Introgressive hybridization

Abstract

This little book is concerned with hybridization under those circumstances which we so glibh^refer to as "natural conditions," that is, with the results of hybridization outside the laboratorjâ nd the breeding plot.It passes no judgments on the importance of hybridization in evolution but attempts to take this whole problem outside the area of argument and opinion and into the zone of measurement and analysis.It is verj^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 thej^can be employed for anah^sis 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. 43 to 48 and 92 to 99).An}^field of stud}^that is in the process of shifting from the descriptive phase to the analj^tic 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 improvement.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 taxonomic journals (Anderson and Turrill, 1938; Anderson and Hubricht, 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 merety vii 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 studjdng 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, pedigree culture, 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.Furthermore, the general method (pp. 92to 99) of extrapolated correlates (and the more specialized techniques here described as '^pictorialized scatter diagrams," radiate diagrams, standardized photographs, etc.) have proved to be of wide adaptability in ana- lyzing the effects of such hybridization.Though these methods are here described in full for the first time, they have been rather widely used by my students and colleagues.