202 PROTOZOA 



Paramecium aurelia: Breeding Systems and Evolution 



Disagreements concerning terminology must not be permitted 

 to obscure the biological problems that underlie them. The ques- 

 tion discussed at the end of the preceding section — whether the 

 varieties of P. aurelia should be called species or should be given 

 some other designation such as syngens — is of relatively minor 

 importance. More important are the evolutionary questions raised 

 by the recognition of these biological units of organization and 

 by the nature of the diversities among and within them. These 

 are the questions to be dealt with in the present section. In doing 

 so the biological units will have to be referred to by some term. 

 I shall continue to call them varieties so that the reader will not 

 have to change horses in the middle of the stream. However, I 

 urge that a term such as syngen eventually be agreed upon both 

 for the varieties of Ciliates and for the so-called biological species 

 of other organisms. 



Basic Biological Frame of Reference in P. aurelia. Probably 

 the most fundamental evolutionary requirements of any organ- 

 ism are to take full advantage of opportunities for reproduction 

 and to possess and maintain mechanisms for providing enough 

 genetic variability to meet the demands imposed by changes in 

 the environment. The common mechanism for the latter is of 

 course mutation. But organisms differ in the way mutations are 

 handled. Among lower organisms, two major ways are observed. 

 Some combine haploidy with large and rapidly produced popu- 

 lations. This permits both an adequate supply of mutations and 

 an effective way of bringing them to immediate or rapid expres- 

 sion. Other lower organisms, like most higher organisms, combine 

 diploidy with genetic recombination by sexual means. This per- 

 mits storing an accumulation of mutations and selecting among 

 their various combinations. The latter is the alternative generally 

 adopted by Ciliates, including Paramecium. It is further rein- 

 forced by the occurrence of asexual reproduction between sexual 

 processes and by the existence of nuclear dimorphism. The new 

 mutations are stored in the micronuclens, which has little or no 

 effect on the phenotype, and they are multiplied during asexual 



