TAXONOMIC PRINCIPLES 



29 



detected or not, would hardly affect classificatory systems since, in 

 both the phylogenetic and typological approaches, the taxa con- 

 cerned would be grouped in about the same relative systematic posi- 

 tions. The type of parallelism shown in Fig. 2- 10a poses a more 

 difficult problem, but, except where one or only several criteria are 

 selected for emphasis over other kinds of data, such cases are 

 apparently uncommon. When autonomous paralleHsm^ following 

 "convergence" has been a factor in the evolution of a plant group, its 

 discovery is more likely to reflect the soundness of a broad, synthetic 

 (albeit predominately morphological) approach to higher plant classi- 

 fication. 



The case for convergence in most closely related taxonomic 

 groups usually rests upon the quantitative features in one or at most 

 only a few characters. If these characters are important "key" charac- 

 ters (discussed below), then any systematic treatment based on such 

 features is hkely to be more artificial than natural, and to cite such 

 examples as instances of erroneous phylogeny resulting from conver- 

 gence and parallelism is to stretch the case. If two taxa have diverged 

 sufficiently to be recognized by their phenotypic differences, reflecting 

 multiple gene differences, then, on a priori grounds, the chance for 

 absolute genetical convergence seems most unlikely in view of our 

 present knowledge of mutational rates and the subtleties involved in 

 the selective forces having to do with character fixation. 



Several workers have mentioned examples of what appear to 

 be autonomous convergence and parallelism for certain characters of 

 different taxa of higher plants (Bailey, 1944). An even more striking 

 parallelism has been described for some chemical components of 

 otherwise widely differentiated taxa. One rather striking example is 

 the occurrence of the hemoglobin molecule in cells of fungi and in the 

 root nodules of legumes (White, et al, 1959). Several additional 

 examples of chemical convergence and parallelism will be discussed 

 elsewhere in the present text more fully. 



The argument that convergence and parallelism make it im- 

 possible to achieve a meaningful phylogenetic system can be appro- 

 priately countered with the following remark from Crow (1926): 



The problem of the cause of convergence and parallel development is, 

 of course, an extremely important one. But inasmuch as convergence 

 itself was discovered by systematic and morphological investigations, 

 and is itself a phylogenetic conclusion from the systematic and 

 anatomical facts, the necessity of making more detailed study of 

 phylogeny is all the more necessary. ... To use the polyphyletic origin 



5 As distinguished from parallelism due to hybridization and subsequent back- 

 crossing. 



