THE EVIDENCE FOR EVOLUTION 733 



they would undoubtedly be considered separate species. They are all 

 inierfertile and are known to come from common ancestors, so they are 

 regarded as varieties of a single species. A comparable range of varieties 

 has been produced by artificial selection in cats, chickens, sheep, cattle 

 and horses. Plant breeders have established by selective breeding a tre- 

 mendous variety of plants. From the cliff cabbage, which still grows wild 

 in Europe, have come cultivated cabbage, cauliflower, kohlrabi, Brussels 

 sprouts, broccoli and kale. 



Geneticists have been able to trace the ancestry of certain modern 

 plants by a combination of cytologic techniques, in which the morphol- 

 ogy of the chromosomes is compared, and breeding techniques which 

 compare the kinds of genes and their order in particular chromosomes 

 in a series of plants. In this way the present cultivated tobacco plant, 

 Nicotiana tabaciun, was shown to have arisen from two species of wild 

 tobacco, and corn was traced to teosinte, a grasslike plant which grows 

 wild in the Andes and Mexico. The details of the structure of the giant 

 chromosomes of the salivary glands of fruit flies have been of prime 

 importance in unraveling the evolutionary history of the many species 

 of Drosopliila. 



31 5. Evidence from the Geographic Distribution of Organisms 



In the course of the voyage of the Beagle, Darwin was greatly im- 

 pressed by his observations that the plants and animals of South America 

 and the Galapagos Islands were not found everywhere that they could 

 exist if climate and topography were the only factors determining their 

 distribution. The facts of biogeography, the geographic distribution of 

 plants and animals, were of prime importance in leading both Darwin 

 and Alfred Russell Wallace to the conclusion that organic evolution had 

 occurred by natural selection. The present distribution of organisms, 

 and the sites at which their fossil remains are found, are understandable 

 only on the basis of the evolutionary history of each species. 



The range of each species is that particular portion of the earth in 

 which it is found. The range of a species may be restricted to a few 

 square miles or less, or, as with man, may include almost the entire 

 earth. In general, the ranges of closely related species or subspecies are 

 not identical, nor are they widely separated, but are adjacent and sep- 

 arated by a barrier of some sort. This generalization was stated by David 

 Starr Jordan and is known as Jordan's rule. The explanation for this 

 should be clear from the discussion of the role of isolation in species 

 formation. A single species cannot be subdivided as long as interbreeding 

 can occur throughout the whole population. But when some barrier is 

 interposed between two parts of the population so that interbreeding 

 is prevented, the two populations will, in the subsequent course of time, 

 accumulate different gene mutations. , . u u 



One of the fundamental assumptions of biogeography is that eacfi 

 species of animal or plant originated only once. The place where this 

 occurred is known as its center of origin. The center of origin is not a 

 single point, but the range of the population when the new species was 



