EVOLUTION OF NUMBER AND MORPHOLOGY OF MAMMALIAN CHROMOSOMES

Numerous hypotheses have been advanced to explain how selection might operate on the level of the chromosomal complement (chromosome selection) and how this process could be related to morphological divergence and speciation. Hypotheses emphasizing a general trend of increasing or decreasing diploid numbers in mammalian evolution are contradicted by documented studies from diverse families of mammals. However, certain selective forces were identified that could drive chromosomal evolution in certain lineages under limited conditions. Species with higher fundamental numbers and/or higher diploid numbers are expected to have increased genetic recombination. Increased recombination is advantageous in increasing variability and thus utilization of a wider niche, but decreased recombination allows fixation of new mutations (both genic and chromosomal) and thus increased potential for phyletic divergence and speciation. The species with higher diploid numbers are also less likely to undergo deleterious translocation and inversion events because of the position of the chromosomes in the nucleus. A decrease in diploid or fundamental numbers by fusions or inversions would be important as an initial event in phyletic divergence in some groups. A rearrangement in one chromosome also could affect nuclear structure or recombination in other chromosomes and, in some lineages, could drive the acquisition of additional rearrangements. It is suggested that changes in the level of recombination and in nuclear stability played a pivotal role in mammalian chromosome evolution.