A Comprehensive Multigene Phylogeny of Chipmunks (Rodentia: Tamias): Testing Divergence with Gene Flow
What is a species? The "biological species concept" defines a
species as members of a population that are capable of
interbreeding. But according to Dr. John Demboski of the Denver
Museum of Nature & Science, this is not entirely accurate.
"It's not as easy as what you learned in your high school text
book," Dr. Demboski said, explaining that the theory does not
accommodate asexual organisms or interbreeding between distinct
species like wolves and coyotes.
Dr. Demboski and collaborator Dr.
Jack Sullivan of the University of Idaho aim to solidify the
term "species" through a comprehensive study of the phylogeny -- or
evolutionary tree -- of chipmunks, which are small ground
squirrels. According to Dr. Demboski, western chipmunks are a great
study system because they are a recently evolved group that still
appears to be in the middle of speciating-the process through which
new species arise.
"Chipmunks give us a snapshot of the process," Dr. Demboski
Chipmunks are among the most diverse mammals in the West with 23
currently defined species and extensive diversity within species.
Many chipmunks partition into distinct habitats or niches to
minimize competition between species. Some of these habitats
overlap, however, creating contact zones where different species
meet and potentially interbreed. When this occurs and chipmunk
species exchange genes, it creates a complex family tree.
"Because chipmunks tend to be very similar in appearance, it can
be difficult to figure out how these 23 species are related to each
other and how they came to be so diverse in the West," Dr. Demboski
But there is one way to tell them apart: chipmunk species have
very distinct genital bones. This has been thought to promote
reproductive isolation among species through a "lock and key"
mechanism. However, as Dr. Demboski and his colleagues have
discovered, interbreeding still occurs.
"We have identified numerous instances where gene flow occurs
between ecologically and anatomically distinct species, suggesting
hybridization may be important during the evolution of this and
other groups," Dr. Demboski said.
Hybridization in chipmunks is not easy to detect. Dr. Demboski
uses genetic data (e.g., DNA sequences) to identify the otherwise
cryptic occurrences. Molecular techniques can also be used to
date hybrid events and determine the lineage of individual
Over the last 10 years, Dr. Demboski has collected specimens in 12
western states and Canada, discovered new hybrid zones (one in
Colorado's Front Range), and uncovered previously unknown genetic
diversity in chipmunks, possibly including new species.