Watching Populations Melt Down.

One of the curious features of Darwinian Evolution is that it requires errors to be made during genome replication. Otherwise, beneficial mutations could not occur and adaptation would arrest. Not surprisingly, however, random DNA copy errors usually do not lead to improvement—most mutations are deleterious or have little effect (1). Because, as far as we know, evolution has not found a way to selectively boost beneficial mutations, one wonders how large replication error rates could possibly be without overwhelming the population with deleterious mutations (2). This question of maximal sustainable mutation rates has fascinated theoretical evolutionary biologists for decades (2,3), yet experimental results are scarce. Now, Max Lavrentovich (a theorist), Mary Wahl (an experimenter), and their colleagues (4) describe an elegant way to watch microbial populations on the verge of succumbing to deleterious mutants. The simplest model for the tradeoff between natural selection and deleterious mutations is illustrated in a two-state population model in Fig. 1. One-way mutations occurring at rate m per generation supply a continuous flux from the wild-type to the mutant type. This flux is compen-