24 GENE RECOMBINATION 



a major generalization, let alone the years of contemplation and 

 testing which precede general acceptance of any such idea. In 

 the present instance, an initial delay was imposed by World War 

 II. Only now is it just beginning to be possible for the new gen- 

 eration of evolutionists to start critical evaluation of what appears 

 to have been the most spectacular era in basic biological thought 

 about evolution since the years following the publication of the 

 Origin of Species. 



The most immediate effect of the syntheses mentioned above 

 was to stimulate a rash of investigations designed to answer 

 new questions about the species and especially about its popula- 

 tions. These new questions arise primarily from purely theo- 

 retical consideration of the consequences of Mendelian inheri- 

 tance in interbreeding communities of organisms. A geneticist, 

 in most instances, can work only with organisms that will inter- 

 breed freely so as to produce abundant offspring. His work is 

 thus of necessity usually at or below the species level. It will 

 therefore be a foregone conclusion that the evolutionarily in- 

 clined geneticist will have developed his own perspective — his 

 own particular way of looking at the species. The geneticist who 

 is oriented toward populations is really attempting the experi- 

 mental study of heredity on a level above that of the individual. 

 The present effort will be a somewhat personal picture — one 

 geneticist's view of the accomplishments of this approach in the 

 last fifteen years and the effects that it has had on the species 

 concept. 



Genotype, Phenotype, and Wild Type 



The term genotype is sometimes used in a strictly operational 

 way in experimental genetics to refer to the genie condition 

 under observation at a single locus or sometimes at a relatively 

 small number of loci (e.g., Aa or Aa Bb Cc Dd). In the sense 

 used in this paper, however, the word refers to the entire genetic 

 constitution or the sum total of the genes of an individual organ- 

 ism. This point requires emphasis because, as will be pointed out, 

 from the evolutionary point of view, it is unrealistic to select a 



