Introgressive Hydridization

INTROGRESSIVE HYBRIDIZATION Edgar Anderson GENETICIST, MISSOURI BOTANICAL GARDEN ENGELMANN PROFESSOR OF BOTANY WASHINGTON UNIVERSITY ST. LOUIS, MISSOURI 1949 John Wiley Sons, Inc., New York Chapman Hall, Limited, London COPYRIGHT, 1949 BY JOHN WILEY SONS, INC. All Rights Reserved This book or any part thereof must not be reproduced in any form without the written permission of the publisher. PRINTED IN THE UNITED STATES OP AMERICA To My Students With pleasure in what they have learned With pride in what they have taught me Foreword This little book is concerned with hybridization under those circumstances which we so glibly refer to as natural conditions that is, with the results of hybridization outside the laboratory and the breeding plot. It passes no judgments on the importance of hybridization in evolution but attempts to take this whole prob lem outside the area of argument and opinion and into the zone of measurement and analysis. It is very largely concerned with how the effects of hybridization can best be measured in natural populations and with a discussion of the forces at work in such populations. Most of the techniques presented here are comparatively simple ones that have been developed for analyzing interspecific and intraspecific variation. Observation and measurement are used much as in traditional taxonomic work but refined to a point where they can be employed for analysis as well as for description. By means of such techniques it is now possible for a trained observer to work intensively with a hybrid population in a region completely new to him and from it to deduce exact descriptions of the hybrid izing species, even when he has never seen that species see pp. 43to 48 and 92 to 99. Any field of study that is in the process of shifting from the descriptive phase to the analytic phase is certain to suffer from growing pains. This one is no exception. The first methods used were crude, and the ones described below need further improve ment. This book is a step forward in that the relevant literature is now brought together for the first time. Previous presentations have been piecemeal. The basic theory appeared in genetic journals Anderson, 19396 applications to taxonomic problems, in taxo nomic journals Anderson and Turrill, 1938 Anderson and Hu bricht, 1938 and practical applications to plant breeding prob lems, in still other places Anderson and Hornback, 1946. This previous division of the subject matter was not capricious. It resulted from the fact that the concept of introgression was merely viii FOREWORD a by-product of my long-continued and still continuing absorp tion with the genetics of multiple-factor characters. Therefore, not only has a well-rounded discussion of the work on introgression never previously been attempted, but also a good deal of what is presented below has never appeared in print. On the other hand, the bibliography is limited to cited works, since an inclusive bib liography on introgression by Dr. Charles Heiser is shortly to appear. This is largely a book about methods for studying hybridization in the field. It is to be hoped that application of these methods and their consequent refinement will produce data from which eventually we can estimate the relative importance of hybridiza tion in evolution. In this book the more usual methods of analyzing hybridization transplant studies, cytological analysis, pedigreeculture, repe tition of suspected hybridization receive little more than passing mention. It goes without saying that these methods should be used whenever the facilities for them are at hand. All these tech niques were employed in the special studies of Tradescantia, Iris, and Nicotiana, from which these newer methods derive their theo retical and experimental verification. It should be emphasized, however, that from a corollary of the demonstration of multiple factor linkage see p. 43 we have a new and powerful criterion for hybridity...