The basis of biological diversity continues to be the subject of intense study. Numerous causes of biodiversity have been identified, including adaptation, spatial heterogeneity, ecological interactions, genetic drift, and phylogenetic history. However, the general importance of these and other possible causes has proved difficult to quantify. Much of the difficulty arises from the historical nature of evolution. Evolutionary change can take many generations, which severely limits studies that examine the dynamics of diversification. Compounding this problem are limits to true replication in studies of extant diversity and an inability to examine directly the past selective conditions, both of which again result from the historical nature of evolution. While several approaches have been successful in overcoming some of these difficulties (e.g., Schluter 1996a), general conclusions have been hard to obtain. A complementary approach to examining extant populations (Endler 1986) is to perform experimental evolution under controlled laboratory conditions (Dykhuizen 1990; Lenski 1995). By performing natural selection experiments in the laboratory, both the selective conditions and the initial genetic composition of the evolving populations can be controlled. Thus, difficulties that arise from limited replication and uncertain selective history are circumvented and evolutionary responses can be attributed directly to the known conditions under which the experiment was performed. Experimental evolution is particularly powerful when coupled with organisms of short generation times, such as bacteria. With appropriately chosen bacterial species, several thousand generations of evolution can be accomplished during a single year.