26 GENE RECOMBINATION 



the reader is referred to the review of Dobzhansky (1955). The 

 very existence of this extensive variability, however, is the real 

 focal point in the unique view of the species that the geneticist 

 is beginning to take. The statement may safely be made that in 

 all probability, no two individuals in a sexual population are 

 genotypically identical. The geneticist thus sees, in every popula- 

 tion, arrays of individually different genotypes. He sees the 

 group as a whole, as a statistically varying array. The newer 

 methods of population genetics are attempting to deal directly 

 with the laws governing the behavior and fate of genotypes in 

 populations. 



The Species as a Gene Pool 



Those who have been bold enough to advance genetical defini- 

 tions of species are on safest ground when they deal with species 

 which can be shown to be sexually reproducing, cross-fertilizing 

 arrays of individuals. When a species has such a breeding struc- 

 ture, it is tempting to view the hereditary material possessed by 

 the species as a whole as a gigantic pool of genes. Reproducing 

 individuals may be viewed as throwing random assortments of 

 their genes, packaged in gametes, into the pool. The individuals 

 of the next generation are thus genetically chance recombinants 

 drawn from this gene pool. Dobzhansky (1950) has utilized 

 Wright's term "Mendelian population" for the reproductive com- 

 munity of individuals who share in a common gene pool. 



Such a formulation as the above should serve only to direct 

 thinking along the general path followed by the population ge- 

 neticist. If it is taken more seriously than this, it is likely to be 

 misleading. Simple concepts of gene frequency and the conse- 

 quent related predictions of the Hardy- Weinberg law may be 

 demonstrated in a very simple model of a gene pool. A hundred 

 poker chips, 30 red and 70 white can be pooled in a fishbowl. The 

 two colors represent the two alternative alleles at one locus. 

 Drawing out chips in pairs thus represents zygote formation for 

 a single locus; the homozygotes will be red-red and white-white 

 and the heterozygotes red-white. The relationship of gametic 



