ABSTRACT
The genetic analysis of taxonomic differences is still in its infancy; by far the greater majority of animals and plant species are genetically unknown, though cytologists have already advanced rather farther in the comparative study of their chromosomes, particularly in plants. However, the comparatively small number of well-analysed cases are already enough to show us that species, and other taxonomic groups, may differ in any of the ways genetically possible. We find a rough, but only a rough, correspondence between the magnitude of the genetical differences and the taxonomic interval between different groups. The imperfections of the correlation are perhaps only to be expected when we reflect that the word species covers groups which are very different in genetical status. Darlington 2 lists six types of species, which will be affected in different ways by the evolutionary mechanisms of variation and inheritance. They are given in order of increasing hybridity (i.e. genetical inhomogeneity) of individuals.
Simple diploid monoecious or hermaphrodite species.
Polyploid species.
Mixed species which include several different races, which may differ by polyploidy, or by chromosome translocations, inver-versions, etc.
Diploids with a sex chromosome mechanism in which one sex is permanently heterozygous. These are clearly near to (1) and (5).
Complex heterozygous species such as those of Oenothera (p. 110).
Clonal species, which reproduce vegetatively or by apomixis; they are frequently aneuploid, i.e. contain chromosome sets which are unsuited to performing regular reduction, as in triploids, trisomics, etc.
