The cytogenetic mechanisms discussed in the preceding chapters 

 provide for the production of offspring sufficiently similar to the 

 parental types that, barring unduly rapid environmental change, 

 the group survives. They provide also for sufficient variability that, 

 when the environment changes gradually, the organisms change also 

 and thus survive. The persistence of an organism depends on the 

 proper balance between these two phenomena: fitness for the im- 

 mediate environment and fitness, in the long-range view, for what- 

 ever changes take place in the environment in the course of time. 

 It has already been pointed out that the genetic mechanism of most 

 organisms provides for storing variability in unexpressed form, as 

 well as for regulating the release of this variability. Diversity of 

 means of storage and release of variability, which, like other traits, 

 must be under genetic control, implies that these means change in 

 the course of time; in other words, they evolve. The collective ways 

 in which the amount and type of new gene combinations are con- 

 trolled may be referred to as the genetic system. Thus one may 

 speak of the evolution of genetic systems, which is the evolution of 

 those mechanisms effecting and affecting variability. 



In this chapter the various sorts of genetic systems found in 

 plants and animals will be described. In the following chapter sev- 

 eral specialized genetic systems which have become important in 

 certain organisms will be discussed in greater detail. Elucidating 

 the interrelationships and ultimate significance of these systems and 

 integrating them into evolutionary theory are among the greatest 

 challenges facing the modern evolutionist. 



Although the actual course of evolution of genetic systems is not 

 known, there has been considerable speculation as to the main lines 

 that it may have taken. Here genetic systems will be discussed, 

 starting with the simplest and concluding with the most complex. 

 This does not indicate an evolutionary sequence. The simplest bio- 

 logical phenomena often can be interpreted as reduced to this state 

 from something more complex. 



GENETIC SYSTEMS IN MICROORGANISMS 



Transformation 



Transformation is a phenomenon involving genetic change in some 

 bacteria. For example, studies of strains of pneumococcus bacteria 

 (Diplococcus) have shown that the DNA extracted from a strain | 161 



