  George Williams defined an evolutionary unit as hereditary information for
which the selection bias between competing units dominates the informational
decay caused by imperfect transmission. In this article, I extend Williams'
approach to show that the ratio of selection bias to transmission bias provides
a unifying framework for diverse biological problems. Specific examples include
Haldane and Lande's mutation-selection balance, Eigen's error threshold and
quasispecies, Van Valen's clade selection, Price's multilevel formulation of
group selection, Szathmary and Demeter's evolutionary origin of primitive
cells, Levin and Bull's short-sighted evolution of HIV virulence, Frank's
timescale analysis of microbial metabolism, and Maynard Smith and Szathmary's
major transitions in evolution. The insights from these diverse applications
lead to a deeper understanding of kin selection, group selection, multilevel
evolutionary analysis, and the philosophical problems of evolutionary units and
individuality.
