Such changes of the karyotype are therefore a visible expression of the genome s rearrangement. In order to find rules or tendencies correlated with the systematic position of a species or a higher taxon, a comprehensive survey of the karyotypes of as large a number of species as possible was required. Considerable progress in clarifying the phylogenetic relations between species came about as a consequence of the analysis of the karyotypes, and the simultaneous study of the geographic distribution and the ecological preferences of closely related species. A general answer, however, is still far away, since the discovery of certain evolutionary strategies is no evidence for their general validity. 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.