© 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e CHAPTER 7 Selective Sweep, edited by Dmitry Nurminsky. Abstract B iodiversity in the bacterial world is strongly influenced by "periodic selection," in which natural selection recurrently purges diversity within a bacterial population. Owing to the extreme rarity of recombination in bacteria, selection favoring an adaptive mutation eliminates nearly all the diversity within an ecotype

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... ed as the set of strains using about the same ecological niche, so that an adaptive mutant or recombinant out-competes to extinction strains from the same ecotype). Diversity within an ecotype is only transient, awaiting its demise with the next periodic selection event. Ecological diversity in bacteria is governed by three kinds of mutations (or recombination events). Niche-invasion mutations found a new ecotype, such that the new genotype and its descendants escape the diversity-purging effect of periodic selection from their former ecotype. Periodic selection mutations then make the different ecotypes more distinct by purging the diversity within but not between ecotypes. Lastly, speciation-quashing mutations may occur, which can extinguish another ecotype even after it has had several private, periodic selection events. For example, an ecotype that shares all its resources with another ecotype, albeit in different proportions, may be extinguished by an extraordinarily fit adaptive mutation from the other ecotype. Sequence clusters, as determined by a variety of criteria, are expected to correspond to ecotypes. Sequence-based approaches suggest that a typical named species contains many ecotypes. That periodic selection occurs in nature is evidenced by the modest levels of sequence diversity observed within bacterial species, levels that are too low to be explained by genetic drift. Also, a special kind of periodic selection event, driven by "adapt globally, act locally" mutations, is inferred when strains fall into discrete sequence clusters over most of their genomes, but are aberrantly homogeneous in a small chromosomal region. Beyond establishing a history of periodic selection, this pattern can help corroborate that a set of sequence clusters correspond to ecotypes. © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e Selective Sweep 2 © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e Selective Sweep 4 © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e Selective Sweep 10 © 2 0 0 4 C o p y r i g h t E u r e k a h / L a n d e s B i o s c i e n c e D o N o t D i s t r i b u t e Selective Sweep 16