  Fluctuations in diversity and extinction sizes are discussed and compared for
two different, individual-based models of biological coevolution. Both models
display power-law distributions for various quantities of evolutionary
interest, such as the lifetimes of individual species, the quiet periods
between evolutionary upheavals larger than a given cutoff, and the sizes of
extinction events. Time series of the diversity and measures of the size of
extinctions give rise to flicker noise. Surprisingly, the power-law behaviors
of the probability densities of quiet periods in the two models differ, while
the distributions of the lifetimes of individual species are the same.
