166 I The Process of Evolution 



the blue-green algae ( Cyanophycophyta ) is recombination un- 

 known. In their rather specialized habitats the blue-green algae seem 

 to be a successful and widespread group. One is led to suspect that 

 they must have some mode of genetic recombination at present not 

 detected. Perhaps it is related to that in bacteria, and possibly it 

 involves viruses as well. 



Sexuality and Diploidy 



The phenomenon of sexual reproduction is so widespread and its 

 evolutionary significance is so immediately apparent that most bi- 

 ologists place its origin very early in the evolution of life. In many 

 groups of plants and animals, every individual produces both female 

 and male gametes. Even so, cross-fertilization is frequently the rule. 

 Where the organisms are not hermaphroditic, the nature of sexual 

 development may be affected by environmental factors such as light 

 intensity, photoperiod, temperature, chemical composition of the 

 medium, etc. It would seem that, in the course of evolution, this 

 rather variable sex determination is replaced with more precise con- 

 trol systems or, at least, have such imposed upon them. 



The first question to be considered is the origin of diploidy. It is 

 usually assumed that diploidy has high selective value because of the 

 opportunity it provides for the storage of recessive genes and thus 

 of variability. There seems to be little doubt that diploidy also pro- 

 vides necessary buffering in development and thus greater freedom 

 from environmental effects. One would expect that this would be 

 increasingly important as the complexity of organisms increases. 

 Fusion of free cells occurs spontaneously in tissue cultures and cul- 

 tures of unicellular organisms with such frequency that it is not 

 difficult to imagine the origin of syngamy. Perhaps the resulting 

 buffering effect in the diploid cell would have immediate selective 

 value. 



In many instances, fusion of cells leads to instability which is re- 

 solved by division. It was suggested in Chap. 1 that in early proto- 

 organisms a sort of protorecombination might have taken place. 

 Since a great many of the simpler algae and fungi are haploid for 

 most of their lives (the only diploid cell being the zygote which 

 immediately divides), one might conclude that recombination oc- 

 curring with division immediately after syngamy had high selective 

 value. 



Sexual reproduction is a complicated process having many com- 

 ponents that must be integrated in function. The stages in its evo- 

 lution are not known. With the completely unstable genetic 



