GENERAL ZOOLOGY 



changed to such an extent that they no longer successfully interbreed. One 

 of the most significant factors conditioning the establishment of new genetic 

 combinations, and thus contributing to the formation of species, involves 

 various forms of isolation. 



Free interbreeding tends to promote uniformity in a population. There- 

 fore, some degree of isolation is necessary for new combinations of character- 

 istics to become established and to form steps in evolutionary change. From 

 the standpoint of genetics, the conditions in nature that eflfect such isolation 

 are many and varied. For example, the production of hybrid zygotes may be 

 prevented by ecological, seasonal, or geographical factors operating in such 

 ways that adult members of different populations never encounter each other. 

 This effectively prevents interbreeding and the consequent exchange of 

 heritable characteristics between the groups. A classical example of 

 ecological isolation is furnished by snails of the family Achatinellidae, found 

 in the Hawaiian Islands during the 1850's by John T. Gulick (1832-1923). 

 Snails of this family live in trees. Since they cannot travel any distance over 

 a land surface devoid of shade or moisture, their distribution is restricted. 

 Along the sides of the principal mountains on the island of Oahu there are 

 small valleys in which these snails find suitable habitats. But the snails can- 

 not easily cross the ridges between adjacent valleys, or the crest of the 

 mountain; neither can they move out upon the plain below. A population 

 that becomes established in any valley tends to remain isolated from those 

 in other valleys as long as similar climatic and topographical conditions 

 prevail. At the time of Gulick's original collections, he found that almost 

 every valley had its particular species or subspecies, differing in size, color, 

 and shape of shell. More recent collectors have reported a species that seems 

 to have been restricted to a single tree, sufficiently isolated to prevent migra- 

 tion and contact with other groups. 



It is diflficult to regard difierences of the kind shown by these snails as 

 useful or adaptive, and thus as having a selective value in the environments 

 observed; the environment in all the valleys appears to be essentially uni- 

 form. The varied characteristics of these populations of snails may be inter- 

 preted as non-adaptive and hence as having no significance in the process of 

 natural selection. Their appearance may best be accounted for on the basis 

 of the isolation of the breeding populations. Under such conditions of isola- 

 tion, each population evolves independently of the others, and because of 

 the somewhat different inviduals originally present or subsequently appearing 

 at random in the different groups, all are likely to evolve in different direc- 

 tions. Genetically speaking, each population consisted originally of in- 

 dividuals with certain combinations of genes, no two populations being 

 identical. The mere sequence of generations produced, by random assort- 

 ment and recombination, increasingly varied gene complexes in the different 

 populations, and mutations continued to occur at random in each of the 

 groups. From these possibilities, in accordance with the laws of probability, 

 different types, and hence evolutionary changes, resulted. Even from popula- 



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