180 I The Process of Evolution 



Y's) may evolve by this process, as has been the case apparently in 

 Drosophila miranda. Karyotype evolution in the Drosophila virilis 

 group also involves translocations between autosomes and sex chro- 

 mosomes. An extreme case of multiple sex-chromosome mechanisms 

 is found in the Palestinian beetle Blaps polychresta. In the males 

 there are 12 X chromosomes and 6 Y chromosomes. The females have 

 24 X chromosomes. 



Genetic systems based upon regularly occurring structural hybrid- 

 ity for reciprocal translocations are characteristic of a few groups 

 of plants. The behavior of translocation systems in nature is not well 

 understood for most plants, although rings are reported for a num- 

 ber of genera, for example, Rhoeo, Paeonia, Datura, Hypericum, and 

 many Onagraceae. Progress toward an understanding of such mech- 

 anisms in natural populations of Clarkia and Oenothera is being 

 made by Lewis, Raven, and their associates. Clarkia has been studied 

 in some detail cytologically. Species invariably differ by chromo- 

 some rearrangements, and interspecific hybrids are often highly 

 sterile. It has been estimated that in many species of Clarkia more 

 than 20 percent of the plants in nature have rings of chromosomes, 

 indicating the presence of translocations. Such rearrangements form 

 a part of the genetic system and may characterize whole populations. 



An interesting example of evolution involving chromosome re- 

 arrangements is Clarkia franciscana, studied by Lewis and Raven. 

 This largely self-pollinated plant is restricted to an area of serpentine 

 rock in San Francisco which is within the geographic range of the 

 closely related C rubicunda. Apparently C franciscana is also re- 

 lated to C amoena, a northern species. Studies of meiosis in hybrids 

 show that C. franciscana differs from C. amoena by at least two 

 translocations and two paracentric inversions. It difiFers from C. 

 rubicunda by at least three translocations and four inversions (see 

 Fig, 9.2). (Clarkia amoena and C. rubicunda differ by at least three 



Fig. 9.2 I (see opposite page) Evolution involving chromosomal re- 

 aiTangements in Clarkia. a, seven bivalents in C. franciscana; h, ana- 

 phase I showing two bridges and two fragments in cross between 

 C. rubicunda and C. franciscana; c, anaphase I showing chain of five 

 chromosomes, a chain of three chromosomes, and three bivalents in 

 cross between C rubicunda and C franciscana; d, anaphase I showing 

 two bridges, two fragments, and a lagging chromosome in a cross be- 

 tween C. amoena and rubicunda. Map shows distribution of three spe- 

 cies in San Francisco Bay area. ( After Lewis and Raven, 1958, Evolution 

 12, Brittonia 10; and Lewis and Lewis, 1955, Univ. Calif. Publ. Bot. 20. ) 



