Selection/Recruitment Table 1: Dissemination of genes and microorganisms throughout Earth’s Critical Zone. Three phenomena, or drivers, affect microbial/gene spread. These are: opportunity for dispersal; stochastics (the number of foreign cells landing at a particular location); and recruitment (the persistence of cells at the new location, often driven by local selection). Historically, these forces generate biogeographic patterns for microorganisms that are similar to those of animals and plants. Human impacts have changed the dynamics of these phenomena, and are altering microbial biogeography in the process. Drivers Dispersal Stochastics Selection/Recruitment Natural forces Wind transport Low cell numbers Favours endemic species Ocean currents Population drift Local differentiation Freshwater flows Human impacts Human movement High cell numbers Co-dispersal with selective agents Animal movement Increased evolvability Co-selection with domesticated hosts Agriculture Ballast water Moving soil / sediment Wastewater flows