rly regularly. Postgate reveals that a mutation arises "simply as a chemical change in the DNA of an organism," and he is equally blasé in announcing, "In Escherichia coli, about one organism in every ten million is a mutant of some kind" (10:179). When E. coli cells are in a rich and plentiful culture medium (have a lot of available food and great environmental growth factors like heat and the absence of ultraviolet light), they can double their numbers (divide) once every 20 minutes, otherwise once every hour or so (8:114-15). Other bacteria have much slower growth rates; Mycobacterium tuberculosis double once every 18 hours or longer (8:115).
Schleifer and Stackebrandt have edited an entire book about Evolution of prokaryotes, comprised of 15 papers about evolution that is still taking place among these primitive life forms (12). Woese explains why studying continuing evolution among the bacteria is still important (18); Rao et al.
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