Experimental adaptive radiation in bacteria

Bacteria are readily dispersed, so a lineage will often encounter unfamiliar conditions of growth to which it must adapt or die. This process can be studied in the laboratory by supplying a microcosm with medium containing a single limiting substrate and then inoculating it with an isogenic culture of bacteria. The culture will die if the substrate is refractory, but in practice often adapts to novel conditions through the successive substitution of beneficial mutations that confer a metabolic capability it previously lacked. This constitutes periodic selection, with one mutation after another sweeping through the population. This kind of selection experiment is particularly valuable because it allows the mechanism of adaptation to be studied directly, and in the last 20 years it has enabled us to discover a great deal about how bacteria adapt to simple environments (reviewed by Mortlock 1984; Elena & Lenski 2003). Fitness typically increases rapidly at first and then slows down, because beneficial mutations of larger effect are likely to be substituted earlier, so that over time there a non-linear approach to a plateau, with a half-life of some hundreds of generations. Hence, if a bacterial culture is inoculated into a spatially structured environment (such as a set of glass vials) in which each site offers a different substrate it will undergo an adaptive radiation through divergent specialization to each of the available substrates.