328 Cell as Basis of Organic Activity 



pangene, it cannot be considered even plausible in the light of later 

 genetical discoveries. 



Neo-Darwinism. — Since Darwin's time, there has been no lessen- 

 ing of the interest in the manner in which evolution has occurred. In 

 fact it has been pointed out that nearly all biological research is di- 

 rected toward an explanation of this very fact. The most widely held 

 present-day theory of evolution incorporates many of Darwin's ideas 

 into a framework of genetics. 



This theory, termed Neo-Darwinism, regards the breeding popula- 

 tion as the unit of study rather than the individual animal or plant. 

 The population is regarded as having a "pool" of genes which are con- 

 tinually being combined and recombined. Each individual thus is only 

 an expression of one of many possible gene combinations. In this way 

 there is constant variation among the progeny. Further variation is 

 made possible by the fact that mutations are constantly arising. While 

 most of these mutations will be unfavorable and have little or no sur- 

 vival value, some few may be retained in the "pool" to be passed on to 

 succeeding generations. If no part of the population becomes isolated 

 from any other, no new forms can develop and there will be relative 

 homogeneity. While a single individual may look quite different from 

 others, the next generation will reshuffle the genes and the aberrant 

 form will not be perpetuated. 



According to this theory, these variations can become new forms 

 only if isolation occurs. If a single breeding population in some way 

 becomes divided into a series of populations, each of these resulting 

 groups will then have its own pool of genes. Under possibly new and 

 dififerent environmental conditions, these populations will gradually 

 become less similar in appearance. At first the differences will be only 

 slight, and the populations might be termed subspecies. With longer 

 isolation, the differences become more pronounced, subspecies become 

 species, and species become new genera. 



Isolation may be of many types. The most common is spatial, 

 resulting from migration, extension of range, and geological or climatic 

 changes. Other isolating mechanisms are food preferences, seasonal 

 appearance, abundance or differing habits. 



The leopard frog, Rana pipiens, illustrates some of the principles 

 here involved. This frog has a wide distribution from Canada south to 

 Panama. Throughout this extensive range the animals are quite simi- 

 lar in appearance, but nonetheless animals from widely separated parts 



