Chromosomal Variation and Evolution

Abstract

Chromosome numbers suggest that the early history of the angiosperms was divided into 3 stages, characterized by different kinds of chromosomal changes. During the first of these stages, the earliest woody forms, now largely extinct, underwent aneuploid changes of basic number at the relatively low levels of x= 9 to x=5, perhaps through both upward and downward progression from an original number of x= 7 or x=6. Then ensued a period when the woody forms built up higher basic numbers by means of polyploidy. At the same time some of them probably gave rise to the earliest herbaceous angiosperms, such as the Ranunculaceae, Saxifra-gaceae, and Berberidaceae, without alteration of the basic number. During the 3rd period of evolution the predominant process was specia-tion and generic differentiation of woody forms at the higher chromosomal levels which they had reached by polyploidy. During this time, the primitive herbs were probably few in number and restricted in dis- . tribution, so that they did not evolve extensively. A 4th period of angio-sperm evolution, which probably took place during the middle and latter part of the Tertiary Period, saw the decline of the woody groups except in the moist tropics and the rise to prominence of the previously subordinate herbaceous vegetation. This last phase was accompanied by extensive polyploidy in the herbaceous groups. These 4 periods almost certainly overlapped extensively at various times and places.