182 I The Process of Evolution 



translocations and two inversions. ) Lewis and Raven have concluded 

 that C. franciscana has originated relatively recently from C. rtibi- 

 ciinda and that it may have been the result of a rapid repatterning 

 of the chromosome set, producing many differences in a relatively 

 short time. It also seems likely that C rubicunda may have arisen 

 from C. amoena at an earlier time. 



Some plants, such as Oenothera, subgenus Euoenothera, have 

 evolved translocation systems of amazing complexity. The develop- 

 ment of these systems seems to have involved selection for hybrid- 

 ity. This group is well known through the work of Cleland and 

 others. Indeed, the "mutation hypothesis" of De Vries was based 

 upon studies of Oenothera erythrosepala (O. lamarckiana) . Some 

 species of the subgenus, for example, O. hookeri, have seven pairs 

 of chromosomes (Fig. 9.1) or small rings (floating translocations). In 

 others, such as O. biennis, all their chromosomes are in a ring of 14 

 at meiosis (Fig. 9.1 ). In addition to the reciprocal translocations that 

 involve all the chromosomes, there are balanced sets of lethal genes, 

 the Renner complexes, which prevent the survival of homozygous 

 offspring by acting to produce nonviable gametes or zygotes. (As 

 mentioned above, there is directed alternate disjunction.) The 

 ring-forming species have small flowers and are largely self-pol- 

 linated (in contrast to O. hookeri, for example). They consequently 

 form a very large number of highly heterozygous, mostly true-breed- 

 ing, and partially isolated races. Occasional outcrossing between the 

 races leads to the origin of new ring systems and new racial types. 

 The great number of such races, which are the primary evolutionary 

 units in the ring-forming Oenotheras, makes the taxonomy of the 

 group very difficult. 



As Cleland has pointed out, these ring-forming Oenotheras illus- 

 trate that various types of "mutations," which individually might be 

 considered harmful, may, in combination, produce a workable 

 genetic system. They form a widespread and even weedy group. 

 Reciprocal translocations (which usually lead to sterility), lethal 

 genes, and self-pollination combine to form a system in which hetero- 

 zygosity of all the chromosomes is preserved and the plants are 

 highly fertile. There are what appear to be disadvantages of such a 

 genetic system. Recombination is reduced, and there may be wastage 

 of gametes and zygotes. The failure of any part of the system de- 

 stroys the whole. Polyploidy apparently is excluded since it is in- 

 compatible with a complete ring system and lethal gene complexes. 



Changes in Chromosome Size and Shape 



By their nature, inversions and translocations also change the size 

 and appearance of the chromosomes. Pericentric inversions may 



