The Origin of Crepis and Related Genera 33 



genetic evolutionary processes involved in speciation within 

 these groups are processes v^hich are not accompanied by many 

 visible changes in the chromosomes. Factor or gene mutation 

 (genovariation) is one such evolutionary process of general 

 occurrence. Experimental genetic evidence is thus far very lim- 

 ited from this group of plants. It can be stated, however, that 

 Fi hybrids from C. pulchra x C. palaestina are highly fertile and 

 produce F2 progeny exhibiting numerous combinations of char- 

 acters from the original parents. This is just the result that 

 would be expected if the two species differ in numerous genes 

 and yet are homologous with respect to many others. On the 

 contrary, Fi hybrids from C. pulchra x C. Stojanouii are nearly 

 if not quite sterile, which suggests an almost complete lack of 

 homology or some combination of genes in the hybrid which 

 prevents sexual reproduction. 



Similar evidence from various groups of related species leads 

 to the definite conclusion that gene mutation is an evolutionary 

 process of general occurrence throughout this genus. But it must 

 be viewed as superimposed upon or acting concurrently with 

 those more profound genetic processes involving changes in 

 chromosome number and chromosome morphology. 



Changes in size of the chromosomes have occurred in Crepis 

 independently of changes in shape. Chromosomes have both 

 increased and decreased in size in the evolution of the genus, 

 but there is a general tendency toward reduction in size of the 

 chromosomes concurrendy with reduction in size of the plant 

 and reduction or specialization of parts. In figure 8 the genoms 

 of another group of Eucrepis are shown. The differences in size 

 of the chromosomes are striking and they illustrate the general 

 tendency toward reduction in the more recent and highly special- 

 ized species. C. oreades and C. Rohertioides are perennials but 



