INTRODUCTION TO THE PROBLEM 



dysploidy)* by which is meant a numerical change of some type other than that of a 

 mere mukipUcation of whole gametic sets. The genus Biscutella, from which so much 

 information has already been obtained, may again be quoted in illustration. In the 

 Laevigatae section one species of very limited geographical range in southern Spain has 

 a haploid chromosome number not of 9 but of 6. At first sight it might be suggested 

 that the numerical relation between 9 and 6 is such that both could be parts of a poly- 

 ploid series on 3. In this case, however, there is no direct evidence to support such a 

 contention and much which directly contradicts it. A gametic number of 3 is not 

 known in the Cruciferae at all, and the species in question has all the appearance of 

 being recent. Moreover, all other species of Biscutella in sections other than the 

 Laevigatae, together with species of a number of neighbouring genera, have a basic 

 haploid chromosome number not of 3, 6 or 9 but of 8. It therefore seems almost 

 certain that the Laevigatae section itself arose at some remote period, probably in the 

 Tertiary, by an aneuploid nuclear change which produced a numerical difference of 

 one chromosome in the basic haploid count. This change presumably at first was 

 associated with the production of one new species. This species must, however, have 

 been somewhat more different from its fellows than usual,! for when sufficient time 

 had elapsed for further new forms to develop from it in ways that we have recently 

 touched upon, the taxonomists studying the group with only morphology as a guide 

 were constrained to segregate them together into a separate section. In other cases, 

 no doubt, the size of the unit might have been not a section or subgenus but a genus 

 or larger group. Here, therefore, we are confronted suddenly with something very 

 Hke the old mutation theory in one of its simplest forms. And the least that we may 

 conclude is that new species are not all equally potent as evolving units, but that the 

 precise means by which they have come about may have a decisive influence on their 

 subsequent fate. 



The difference between the relative importance of aneuploidy and polyploidy was 

 probably the most significant general conclusion which the work on the Cruciferae 

 brought out (cf. Manton, 19320), and the evidence can be repeated again and again 

 in other families. Polyploidy in the flowering plants abounds as a means of formation 

 of the type of species which do not fundamentally break new ground and the chromo- 

 some numbers reached maybe high; in the Cruciferae some of the highest known are 

 9n = 8i in nonaploid Biscutella and 8n= 120 in some varieties of Crambe, in which the 

 basic haploid number appears to be n=i^. The aneuploid changes, on the other 

 hand, are, with very few exceptions, characteristically associated not with species but 

 with groups of species, i.e. subgenera, genera, or larger units. They are far less frequent 



* See footnote on p. 5 for further definition of terms. 



t Very important recent evidence from Crepis summarized by Babcock (1947) indicates that some- 

 times the initiation of sterility barriers and morphological changes in the form of an organism may occur 

 in the reverse order to that postulated above, the sterility barrier associated with aneuploidy being 

 produced at first without external morphological changes which only ensue subsequently as a result 

 of the isolation imposed. If this can be shown to be the usual order, it will represent a considerable 

 advance in our knowledge of evolutionary mechanisms, though the essential point which is being made 

 above, namely, that the morphological changes ultimately associated with aneuploidy are of a more 

 far-reaching kind than those accompanying polyploidy, would remain unaffected. 



15 



