Can Artificial Selection Produce Unlimited Change?

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Abstract
Selection experiments on bristle number in Drosophila melanogaster (Payne, 1918) and on number of white hairs in a spot on the forehead of mice are described. It is hard to understand how the large changes produced by selection could have arisen from an accumulation of favorable genes present in, but dispersed among, the 2 (Drosophila) or 5 (mouse) foundation animals. In the Drosophila expt., selection, beginning with a race almost without variation (99.5% of 3861 flies had 4 bristles), resulted in a marked increase in variation and a mean more than double the original number of bristles. In the mouse expt. the original maximum size of the character was multiplied 5 times. A method of triangulation is described which demonstrates that this increase cannot well be explained by the accumulation of genes scattered among the 5 foundation animals (multiple factor theory). The rarity and erratic nature of point mutations exclude them from consideration. It appears that selection, in some way not understood, can cause large, perhaps unlimited, changes in favorable material.

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