Genetic Systems I | 169 



dikaryosis apears to be requisite for great morphological complexity 

 in the fungi. 



Recombination and Genetic Systems 



To recapitulate, when the many different genetic systems that have 

 evolved are compared, certain common features stand out. Except 

 where simple unicellular structure and rapid reproduction are found, 

 it seems that diploidy has had selective value. Simple, rapidly re- 

 producing organisms have great flexibility for these reasons alone, 

 and mutation rate is the chief source of variability. Such organisms 

 as yeast and Faramccium are exceptions. With increased complex- 

 ity, integrated combinations of genes controlling the developmental 

 pathways are built up. The diploid state (or dikaryosis) provides 

 the necessary buffering. It also permits the storage of variability in 

 the form of recessive genes and of polygenes in balanced systems. 

 Meiosis and syngamy produce the release of variability as new gene 

 combinations. 



It is usually assumed that, without environmental change, most 

 new gene arrangements will have lower selective value than exist- 

 ing ones. Thus a certain wastage of zygotes occurs in addition to 

 the wastage of gametes in sexual reproduction. Nevertheless, the 

 wastage in diploid outcrossing organisms is surely much less than 

 that in haploid organisms. The wastage of recombinants in the bac- 

 teria-bacteriophage systems is probably very much higher still. This 

 suggests that there has been selection, also, for genetic systems that 

 not only provide for buffering and the build-up of gene complexes 

 and for storage and gradual release of variation but also reduce the 

 wastage of biological materials and thus energy in the course of 

 evolution. (It must not always be assumed, however, that what 

 appears to be "wastage" is disadvantageous. ) 



Reduction of Recombination 



In nearly all genetic systems, modifications that reduce the amount 

 of recombination have occurred. The result is reduction of wastage, 

 as immediate fitness is increased. It has been generally assumed that 

 the most primitive organisms were asexual, and these modifications 

 are usually referred to as reversions to asexualitij. Since recombina- 

 tion mechanisms have been found in nearly all organisms and since 

 the ability to effect recombination appears to be a fundamental 



