INTROGRESSION AND EVOLUTION 



77 



Washington, a variety of garden lettuce (Laduca saliva) with 

 dark red leaves is widely grown. This color difference is 

 dominant in crosses with weed lettuce {Laduca serriola), and 

 one can therefore recognize naturally occurring hybrids be- 

 tween the two lettuces. Ordinarily, because so many of the 

 characteristics of cultivated lettuce are recessives accumu- 

 lated under domestication, the hybrid looks so unlike garden 



Table 3 * 



Comparison of Morphological Features of Helianthus annuus, 

 H. Bolanderi, and Their Hybrid 



* Adapted from Heiser (19476). 



lettuce that it escapes critical notice. Using the red-leaved 

 character as a marker, Ownbey has been able to demonstrate 

 the extensive introgression that is continually going on from 

 garden lettuces into weed lettuces, previously largely un- 

 suspected because the hybrids and hybrid derivative mon- 

 grels were superficially so similar to wild lettuce and so un- 

 like garden lettuce. 



An effective demonstration of the role of introgression in 

 building up weed complexes is afforded by two species of 

 fleabane, Erigeron annuus and Erigeron strigosus {= E. 



