In this paper three formal models of selection operators (two known from the literature and one newly porposed) for genetic algorithms, used to learn structured concepts descriptions containing small disjuncts, are presented. The evolution of a population, according to these operators, with a generation gap equal to or less than one, is investigated in order to assess their ability to let different species emerge and coexist; finally, the average asymptotic behaviour of the population is determined. Experimental results confirm both bevahiours previously reported in the literature and the prediction of the new model.