Williams's theory of the evolution of senescence: still useful at fifty.

George Williams indicated that he would not expect senescence to evolve in organisms that lack a distinction between germ line and soma. Escherichia coli--long assumed to lack even a hint of this distinction--is now known to senesce, posing what would seem to be a challenge to Williams's well-known theory of the evolution of senescence. However, in this review, I will show that cell division in E. coli produces a degree of germ-soma modularity sufficient to generate age structure and antagonistic pleiotropic effects, thereby satisfying the requirements of Williams's theory. From this perspective, senescence in E. coli is supportive and points the way to a better understanding of the pleiotropies that connect adaptive complexity and senescence. Sexual reproduction is but one of the complex adaptations illuminated by this approach.