(32 THE PLANT SPECIES 



Significance of Hybridization for the Species Concept 



The bearing of natural hybridization on the species concept 

 may be illustrated by three concrete examples selected from 

 Gilia. All these examples involve diploid sexual organisms with a 

 normal chromosome cycle and a rate of natural cross fertilization 

 which varies from high to low but never reaches exclusive in- 

 breeding. The integrity of the species as a biological unit is 

 slightly impaired in the case of the first two examples but it 

 breaks down in a more serious way in the third example. It is 

 this third case that proves the most instructive for the question 

 of the objective reality of species limits in a hybrid complex. 



The first case is that of Gilia capitata, a widely distributed spe- 

 cies on the Pacific slope of North America, and Gilia millefoliata, 

 a species of the coastal sand dunes. The two species come into 

 contact in a number of places along the Pacific coast. Morpho- 

 logically, they are amply distinct, G. capitata being an erect plant 

 with globose flowering heads and G. millefoliata a prostrate 

 spreading plant with spotted flowers borne in small clusters. They 

 are also well isolated by sterility barriers and can be crossed only 

 with difficulty in the experimental garden. The hybrids then are 

 highly sterile and are characterized by a low degree of chromo- 

 some pairing. On Cape Mendocino in northern California a local 

 breakdown in the complex of reproductive isolating mechanisms 

 has led to a limited amount of natural hybridization. The bound- 

 aries of the species are, however, quite clearly defined even here, 

 which suggests that the populations are maintaining themselves 

 as two distinct species. 



The second case concerns Gilia capitata itself (Grant, 1950). 

 This is a polytypic species composed of eight subspecies which 

 align themselves morphologically and ecologically into three 

 main groups. These major subdivisions of the species in all proba- 

 bility represent a trio of formerly distinct species. Vestiges of the 

 original reproductive isolation between the ancestral species still 

 persist in a few areas ol syinpatric overlap. The extreme popula- 

 tions are connected by a complete series of intergradations. Sinee 

 the intermediate populations occupy geologically more recent 



