CHROMOSOMES AND MICROEVOLUTIONARY PROCESSES

The role of chromosomal rearrangement in microevolutionary processes is discussed, considering numerous aspects of the complex mechanisms of chromosomal speciation. The different degrees of chromosome-derived subfertility, a consequence of the different kinds of structural heterozygosites in the inter-racial hybrids, are discussed with reference to the effectiveness of the postmating reproductive barrier. A ''non meiotic'' view of the reproductive fitness of the Robertsonian heterozygotes and homozygotes is proposed. It is based on the possibility that Robertsonian fusion, and chromosomal structural rearrangement in general, may alter the internal topography of interphasic nucleus. Consequent upon any such change would be an alteration in the collocation of the gene clusters in the nuclear domains programmed for their regular function. It is claimed that these considerations explain the selective advantage that the new homokaryotype must have over the non-rearranged type for the new chromosomal variant to be fixed and maintained within a panmyctic population. Also discussed are the role of the demographic factors involved in chromosomal speciation and the molecular mechanisms responsible for the chromosomal rearrangement. An alloperipatric model of chromosomal speciation of seems not to conflict with the ''geographical'' speciation principle of Mayr, and to be fully justified in terms of biological theory.