THE ORIGIN OF SPECIES 



isms which show it, they may have a death rate somewhat higher than 

 their more fortunate cousins, with the result that relatively fewer of them 

 will reach reproductive age, and they will leave somewhat fewer progeny. 

 Conversely, a variant which has even a slight advantage will have a some- 

 what higher survival rate and hence will leave somewhat more descend- 

 ants. In either case, the net result will be a gradual change in the propor- 

 tions of each variant in a natural species. Once it was realized that gradual 

 changes, apparent only on statistical analysis, might be effected by selec- 

 tion rather than all-or-none changes, it became possible to study selective 

 changes in natural populations, and, to some extent, to perform experi- 

 ments in selection. 



EXPERIMENTAL DEMONSTRATION OF 

 NATURAL SELECTION 



Sukatchew analyzed such changes experimentally in the dandelion, Tarax- 

 acum officinale. Strains tested were obtained from the Crimea, Leningrad, 

 and Archangel. As the latitudes are respectively 45°, 60°, and 64°, the 

 Crimea is comparable to Minneapolis, Leningrad to Seward, Alaska, and 

 Archangel to Dawson, the gold rush city of the Yukon Valley. The dande- 

 lions tested were then adapted to temperate, subarctic, and arctic condi- 

 tions, respectively. The three strains were planted in mixed plots at 

 Leningrad at densities of every three centimeters, and every eighteen 

 centimeters. The Crimean strain survived to about 60 per cent in the 

 sparsely planted plots, but in the densely planted plots, where competition 

 ( and hence selection ) was more severe, its survival rate dropped to about 

 1 per cent. At high densities, the Archangel sti-ain had a greater survival 

 rate than did the Leningrad strain (70 as against 11 per cent), but at low 

 densities, there was a small but significant margin in favor of the local 

 strain (96 as against 88 per cent). These observations are important for 

 two reasons. First, they do demonstrate that the differences between pop- 

 ulations of the same species result in differential survival when a mixed 

 population is exposed to identical conditions. And second, they demon- 

 strate that selection need not be an all-or-none phenomenon, but that it 

 may operate by the statistical transformation of populations. 



Timofeeff-Ressovsky, working at Berlin, similarly tested the relative 

 viability of Drosophila meJanogaster and D. funebris at temperatures of 

 15°, 22°, and 29° centigrade. D. funebris is an originally northern species, 

 while D. melanogaster is originally tropical. His method was to put 150 

 eggs of each species together in a culture bottle containing insufficient 

 food for 300 larvae. The numbers of adults of each species which emerged 

 were then compared. As D. mekinogaster usually showed the greater vi- 

 ability, results were expressed by stating the viability of D. funebris as a 



r 1 r -^ 7 /surviving funebris ^^^ \ 



per cent of that of D. niclanopaster . . ^ \ ; — X 100 



^ ® \siuvivmg melanogaster / 



As already mentioned, the survival rate of D. melanogaster was almost 



always higher than that of D. funebris. But northern races of D. funebris 



did much better relatively, when tested at 15°, than did southern races. 



2S4 



