Schematics of evolution experiments using serial transfer of phage populations in the laboratory. Typically, an ancestral clone or population of phages (circles) is allowed to infect bacterial host cells under novel or control environmental conditions. At end of the passage (e.g., after 24 h of incubation), a sample of the phage population is transferred with dilution (e.g., 1:100, creating a bottleneck) into a fresh culture vessel with host cells, and a second sample is stored frozen to permit later analyses. Over time, the phage population is expected to fix mutations (red circles) that increase fitness (survival and/or reproduction) in the novel environment. (Top): Phages are serially transferred in liquid cultures containing bacterial host cells (rectangles). (Bottom): Phages are grown on lawns of bacterial hosts to form plaques (white circles, indicating zones of lysed bacterial cells). One or more plaques are collected to obtain a cell-free lysate, followed by plating with dilution onto a fresh host lawn.