CHAPTER EIGHTEEN 



Isolatiiig Mechanisms 

 and Species Formation 



Darwin and his contemporaries gave much thought to the possibihty 

 that the variations which form the raw materials of evohition might be 

 "swamped" by crossing to the original type, with the result that no actual 

 change in the species could occur unless such hybridizing were prevented 

 by isolation of the new variant from the parent stock. This was based upon 

 a pre-Mendelian conception of heredity, the blending theory of inherit- 

 ance, according to which offspring should always be intermediate between 

 the parents. If this theory were correct, then repeated backcrosses of a 

 hybrid stock to the original type should result in an ever closer approach 

 to the original type. In the hands of Moritz Wagner ( 1868 ff. ) isolation 

 of variant races became the necessary prerequisite and the inevitable 

 cause of speciation. (This term, generally used as a synonym for species 

 formation, is, unfortunately, etymologically incorrect. But it seems cer- 

 tain to remain a permanent part of our evolutionary vocabulary. ) 



MENDELIAN GENETICS. ISOLATION. AND SUBSPECIATION 



With the rise of Mendelian genetics, the original idea of Darwin and 

 Wagner became untenable. A gene-determined character could never be 

 destroyed by crossing to the original type: it could only become hetero- 

 zygous, with the possibility always present that homozygosity would be 

 re-established. The gene might thus become evenly spread through a 

 species, so that no tendency toward formation of a subspecies could be 

 observed, but this would not entail loss of the gene, nor of the phenotype 

 for which it was responsible. While breeding to the original type could 

 never destroy a Mendelian gene, it could break up comliinations of such 

 genes. Selection can only operate on whole organisms, and so particular 

 combinations of genes may have a value which would not be possessed 

 by the separate genes of the genotype. Thus, mutants for a keen sense of 

 smell and for rapid running (such as longer leg Ijoncs) would both be of 

 selective value to a chase predator like the wolf. But the two in combina- 

 tion would be of much grcuiter value than either one alone. While neither 



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