This concluding chapter argues that experimental evolution with microbes has emerged as a very attractive alternative to overcome the problem of long time scales in empirical studies of evolution. This is exemplified by the long-term evolution experiments of Richard Lenski, whose experimental Escherichia coli lines have evolved for more than 40,000 generations to date. Lenski and his many collaborators convincingly argued that the diversified strains have coexisted over long time periods, and hence that this diversification represents a case of asexual speciation. The ecological mechanism for diversification in this case appears to be related to crossfeeding, a scenario in which one strain or species persists by scavenging on nutrients that accumulate in the environment as metabolic byproducts of the coexisting strain. With crossfeeding, polymorphisms can be maintained even in simple environments with a single limiting resource such as glucose. This is an excellent example of frequency-dependent selection, as the fitness of the crossfeeder depends on the presence or absence of the glucose specialist.