Important ideas about mutations, genetic drift (survival of the luckiest) and natural selection (survival of the fittest), originally developed in population genetics, will be reviewed in a form suitable for physicists, with the aim of understanding the growth of bacterial or yeast colonies in a laboratory environment. When migrations of one- and two-dimensional populations are considered, results for mutation, selection and genetic drift are closely related to 'voter models' of interest in nonequilibrium statistical mechanics, suitably extended to allow for inflation of a thin layer of actively growing pioneers at the frontier of a colony of microorganisms undergoing a radial range expansions on a Petri dish.