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8.

inversions and translocations of sizeable parts of the chromosomes, a slow
and erratic process which is little known in plants but reasonably well
understood in Drosophila. It has been named gradual speciation, and it
certainly is the predominant process by which species differentiate. Another
process, which is efficient in isolating gene pools, is polyploidy, or the
sudden duplication of the chromosome set of an individual, usually a hybrid,
a process which is termed abrupt speciation. Although small variations in
chromosome number within a gene pool often can be tolerated, duplications
of an entire chromosome set effectively isolate the new gene pool from the
old, either by preventing all intercrossing because of incompatibility, or,
more frequently, by making it impossible for the hybrid to reproduce because
of difficulties in distributing evenly the chromosomes of an individual
with unequal sets of chromosomes, as will be typical of the hybrid. Plants
with different multiples of the same chromosome set are often able to cross,
but their mixing of genes is made difficult or prevented by sterility of
the hybrids. It is the genetical miscibility which is the most important
characteristics of a good Linnean and biological species.

It is this observation of the chromosomes as the material basis of
evolution that the species level which has caused the success of the
cytotaxonomical method. This approach was originally invented as a
subsidiary to genetical studies of variation within the species. The success
of the model in interpreting the most important entity of evolution, and in
identifying this entity with the truly Linnean species, in due course
reinforced the initial advantage of the method. The greatest strength of
the cytotaxonomical approach lies in the fact that it has given biologists
the first universally applicable definition of this basic taxonomical
category as it was understood by Linnaeus, at the same time as it has