A dynamic model has been constructed in an attempt to disclose sufficient conditions for sympatric speciation. It was demonstrated that under a particular set of conditions the speciation may be triggered as a result of a sequence of time-related events, including a decrease of heterozygote fitness (which, however, must remain within the defined limits), followed by a mutation originating assortative mating. Then the prezygotic isolation mechanism can be effectively established provided that heterozygote fitness remains low. The model was simulated and led to conclusions that sympatric speciation has limited chances both to commence and to be concluded. From the model it follows that, apart from overdominance and the balancing selection in the absence of overdominance, a third kind of stable heterozygosity does exist: the double (many) niche overdominance. This type of overdominance effectively blocks sympatric speciation.