236 I The Process of Evolution 



produce only scattered Linanthus populations. This, combined with 

 a possible low gene flow in the species ( caused by pollinators trans- 

 ferring pollen mostly between nearby flowers), would help to 

 maintain the established pattern. 



Cessation of Gene Exchange 



When the distribution of an organism becomes broadly discontinu- 

 ous, exchange of genetic information between populations (or 

 groups of populations ) may cease entirely. Such situations may de- 

 velop in many ways. Emerging land may divide a marine habitat. 

 The Isthmus of Panama has been repeatedly submerged through 

 geologic time. With each new emergence some populations of 



Fig. 10.12 I Frequency ot samples of Linanthus parryae containing 

 different proportions of plants with blue flowers. Ordinate, percentage of 

 samples; abscissa, percentage frequency of blues. ( From Epling and 

 Dobzhansky, 1942, Genetics 27.) 



30 



28 



26- 



24- 



22- 



20 



18H 



16 



14 



12 



10 

 8- 

 6- 

 4- 

 2 

 



10 20 30 40 50 60 70 80 90 



100 



30 

 28 



26 

 24 

 22 

 20 



hl8 

 16 

 14 

 12 

 10 



-8 



-6 

 4 

 2 

 



