In contrast to SPEA, SPEA2 uses a fine-grained fitness assignment strategy which
incorporates density information as will be described in Section 3.1. Furthermore, the
archive size is fixed, i.e., whenever the number of nondominated individuals is less
than the predefined archive size, the archive is filled up by dominated individuals; with
SPEA, the archive size may vary over time. In addition, the clustering technique, which
is invoked when the nondominated front exceeds the archive limit, has been replaced by
an alternative truncation method which has similar features but does not loose boundary
points. Details on the environmental selection procedure will be given in Section 3.2.
Finally, another difference to SPEA is that only members of the archive participate in
the mating selection process.