T. M. SONNEBORN 287 



than those encountered by most sexual organisms. A large body 

 of water or a particular host, for example, acts as a buffer against 

 radical variation in the milieu. But some variations in conditions 

 are inevitable and the organism has to be prepared to adapt to it 

 or run the risk of extinction. Lacking the rapid and wide-ranging 

 variability obtainable by genetic recombination, other sufficient 

 mechanisms for yielding adaptive variations have been selected. 

 One of these is the combination of haploidy and large, rapidly 

 produced populations with mutation. This has apparently been 

 adequate for many asexual organisms. 



Another and faster-operating mechanism is one which is less 

 widely appreciated. It is the one illustrated by the serotype 

 systems of Trypanosomes and Paramecium. This mechanism in- 

 volves ( 1 ) including in the common genotype of the species sets 

 of loci for alternative and mutually exclusive phenotypes and (2) 

 incorporating mechanisms for shifting phenotypic expression 

 from one to another on demand. This is clearly a very satisfactory 

 adaptation to the need for rapid variation during asexual repro- 

 duction. As such, it is found not only in organisms that lack sexual 

 processes, but also in those that have sexual processes along with 

 a well-developed asexual phase, such as the Ciliates. Further, 

 there is some evidence that it plays an important role in develop- 

 mental differentiation, the asexual phase of higher organisms. 

 For example, genes for both sexes are present in hermaphrodites, 

 but come to expression in different cells; and other aspects of 

 differentiation probably have a comparable genetic basis. Thus, 

 at every level of asexual reproduction, this mechanism of var- 

 iability seems to be in operation. 



Speciation in both asexual and sexual organisms is probably 

 fundamentally the same. In both cases, the basic event is genetic 

 adaptation to different conditions. Perhaps asexual organisms 

 selectively live under relatively constant or very slowly changing 

 conditions. This selection of constant environments would tend to 

 retard conspicuous speciation, but beneath the visible uniformity 

 there might well be much accumulated genie and chromosomal 

 change, such as exists among local populations of strongly in- 

 breeding varieties of P. aurelia. On the other hand, selected vari- 



