H. L. CARSON 35 



the mating system, which is the external and ecologically affected 

 phase of genetic recombination. Through it, chance recombina- 

 tion of gametes is effected. In human populations, the mating 

 systems are intricate in the extreme. Suffice it only to say here 

 that we are witnessing in a few generations the rapid recombina- 

 tion of large segments of the human gene pool which were once 

 separate. 



Flies of the genus Drosophila may serve as an example of a 

 recombination system which is much more restricted than the 

 above example ( Patterson and Stone, 1952 ) . The highest chromo- 

 some number in the group is 2n = 12, and most species have suf- 

 fered a diminution in whole chromosome recombination potential 

 by further reduction of the effective chromosome number by 

 centromere fusions. Secondly, this genus of flies, together with a 

 large segment of the higher Diptera have no crossing over in the 

 male sex. This condition, despite the distributed crossing over 

 in females, reduces the effective recombination due to crossing 

 over by one-half compared to organisms which have dis- 

 tributed crossovers in both sexes. In certain species, the accu- 

 mulation of inverted sections in the chromosomes in natural 

 populations results essentially in the isolation of blocks of the 

 germ plasm from effective genetic recombination. This system is 

 particularly evident and important in many endemic species of 

 Drosophila. The outcrossing potential of Drosophila is also rela- 

 tively low. Many species crosses can be made in the laboratory, 

 but, with rather insignificant exceptions, do not occur under 

 natural conditions. 



Drosophila as a genus shows an interesting variety of recombi- 

 nation systems. In some species, inversions are absent, or nearly 

 so, from natural populations, whereas in others inversions are 

 abundant. Even within an individual species, there are some 

 populations which are highly heterozygous for inversions and 

 others which are essentially homozygous. For instance, Carson 

 (1954, 1955) has devised a means of comparing quantitatively 

 the amount of genetic recombination which occurs in different 

 populations of Drosophila rohusta of the eastern United States. 

 Marginal populations of this species display a much greater po- 



