Is cross breeding of species part of evolution or seperate from it.?

My thoughts were on how cross breeding of species added to development of species. Most cross bred species like the mule for example are sterile. On investigating though 60 females mules have produced there own offspring and possibly 1 male. 60 reproductive females could easily start a new species with the right conditions. Would this be consider part of evolution and how much of it has effected evolution.