206 I The Process of Evolution 



that will give rise to males, the chromosomes behave similarly, ex- 

 cept for the X chromosomes, which pair and are reduced as in 

 meiosis. One X remains in the egg; the other goes into the polar 

 body. 



In spermatogenesis of the male sexuales, the X chromosome is 

 apportioned to one of the secondary spermatocytes in normal 

 fashion. However, those cells without the X chromosome eventually 

 degenerate and only X-containing cells produce sperm. Thus the 

 sexuales males can have only daughters, which complete the cycle 

 as fundatrices. 



Other aphids have similar cycles in which the number of genera- 

 tions may differ, in which there are two kinds of sexuparae: male- 

 producing and female-producing, etc. 



In such complexes as those described for plants and animals, the 

 usual concept of species is very difficult to apply. The sexually re- 

 producing diploids may be comparable to species in other organisms. 

 But the autopolyploids and allopolyploids that combine the char- 

 acteristics of two or more diploids in asexually reproductive and 

 therefore very fertile organisms break down the utility of criteria 

 based upon morphological intergradation, gene exchange, and geo- 

 graphic distribution. Combining the classic techniques of taxonomy 

 with the methods of cytogenetics, however, the biologist may be 

 able to identify the major evolutionary units within the complex. 

 To these he customarily gives the rank of species, while the multitude 

 of apomictic forms may be described, with or without formal tax- 

 onomic recognition, as appears most useful. 



Aside from greatly complicating the work of the biologist, what 

 are the effects of apomixis as a genetic system? It is obvious that 

 apomixis makes possible the survival of many genotypes that are 

 vigorous and well-adapted but sexually sterile for one reason or 

 another, e.g., in unbalanced polyploids. Apomixis also permits the 

 building up of large numbers of genetically similar individuals for 

 the rapid colonization of newly available habitats. One finds apo- 

 mixis often to be the genetic system of weedy or pioneer organisms 

 and of those in habitats subject to frequent or regular catastrophe, 

 such as sand bars, lawns, etc. 



It is also true that apomixis limits the genetic variability of the 

 organisms that have adopted it as their sole mode of reproduction. 

 For this reason, it generally is found to be an alternative or second- 

 ary genetic system. Apomixis usually is not combined with other 

 systems that reduce the long-range flexibility of the organism for 

 the sake of immediate fitness (e.g., self-fertilization). It is interesting 

 that, even in those groups, such as Poa, where apomixis and high 



