154 Increase in Number of Species 



IN APOMICTIC ORGANISMS 



In the completely apomictic or parthenogenetic species the popula- 

 tion consists of numerous lines or clones of mother-daughter rela- 

 tionships, but these lines do not exchange genie constituents through 

 the sexual process. Thus each individual is an independent evolu- 

 tionary line in contrast with the situation in a bisexual species in 

 which the entire system of interbreeding populations is the evolu- 

 tionary line. Each viable mutant in an apomictic line produces a 

 new kind of line which is immediately and permanently distinct 

 from sister lines in respect to the new character. One might expect, 

 therefore, that apomictic lines would vary too much to be grouped 

 into assemblages of like individuals, in other words, into species 

 in the usual sense. However, apomictic species are frequently ex- 

 tremely constant both morphologically and ecologically over a wide 

 geographic range. Pathak and Painter (1959), for example, have 

 segregated four distinctive apomictic strains or species within the 

 corn leaf aphid Rhopalosiphiwi maidis, all four abundant and widely 

 distributed in Kansas and probably over most of North America. 



It has been asserted by many writers that apomictic species may 

 be unusually well adapted to some particular situation but that 

 eventually their lack of plasticity with reference to character mixing 

 will lead to their early extinction. For this reason apomictic species 

 seem to be dismissed by many as of little evolutionary importance. 

 The large number of existing apomictic species may have only a 

 short tomorrow, geologically speaking, but they certainly are im- 

 portant ecologically today. There probably never was a time when 

 apomictic species did not occur in goodly numbers and by their 

 presence exert influential selection pressures on the bisexual species 

 whose lives they touch either as competitors, predators, or bene- 

 factors. Because of these considerations, every new apomictic species 

 is a unit of potential evolutionary importance. 



An increase in the number of these species may occur by two 

 processes: the origin of new apomictic species from bisexual forms, 

 and the diff^erentiation of new distinctive lines from parental apo- 

 mictic species. 



New Apomictic Species from Bisexual Parents 



New apomictic species may arise from bisexual parents in two gen- 

 eral ways: (1) by restriction to vegetative reproduction, or (2) by 

 complete circumvention of normal fertilization. 



