CHAPTER 12 



The Genetic Nature of Taxonomic DifiFerences^ 



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^ 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. 



(i) Simple diploid monoecious or hermaphrodite species. 



(2) Polyploid species. 



(3) Mixed species which include several different races, which may 



differ by polyploidy, or by chromosome translocations, inver- 

 versions, etc. 



(4) Diploids with a sex chromosome mechanism in which one 



sex is permanently heterozygous. These are clearly near to (i) 

 and (5). 



(5) Complex heterozygous species such as those of Oenothera (p. 1 10). 



(6) 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, trisomies, etc. 



It will be convenient to discuss polyploid species first. At the end of 

 the chapter we shall consider whether there may be species differences 

 which are not dependent on the usual chromosome mechanisms. 



^ General references: Dobzhansky 1937a, h, Haldane 1929, 19326, 

 2 Darlington 1932a. 



