60 THE PLANT SPECIES 



hybrid, is a very different situation from a mass flow of genes 

 between species by way of a bridging population of hybrid 

 origin. In the one case the hybridizing entities are still good 

 species in the taxonomic sense and only slightly contaminated in 

 the biological sense; in the second case they have probably 

 become geographical subspecies; and between these extreme con- 

 ditions there is every intermediate stage in the merging of spe- 

 cies. 



If it should prove useful to designate the merging of separate 

 species into a single species by a special term, the process might 

 well be called secondary speciation. Secondary speciation is that 

 stage of the evolutionary process at which two or more repro- 

 ductively isolated populations become combined into one inter- 

 breeding population. The process is a gradual one and, as with 

 primary speciation, presents a formal difficulty during the period 

 of transition. 



A concrete example of secondary speciation documented by 

 fossil records is the progressive fusion of Pinus muricata and P. 

 remorata since Pleistocene time (Mason, 1949). These two 

 closed-cone pines were distinct and sympatric on the Santa Bar- 

 bara mainland of California in the Pleistocene. Since that time a 

 single variable population with the combined characteristics of 

 both P. muricata and P. remorata has developed in this area. In 

 Pleistocene time Pinus remorata was uniform and distinct on 

 Santa Cruz Island thirty miles off the Santa Barbara coast. Dur- 

 ing this period P. remorata was probably the only species present 

 on the island. Today both species exist on the island, P. muricata 

 probably having arrived in the Late Pleistocene or Post-Pleisto- 

 cene. Some of the recent populations of P. remorata are no longer 

 uniform and sharply distinct from P. muricata but, on the con- 

 trary, are variable and introgressive witli characteristics which 

 indicate that they arc merging with P. muricata. 



If the merging of species proceeds at a rapid rate and goes to 

 completion, the number of populations which cannot be assigned 

 to the alternative categories of species or subspecies will lie rela- 

 tively small. Enough cases are actually known, however, of spe- 

 cies groups in an intermediate stage of secondary speciation as to 



