  The symmetric distribution and all other states in the symmetry sector of the
frequency trajectory increase mean fitness during competitive replication at
sublinear propagation rates (parabolic time course). States in the non-symmetry
sector, by contrast, produce negative time variations in mean fitness. The
polymorphic steady state attained in sublinear systems is destabilised by
formation of a variant with above threshold fitness. Evolution in the
post-steady state interval increases threshold fitness. Contrary to the
proposition that 'parabolic growth invariably results in the survival of all
competing species,' only species with sufficient fitness to avoid subthreshold
frequencies survive.
