Frederick M. Cohan
 Professor
Ph.D. (organismic and evolutionary biology), Harvard University
Campus Extension: 3482
Room #: Shanklin Lab 207
E-Mail: FCOHAN@WESLEYAN.EDU
Evolutionary genetics, speciation, adaptive radiation and biogeography of bacteria.
Molecular surveys
suggest that bacterial species may number in the millions or
even billions. The challenge to microbial ecology is to
identify the ecologically distinct bacterial groups (ecotypes)
within a community and to determine what differences allow them
to coexist and to perform different ecosystem functions. The
systematics of bacteria does not aim to demarcate diversity at
this fine level of ecotypes, and indeed, we have shown that the
named species of bacteria typically contain multiple ecotypes.
Much of our recent research has aimed to develop a theory of
bacterial species and speciation, for the purpose of identifying
these ecotypes.
All modern concepts of biological species
attribute a set of quintessential characteristics to species:
that they are each subject to forces of cohesion (limiting the
genetic diversity within species), ecologically distinct (so
that they can coexist into the indefinite future), monophyletic
(invented only once), and that different species are
irreversibly separate (free of cohesive forces). We have
developed the ecotype concept of bacterial species, which grants
bacterial species all of these characteristics. The cohesive
force within an ecotype is not genetic exchange, owing to its
rarity in bacteria, but rather “periodic selection.” Each
adaptive mutant can cause a sweep of an ecotype’s diversity—a
periodic selection event setting the genome-wide diversity to
nearly zero. Because ecotypes differ in their ecological
niches, the periodic selection from one ecotype cannot affect
the diversity in other ecotypes, and thus different ecotypes may
diverge indefinitely.
Under the assumption that ecotypes are
formed only rarely, and that periodic selection events within
ecotypes are much more frequent (the Stable Ecotype model), we
can make a powerful prediction: that ecotypes will correspond
to the sequence clusters within a DNA-based pylogeny. We have
recently developed an “ecotype simulation” approach to determine
the sequence clusters that correspond to bacterial ecotypes. We
have applied the method to three intensively studied systems:
the cyanobacteria within Yellowstone’s hot springs, the
Bacillus in the soil in Israel’s “Evolution Canyon” (an
east-west canyon with microclimatically distinct north- and
south-facing slopes as well as the canyon bottom), and worldwide
environmental and clinical isolates of Legionella, which
causes Legionnaires’ Disease. In each system, we have found
that many of the putative ecotypes demarcated by ecotype
simulation are indeed ecologically distinct.
We have recently developed an Evolution
Canyon in Nevada’s Mojave Desert, and we are attempting to
determine whether the suite of Bacillus ecotypes we
identify is more complete when our sampling is not biased by
what we can cultivate. We are also testing for the existence of
extremely closely related ecotypes adapted to the various
microclimates of the canyon.
We are currently using a metagenome
approach to attempt to fully characterize the cyanobacterial
diversity within two Yellowstone hot springs. Here the entire
diversity of the communities’ DNA is assembled into genomes of
closely related organisms, allowing characterization of each
ecotype’s adaptations by identifying the genes that are unique
to that ecotype.
Our work is supported by a FIBR grant of
NSF, and by NASA.
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