PROCEEDINGS 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volume 3 DECEMBER 15.1917 Number 1 2 
INCOMPATIBILITY OF MUTANT RACES IN DROSOPHILA 
By C. W. Metz and C. B. Bridges 
CARNEGIE STATION FOR EXPERIMENTAL EVOLUTION, AND COLUMBIA UNIVERSITY 
Communicated by C. B. Davenport, October 17, 1917 
Perhaps the most characteristic difference between ordinary mutant 
races in the laboratory or experimental garden, and incipient species in 
nature, is the difference in their ability to hybridize. Mutant races 
usually exhibit complete inter-fertility, v^hile species, as a rule, are inter- 
sterile. If this distinction, based on fertility of mutants, were found to 
be a fundamental one (i.e., of universal application) it would offer a 
serious objection to the hypothesis of evolution through mutations, but, 
conversely, if it were not found to hold universally the objection would 
be removed or greatly minimized. The accompanying observations 
seem to indicate that the distinction does not hold universally. 
In our cultures of Drosophila two cases have arisen in which mutant 
races exhibit an incompatibility that is indistinguishable, so far as we 
can see, from that found in nature between distinct species.^ One of 
our cases appeared in cultures of Drosophila virilife at Cold Spring 
Harbor, the other in cultures of Drosophila melanogaster (ampelophila) 
at Columbia University.^ Each case involves two mutants (apparently 
allelomorphs) that either refuse to cross or else give sterile hybrids — 
a situation comparable to that in the familiar case of the ass and horse. 
The essential data from out experiments are as follows: 
1 . In Drosophila Virilis. — Among eight sex-linked mutant characters 
in this species are two called 'glazed' and 'rugose,' that appeared inde- 
pendently, but almost simultaneously, over a year ago. Pure stock of 
each race breeds readily, and in this form both have been kept in the 
laboratory for over fifteen generations; likewise, each has been crossed 
with several other mutants as well as with the normal, and has shown 
a high degree of fertility in all cases. But when mated together their 
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