536 Cytogenetics and Evolution 



and lack any lethals. There are no true-breeding complex hetero- 

 zygotes, and the situation with respect to species and speciation 

 is comparable to that in most genera. In Arizona, New Mexico, 

 and Utah, however, some or all the plants of a population will 

 have circles of intermediate size. Lethals, both balanced and 

 other kinds, are also present in a few plants. Spreading out into 

 Colorado on the east and Washington on the north, most of the 

 plants have a circle of fourteen and balanced lethals, so that all 

 plants breed true. It is in these areas in which circles are found 

 that the taxonomic problems arise, for a great many genetic types 

 exist that are very difficult to classify. 



According to Cleland, the Eastern Oenotheras are character- 

 ized by the presence of a large number of races, each of which is 

 a complex heterozygote. These races breed true, although they 

 are often highly heterozygous, because of balanced lethals. Sev- 

 eral races frequently are found together in the same area, but 

 they are usually reproductively isolated because of a strong 

 tendency to self-pollination. These barriers must be broken 

 down from time to time, as otherwise heterozygous types would 

 not have arisen. On the whole, however, they are very effective 

 in preventing widespread crossing and the formation of hybrid 

 swarms. A taxonomic study is rendered even more difficult by 

 the fact that in many races one of the complexes masks the other 

 phenotypically so that the presence of the hidden complex can- 

 not be detected without breeding studies. Further complicating 

 factors are the great number of such races, the inability to dis- 

 tinguish certain races phenotypically, and the fact that com- 

 plexes with the same arrangement of their translocated chromo- 

 somal segments may be present in more than one race. Larger 

 groupings exist than the true-breeding races or biotypes, and 

 they have more or less distinctive cytogenetic behavior. L^nfor- 

 tunately, however, these groups cannot always be distinguished 

 from one another by their phenotypes, and for that reason there 

 has been considerable hesitancy in referring them to the cate- 

 gory of species. 



Summarizing the problem of evolution in the Oenotheras, Cle- 

 land points out that as a group they have arisen relatively 

 recently but that they have nevertheless developed a rather 

 unusual combination of characters that results in a very effective 

 isolating mechanism. This isolation has permitted the accumu- 



