  The growth rate of organisms depends both on external conditions and on
internal states, such as the expression levels of various genes. We show that
to achieve a criterion mean growth rate over an ensemble of conditions, the
internal variables must carry a minimum number of bits of information about
those conditions. Evolutionary competition thus can select for cellular
mechanisms that are more efficient in an abstract, information theoretic sense.
Estimates based on recent experiments suggest that the minimum information
required for reasonable growth rates is close to the maximum information that
can be conveyed through biologically realistic regulatory mechanisms. These
ideas are applicable most directly to unicellular organisms, but there are
analogies to problems in higher organisms, and we suggest new experiments for
both cases.
