194 I The Process of Evolution 



There are many groups of plants in which the major evolutionary 

 differentiation has involved allopolyploidy. One of the best examples 

 is the fern genus Aspleniurn (spleenwort), studied by Wagner and 

 his associates. As shown in Fig. 9.5, there are three primary species 

 with n = 36. Meiosis is regular, and 36 bivalents are formed in each. 

 By a study of morphology and by analysis of chromosome behavior 

 in other species, it could be shown that A. bradleyi, A. ebenoides, 

 and A. pinnatifidum are allotetraploids involving the primary species 

 in pairs. It is interesting that the sterile diploid hybrid of the same 

 parentage as A. ebenoides is known; in it there are 72 univalents, 

 instead of 72 bivalents. Asplenium trudellii is a backcross of A. pinna- 

 tifidwii to one of its parents, for it shows 36 bivalents (montanum 

 genome) and 36 univalents (rhizophijllum genome). Asplenium 

 gravesii, on the other hand, is a hybrid between two different allo- 

 tetraploids that share A. montanum as a parent. This is revealed by 

 the behavior of the chromosomes also, for the two montanum 

 genomes form bivalents, while the platyneuron and rhizophijllum 

 genomes remain unpaired (36 bivalents plus 72 univalents). 



In other groups, much more complicated polyploid complexes 

 are known. The diagram of Stebbins (Fig. 9.6) shows the combina- 

 tions of genomes and of autopolyploidy and allopolyploidy that may 

 occur (although higher levels have been found). In some groups, 

 allotetraploids are able to form partially fertile hybrids with auto- 

 polyploid forms of either of their parents. In plants, particularly 

 those with any degree of vegetative propagation, the result may be 

 to blur or obliterate the morphological limits of the taxa originally 

 involved. A group of different degrees of ploidy and varying mor- 

 phology, as well as ecological preferences, may arise. Only by 

 cytological analysis can such complexes be resolved and then some- 

 times only partially. The situation may be further complicated if the 

 plants reproduce apomictically, perpetuating individuals that are 

 sexually quite sterile (see below). 



It is difficult to specify the general importance of polyploidy as a 

 genetic system. It has been estimated that up to one-half of the 

 flowering plants are of polyploid origin. Similarly, the ferns and 

 their allies show much polyploidy, as do the algae and mosses. Only 

 the fungi form an exception and they have been very poorly sam- 

 pled cytologically. Unfortunately very little is known of tropical 

 species, so that the sampling is far from thorough or representative. 

 Whole families or subfamilies (e.g., Pomoideae of the Rosaceae, the 

 apples and their relatives), as well as genera, have numbers that 

 suggest a polyploid origin. Despite the existence of regular pro- 

 gressions of chromosome number in animals, polyploidy generally 



