532 



Cytogenetics and Evolution 



pears to have resulted in both a shortening of the length of the 

 chromosomes and a general reduction in the size of the plant 

 in some species. 



Changes in the shape, number, and size of the chromosomes 

 have been an important factor in evolution in Crepis. One of 

 the effects of these changes is to cause sterility in interspecific 

 hj^brids and thus to erect a barrier to hybridization that can 

 allow the accumulation of mutations in the two isolated strains 



liiiii 



C. kashmirica 100 



llM.J 



ll"M 



C. sibirica 81 



ifill 

 lllli 



C. conyzaefolia 71 C capillaris 38 





If 



C. Mungierii 46 C. leontodontoides 38 C. Suffreniana 21 C. fuUginosa 22 



Fig. 152. Idiograms showing the evolution of karyotypes in Crepis. For 

 discussion, see text. (Redrawn from Babcock, Stebbins, and Jenkins in 

 the American Naturalist.) 



to differentiate them sufficiently so that they are considered new 

 species. Changes in chromosome structure, however, have also 

 led to changes in the number and morphology of the chromo- 

 somes in some species. 



Ninety-six species have been examined for chromosome num- 

 ber. Three species have seven haploid chromosomes, 14 have 

 six, 19 have five, 57 have four, whereas in 3 species, n = 3. Mor- 

 phological studies indicate that the six-chromosome and five- 

 chromosome types are primitive and suggest that the others are 

 derived from them. Babcock believes furthermore that coinci- 

 dent with or following a reduction in the number of the chromo- 

 somes there has been a reduction in the size of the plants and a 

 specialization of the plants. Figure 152 shows the relative 

 lengths and the shapes of the chromosomes in the extreme spe- 

 cies whose haploid chromosome number equals 3, 4, 5, or 6. The 



