Karyotypic fissioning and Canid phylogeny

Abstract

A theory is advanced which holds that mammalian evolution has been largely dependent on extensive chromosomal changes. These take the form of occasional simultaneous massive increase in diploid number through centric mis-division, here called karyotypic fissioning. This event occurs in a single individual and subsequently spreads through a population depending on chance, meiotic compatibility, dynamics of the population and natural selection. The possibility that fusions also occur sporadically is not precluded, although such events are not seen as abundant. This theory accounts for the wide variation in diploid number among the Mammalia and relates the presumed episodes of karyotypic fissioning with known periods of explosive speciation and adaptive radiation. Finally, the traditional evolutionary concept of mutant allele substitution through gene frequency shift under the influence of natural selection is placed in a new perspective. While it is still seen as a primary mechanism of evolution, it is seen as more significant as a “fine-tuning” mechanism, perhaps often responding to exigencies precipitated by chromosomal changes. The theory is tested by integrating its assumptions with the phylogeny of the recent Canidae as previously reconstructed from the fossil record and classical studies of anatomy. The correlation between increase in diploid number and adaptive radiation within this group is obvious. Within the family it is possible to specify approximate times and places of karyotypic fissioning which have been found to be consistent with known paleontogical and zoogeographic facts. It has also been possible to suggest relationships among living species which had not been previously recognized, but which are testable by independent techniques.