  Using an artificial system of self-replicating strings, we show a correlation
between the age of a genotype and its abundance that reflects a punctuated
rather than gradual picture of evolution, as suggested long ago by Willis. In
support of this correlation, we measure genotype abundance distributions and
find universal coefficients. Finally, we propose a simple stochastic model
which describes the dynamics of equilibrium periods and which correctly
predicts most of the observed distributions.
