Speciation 531 



These index methods have received considerable attention of 

 late among geneticists who are interested in a study of evo- 

 lution, speciation, and hybridization. They are useful also to 

 taxonomists and others who may be interested in natural 

 populations. 



Speciation 



We might now consider some of the factors that bring about 

 the formation of new species. Such species formation in nature 

 is called speciation and can well be illustrated by Crepis and 

 Oenothera. 



Crepis. A genus that has been studied intensively for a num- 

 ber of years from the points of view of taxonomy, genetics, cyto- 

 genetics, and plant geography is the composite genus Crepis, 

 which belongs to the tribe Cichorieae and is closely related to 

 Lactuca and Hieracium. We shall point out briefly a few of the 

 important genetic processes which have been operative in speci- 

 ation and have been discovered largely through the extensive 

 and intensive studies of Babcock and his many co-workers. Bab- 

 cock recognizes 196 species of Crepis, of which he has grown and 

 studied 113. 



The genetic processes which have operated in speciation can 

 be divided into the primary genetic processes and the secondary 

 genetic processes. The first group includes gene mutations and 

 changes in the structure of the chromosomes, whereas the sec- 

 ondary processes are interspecific hybridization, polyploidy, and 

 apomixis. 



One of the most important factors in speciation in the genus 

 is gene mutation, a process that appears to occur frequently. 

 Gene mutations have brought about numerous morphological 

 and physiological variations within certain species and have ap- 

 parently also been important in the differentiation of species after 

 reproductive isolation has occurred. In a few species gene 

 mutation has also apparently brought about sterility in certain 

 interspecific hybrids that appear to have resulted from a cross 

 between two parents that do not differ as to any large duplica- 

 tions, translocations, or other structural aberrations. This Fi 

 hybrid sterility is an example of a reproductive isolating mecha- 

 nism that functions solely as the result of gene action. Finally, 

 and this is apparently not too well established, gene action ap- 



