Epilogue 



How important is introgressive hybridization? I do not 

 know. One point seems fairly certain: its importance is 

 paradoxical. The more imperceptible introgression becomes, 

 the greater is its biological significance. It may be of the 

 greatest fundamental importance when by our present crude 

 methods we can do no more than to demonstrate its exist- 

 ence. When, on the other hand, it leads to bizarre hybrid 

 swarms, apparent even to the casual passer-by, it may be of 

 little general significance. When, as described in Woodson's 

 studies of Asclepias populations, it produces clines reaching 

 a third of the way across a continent, it is scarcely per- 

 ceptible in any one locality. Only by the exact comparisons 

 of populations can we demonstrate the phenomenon, yet 

 in such populations the raw material for evolution brought 

 in by introgression must greatly exceed the new genes pro- 

 duced directly by mutation. The wider spread of a few genes 

 (if it exists) might well be imperceptible even from a study 

 of population averages, but it would be of tremendous bio- 

 logical import. Germplasms are proteins, strange and com- 

 plex substances. The introduction of a single alien gene into 

 a new germplasm would be the introduction of one new unit 

 into a gigantic protein complex. Reasoning purely from 

 chemical facts, we might expect such a mixture to have sec- 

 ondary consequences in addition to its primary ones. But 

 even were there no secondary consequences, the wide dis- 

 persal of introgressive genes (perceptible only to the most 

 exquisitely precise techniques) would be a phenomenon of 

 fundamental importance. Hence our paradox. Introgres- 

 sion is of the greater biological significance, the less is the 

 impact apparent to casual inspection. 



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