ABSTRACT
Quantitative estimation of nuclear DNA content, either chemically or by microdensitometry, has added a new dimension in the study of evolutionary relationships. The presence of significant differences in nuclear DNA content within narrow taxonomic groups among higher plants is well recognized. On the basis of the present data the role of relative DNA content in systematics and evolution at the genus or maybe at the family level can be assessed; however, before going into the feasibility of this approach, the nuclear DNA variation at subspecific level has to be given due consideration. Equally significant and much more surprising is the fact that the interspecific variation in DNA content, being far from random, shows a discontinuous pattern and in some cases even geometric progression. The multiplication of selfish DNA is suggested to take place by duplicative transposition, a mechanism originally postulated to account for the increase in copy number of transposed elements.
