Understanding evolutionary mechanisms is fundamental to a balanced biological education, yet practical demonstrations are rarely considered. In this paper we describe a bacterial liquid microcosm which can be used to demonstrate aspects of evolution, namely adaptive radiation, niche colonisation and competitive fitness. In microcosms inoculated with Pseudomonas fluorescens SBW25, evolved mutants such as the Wrinkly Spreader (WS) rapidly arise to form biofilms covering the air–liquid (A–L) interface. WS are readily isolated due to a distinctive colony morphology and reach ∼30% of the population within five days. When re-inoculated into static microcosms, WS preferentially colonises the A–L interface by producing a biofilm, demonstrating a niche preference distinct from the ancestral SBW25 which grows throughout the liquid column. This ability provides the WS with a ∼2.5× competitive fitness advantage over the non-biofilm forming ancestral SBW25. However, WS and SBW25 have similar fitness in shaken microcosms where biofilms cannot form. These practical demonstrations of WS evolution, suitable for secondary or tertiary-level classes, can be linked with a literature-based review of the underlying molecular biology of the WS phenotype to provide a true exemplar of the modern evolutionary synthesis, the current paradigm in evolutionary biology.