Speciation 533 



reduction in chromosome nmiiber has apparently resulted from 

 changes in chromosome structure, especially reciprocal translo- 

 cation. 



Some work of Tobgy has illustrated how reduction in chromo- 

 some number could occur. He compared the chromosomes of 

 Crepis neglecta, which can be designated A, B, C, and D, with 

 the chromosomes of C. juliginosa. This latter species has an A and 

 a D chromosome that are similar 



structurally to the A and D chro- ■ « 



mosomes of neglecta, except for one 5 J J i a Z 1 



reciprocal translocation. Most of I ' II 



the B chromosomes of the two | ■ 



species are homologous, but the B C. neglecta 31 C.fuliginosa 22 



of juliginosa contains also the es- 



, . , i c 1 i-t r Fig. 153. Karyotypes of two 



sential part of chromosome C oi t r^ • ^ ^• 



^ species 01 Crepis. l^or discus- 



neglecta. The other arm of this gioj,^ g^g text (Redrawn from 

 C chromosome and its centromere Babcock, Stebbins, and Jenkins 

 are missing from C. juliginosa in the American Naturalist.) 

 (Fig. 153). Apparently a recipro- 

 cal translocation occurred between chromosomes B and C of 

 either neglecta or a four-chromosome ancestor which placed 

 the large part of a C chromosome on the B. The remainder 

 of the original C was then lost. In considering the effect of 

 changes in the chromosome complement (or karyotype) , Bab- 

 cock, Stebbins, and Jenkins consider that the change in the 

 chromosomes is not a direct cause of speciation, but creates an 

 interspecific sterility that acts as a reproductive isolating mech- 

 anism, since the neglecta-juliginosa Fi hybrids were highly 

 sterile. 



Interspecific hybridization has not been nearly so important 

 a factor in speciation in Crepis as in some other genera. A few 

 species apparently have arisen by amphidiploidy, but allopoly- 

 ploidy has not approached gene mutation or structural changes 

 in importance. The same can be said also of polyploidy and 

 apomixis, which have played a definite but not an extensive part 

 in the evolution of this genus. 



Oenothera. Evolutionary phenomena in Oenothera have in- 

 terested geneticists for many years as they present some prob- 

 lems that were very puzzling for a long time. 



Among the intrachromosomal aberrations that we discussed 

 in Chapter 24 were reciprocal translocations. Their effect on 



