Institute of Zoology, Kunming, China. Jeff Bradley, Collection Manager of the Mammal Section, looks after the general curation of specimens, samples, and databases. Jim Kenagy, Curator of Mammals, oversees the general organization of projects that fall within the program

 


The Dynamic History of the Boreal Forest Biome of North America

Boreal ("northern") coniferous forests cover vast regions of Eurasia and North America, comprising one of the largest terrestrial biomes on earth (see map above). However, the present widespread distribution of boreal forests in North America is a relatively recent phenomenon (see map below left). For example, during the peak of the Wisconsinan glaciation (about 18,000 years ago), vast regions of North America were covered by massive sheets of ice (map below right) The largest of these ice sheets, the Laurentide, extended over much of North America. In western North America, the Cordilleran ice sheet extended from Alaska to the Puget Sound region of Washington and merged with the Laurentide ice sheet in the northern Rocky Mountains.
Numerous smaller glaciers also formed in the mountains of the west-the Cascades, Rockies, and northern Sierra Nevada. The formation of these glaciers fragmented the boreal forest biome of North America, displacing it southward into several areas that served as refuges for the accompanying species of mammals. Following the retreat of the glaciers, beginning about 13,000 years ago, northern North America was recolonized by the boreal forest community. For example, typical trees such as spruces became re-established as far north as Alaska by approximately 8,000 years ago. Based on estimated fluctuations of past global temperatures, this type of scenario apparently recurred ten to twenty times during the past two million years.

Dynamic History of the Boreal Forest Biome of North America
Dynamic History of the Boreal Forest Biome of North America

Boreal forest refugia during the most recent glacial maximum 18,000 years ag
Boreal forest refugia during the most recent glacial maximum 18,000 years ag

How did these major historical changes in the distribution of boreal forest habitat affect the variety of mammal species that inhabit these forests?

This is the fundamental question we are addressing in our project. When major environmental barriers arise, such as formation of mountain chains or fragmentation of boreal forest habitat into disjunct areas by glaciers, gene flow between populations on opposites sides of the barrier can no longer occur. As a result, populations isolated from one another begin to evolve independently, adapting to their own local environmental conditions. If the environmental barriers persist for a long period of time, separated populations may become genetically differentiated from one another. How different they are now can be measured by comparing the genetic code, the DNA, of individuals from each of the modern populations of interest. Maps of mammal species distributions in the Northwest help us to pose interesting questions about these historical questions. For example, are montane populations now isolated on the Olympic Peninsula evolutionarily distinct from populations in the main axis of the Cascades?

Given that a wide variety of mammals is associated with boreal forests, we might expect each species to respond differently to historical environmental events. For example, would a mole be affected in the same way as a squirrel? The only way to explore this question is to compare the genetic structure of a wide range of contemporary species. If they have similar and congruent geographic patterns of genetic variation, we have evidence that they responded in a similar fashion to historical environmental changes. On the other hand, if different species have different patterns, this is evidence that the species responded differently to past events. The research we are doing now in the Mammal Research Program at the Burke Museum contributes to our understanding of how a wide range of species responded to past climatic and geological changes. It can also give us important insights into how mammals may respond to future climate change as well, and what measures, if any, might be appropriate in cases where species conservation is critical. 

 

 


Which species do we study?

We are studying a diverse group of mammals representing 7 families and 3 orders.
Click on the names of these mammals for more information

 

Order Rodentia

 

 

Order Soricomorpha

 

Order Carnivora