  Motivated by the wide range of known self-replicating systems, some far from
genetics, we study a system composed by individuals having an internal dynamics
with many possible states that are partially stable, with varying mutation
rates. Individuals reproduce and die with a rate that is a property of each
state, not necessarily related to its stability, and the offspring is born on
the parent's state. The total population is limited by resources or space, as
for example in a chemostat or a Petri dish. Our aim is to show that mutation
rate and fitness become more correlated, \emph{even if they are completely
uncorrelated for an isolated individual}, underlining the fact that the
interaction induced by limitation of resources is by itself efficient for
generating collective effects.
