CHROMOSOMES AND MICROEVOLUTIONARY PROCESSES

The role of chromosomal rearrangement in microevolutionary processes i s discussed, considering numerous aspects of the complex mechanisms of chromosomal speciation. The different degrees of chromosome-derived s ubfertility, a consequence of the different kinds of structural hetero zygosites in the inter-racial hybrids, are discussed with reference to the effectiveness of the postmating reproductive barrier. A ''non mei otic'' view of the reproductive fitness of the Robertsonian heterozygo tes and homozygotes is proposed. It is based on the possibility that R obertsonian fusion, and chromosomal structural rearrangement in genera l, may alter the internal topography of interphasic nucleus. Consequen t upon any such change would be an alteration in the collocation of th e gene clusters in the nuclear domains programmed for their regular fu nction. 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 withi n a panmyctic population. Also discussed are the role of the demograph ic factors involved in chromosomal speciation and the molecular mechan isms responsible for the chromosomal rearrangement. An alloperipatric model of chromosomal speciation of seems not to conflict with the ''ge ographical'' speciation principle of Mayr, and to be fully justified i n terms of biological theory.