  We investigate an extremal dynamics model of evolution with a variable number
of units. Due to addition and removal of the units, the topology of the network
evolves and the network splits into several clusters. The activity is mostly
concentrated in the largest cluster. The time dependence of the number of units
exhibits intermittent structure. The self-organized criticality is manifested
by a power-law distribution of forward avalanches, but two regimes with
distinct exponents tau = 1.98 +- 0.04 and tau^prime = 1.65 +- 0.05 are found.
The distribution of extinction sizes obeys a power law with exponent 2.32 +-
0.05.
