Hypothetical scheme in which reproductive isolation is determined by the appearance of chromatin diminution in one of the populations. Genome reduction of parental species (P) can take place in its descendants in two ways: population A, in which genome reduction occurred only in somatic line cells, and population B, in which genome reduction took place in somatic and germline cells. In population B due to inversion the linear order of the arrangement of functionally significant DNA sections has changed (population B1). In hybrids of individuals of populations A and B, partial conjugation will take place in meiosis. In hybrids of individuals of populations A and B1 conjugation in meiosis will be impossible and such hybrid individuals will be infertile.