  The Poisson--Dirichlet distribution arises in many different areas. The
parameter $\theta$ in the distribution is the scaled mutation rate of a
population in the context of population genetics. The limiting case of $\theta$
approaching infinity is practically motivated and has led to new, interesting
mathematical structures. Laws of large numbers, fluctuation theorems and
large-deviation results have been established. In this paper,
moderate-deviation principles are established for the Poisson--Dirichlet
distribution, the GEM distribution, the homozygosity, and the Dirichlet process
when the parameter $\theta$ approaches infinity. These results, combined with
earlier work, not only provide a relatively complete picture of the asymptotic
behavior of the Poisson--Dirichlet distribution for large $\theta$, but also
lead to a better understanding of the large deviation problem associated with
the scaled homozygosity. They also reveal some new structures that are not
observed in existing large-deviation results.
