H. L. CARSON 33 



frequencies of distributed crossovers is, in fact, the condition 

 which would appear to be able to release the greatest amount 

 of hereditary variability in the shortest possible time. 



How Much Recombination? 



The amount of recombination permitted in a given species by 

 the genetic system of that species is best looked upon as an adap- 

 tive property which is under the control of natural selection. 

 Too much recombination is inadaptive because it tends to break 

 up adaptive complexes of genes which have been welded to- 

 gether by natural selection. This reduces the immediate fitness of 

 the organism. In order to survive, a species must maintain high 

 immediate fitness and it is not surprising that devices limiting or 

 preventing recombination are common in species populations. 

 Not infrequently, as will be brought out below, the process ap- 

 pears to have been carried to its logical conclusion and recom- 

 bination is inhibited completely. At this point, sexual reproduc- 

 tion ceases to have significance as a biologically important proc- 

 ess. 



Organisms which have completely discarded sexual reproduc- 

 tion, and reproduce exclusively by asexual means are definitely 

 in the minority in nature. Thus it appears that too little recom- 

 bination jeopardizes the ability of the species to meet drastic 

 changes in environmental conditions over very long periods of 

 time (see Thoday, 1953). Most organisms which have survived 

 to the present day display a balance between these two forces. 

 There is neither too much recombination on the one hand, which 

 tends to break up adaptive complexes, or too little, which leads 

 to evolutionary rigidity, specialization, inability to change, and 

 eventually extinction in the face of an environmental challenge. 



Open, Restricted, and Closed Recombination Systems 



If we refer to the sum total of all the devices leading toward 

 recombination as the "recombination system," we may attempt to 

 assess the capacity of a given species to generate recombinations 

 in a typical contemporary local population. The effectiveness of 

 the recombination system obviously varies enormously from one 



