36 GENE RECOMBINATION 



tential for genetic recombination than do populations from the 

 central part of the range. Marginal populations thus appear to 

 have a recombination system that is especially well suited to 

 evolutionary aggression and pioneering. 



In any organism in which reproduction is solely by asexual 

 means, the recombination system may be considered closed. The 

 cases most easily interpreted in this regard are those in which the 

 asexual method of reproduction has clearly come to be irrevo- 

 cably and completely substituted for the sexual method. Such a 

 situation exists, for example, in certain of the obligate apomicts 

 of the genus Crepis (Stebbins, 1950). Adaptation, as it becomes 

 increasingly perfect, may be increasingly interfered with by 

 recombination in such a way that selection will tend to bypass 

 and then gradually eliminate entirely the very process that makes 

 adaptation possible. Organisms which have eliminated sexual 

 reproduction may be looked upon as representing a closed sys- 

 tem, a dead end in evolution. 



To the student of genetic recombination and its importance 

 in evolution it would appear likely that all organisms which repro- 

 duce exclusively by asexual means have acquired this property 

 secondarily and originally reproduced by sexual reproduction. It 

 is perhaps not generally realized in biology that recent findings 

 in population genetics have thrown new light on the basic 

 biological meaning of sex. It may well be that many of the older 

 concepts on the origin of sexual reproduction will have to be 

 reexamined in detail. 



Conclusion and Summary 



The thesis is advanced that the most meaningful approach to 

 the sexually reproducing species that the geneticist may make 

 is by studying and describing it as a system of recombining genes. 

 Such a system is often spoken of as a "gene pool. The breeding 

 structure ol the species is such that there is an almost continuous 

 compartmentation ol tin's larger gene pool into smaller and 

 smaller units ol recombination, with varying degrees ol isolation 

 from one another. At the bottom of the ladder and in a focal 

 position is the local population, which at any one lime level is 



