  We propose a variation of the GMS model of evolution of species. In this
version, as in the GMS model, at each birth, the new species in the system is
labeled with a random fitness mark, but in our variation, to each extinction
event is associated a random threshold mark and all species with fitness lower
than the threshold are removed from the system.
  We present necessary and suficient criteria for the recurrence and transience
of the empty configuration of species; we show the existence of a long time
limit distribution of species in the system, and present necessary and
suficient criteria for the finiteness of the number of species in that
distribution. There is a remarkable symmetry between both sets of criteria.
