Strains and culture conditions
We used the bacterial strain Pseudomonas fluorescens SBW25 (Rainey & Bailey 1996) and the lytic bacteriophage SBW25Φ2 (Buckling & Rainey 2002). Cultures were grown statically in microcosms of 6 mL of KB medium in 25 mL glass vials with loosened lids (glycerol 10 g L-1, proteose peptone no.3 20 g L-1, K2HPO4·3H2O 1.5 g L-1, and MgSO4·7H2O 1.5 g L-1). The optimal growth temperature for this bacterium is 28°C. When grown in batch cultures with a dilution rate of 0.01 every 48 h at a low temperature, 10°C, the bacterium can maintain a viable population with an average population sizes 0.15-fold of that at 28°C (Figure S1; density of the ancestral strain at 10°C was approximately 6.2 × 108 cells mL-1).