At the beginning of the 1960 th , the chromosome numbers of more than 17,000 angiosperms were known. Ferns do also have extremely high values. An increase in the number of chromosomes is usually associated with a reduction in chromosome size. The comparison of the karyotypes of closely related species revealed some conspicuous, though not universally applicable patterns of distribution. We will focus on this topic in a section of its own, because of the eminent importance such ploidy series have in connection with the speciation problem.
Related species do often have the same number of chromosomes.
Identical numbers of chromosomes do not necessarily mean identical shapes of comparable chromosomes. Examples of different shapes at roughly the same chromosomal size occur in species of the genus Lilium . Related species are often characterized by differently sized chromosomes . Homologies of single chromosomes or chromosomal segments occur also in related species. The differences in the karyotypes of Narcissus bulbodicum and Narcissus cantabricus , sympatric species from the south-west Mediterranean, for example, seem to be the result of two inversions and three translocations. .