letters to nature Analyses We determined the population size of the bacteriophage by adding 30 ml of chloroform to a 1 ml subsample of each chemostat to remove bacteria and plating 100 ml of the purified subsample of bacteriophage with 300 ml of an overnight culture of bacteria in 3 ml of soft agar We counted the number of plaques spots cleared of bacteria by the bacteriophage after 24 h Because the bacteriophage were well mixed before plating each plaque was presumed to result from a single viral particle A lawn of ancestral bacteria B0 was used to determine the total size of the bacteriophage population and a lawn of resistant bacteria B1 was used to calculate the number of host range mutants T71 in the bacteriophage population We determined the population size of the bacteria by plating 100 ml of a subsample without chloroform treatment and counting the number of bacterial colonies present after 24 h of incubation To determine the size of the B1 population we plated the bacteria with an equal volume of ancestral bacteriophage To investigate local adaptation in the bacteriophage through time we isolated two to three colonies of B1 from the same day day 9 13 and 19 from each treatment during each run of the experiment For each adaptation assay overnight cultures of the colonies were grown up in 1 mg ml21 glucose media We plated replicate samples of T71 from each time point on a lawn of sympatric from the same chemostat and allopatric from a different chemostat but at the same productivity level B1 from the overnight culture We used the efficiency of plating the number of plaques on each host as a measure of bacteriophage infectivity We calculated adaptation as the ratio of the number of plaques formed by the numerically dominant bacteriophage on the dominant sympatric host to the number of plaques formed on the allopatric host Ratios of the number of plaques formed on the bacterial isolates from each replicate run of the experiment were averaged to give a mean ratio for each run Only one replicate run of the experiment was used in assays of the closed high productivity community due to contamination There were often no plaques on allopatric hosts therefore we coded the data by adding one to both the numerator and the denominator to allow calculation of a ratio A value above one indicates local adaptation and a number below one indicates local maladaptation that is the number of plaques was higher on the allopatric host than on the sympatric host All ratios were logtransformed before analysis Variation in adaptation was assessed by calculating the coefficient of variation of the adaptation ratios Significance was tested using an F test with a sequential Bonferroni correction for five pairwise comparisons25 Inspecting the data in detail showed that the reversed pattern for the open and closed intermediate productivity communities was driven by one of the two replicate runs of the experiment Thus we did not include data from the intermediate productivity communities in the analyses Received 3 July accepted 30 July 2004 doi 10 1038 nature02906 1 Thompson J N The Coevolutionary Process Univ of Chicago Press Chicago 1994 2 Brodie E D Jr Ridenhour B J Brodie E D III The evolutionary response of predators to dangerous prey hotspots and coldspots in the geographic mosaic of coevolution between garter snakes and newts Evolution 56 2067 2082 2002 3 Burdon J J Thrall P H Spatial and temporal patterns in coevolving plant and pathogen associations Am Nat 153 S15 S33 2002 4 Zangerl A R Berenbaum M R Phenotype matching in wild parsnip and parsnip webworms causes and consequences Evolution 57 806 815 2003 5 Thompson J N Cunningham B M Geographic structure and dynamics of coevolutionary selection Nature 417 735 738 2002 6 Benkman C W Parchman T L Favis A Siepielski A M Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine Am Nat 162 182 194 2003 7 Nuismer S L Thompson J N Gomulkiewicz R Gene flow 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