The Cohan Laboratory

racterizing these ecotypes and their origins. For the purposes of our studies in speciation, we define an ecotype as an ecologically homogeneous group of closely related bacteria and we consider each ecotype to be a species. We are using Bacillus from desert soils in Death Valley as a model system to study the origins of bacterial species. In particular, we are studying Bacillus isolated along a salinity gradient and from rhizospheres of various plants to study central issues of bacterial speciation. First, we are aiming to identify the ecotypes that are adapted to different organic resources and/or are adapted to different physical conditions. We are using multilocus sequence analyses to hypothesize ecotype demarcations, using Ecotype Simulation (which our lab developed and is continuing to improve) and other sequence-based ecotype-demarcating algorithms. We are also identifying ecotypes by their unique microhabitat associations, as determined by soil chemistry and physical structure as well as by rhizosphere associations. Finally, we are identifying ecotypes by their genome content, in particular with regard to genes most likely to determine ecological function. Second, we are aiming to determine the ecological dimensions by which ecotypes diverge. Of particular interest is whether newly divergent species have most frequently diverged with respect to the organic resources they utilize or instead by the physical conditions that they can tolerate. Third, we are testing alternative theories of bacterial speciation. Of particular interest are the Stable Ecotype model (where ecotypes are long-standing and are recurrently purged of diversity by periodic selection), the Species-Less model (where ecotypes are not subject to cohesive forces), and the Nano-Niche model (where most-closely-related ecotypes are only quantitatively different in their ecological niches). Our recent papers may be accessed at our BEPress web site: http://works.bepress.com/frederick_cohan/