The Geography of Phytochemical Races

34 2 Examples Within Continents
zones that were hypothesized to have formed as a result of secondary intergradation
of the main groups. The zones of intergradation exhibit different degrees of
sharpness with regard to their boundaries. For example, there is a steep cline for
the occurrence of gl R in a narrow zone, consisting of The Netherlands, Belgium,
and parts of Western Germany. On the basis of available data, it was not possible to
establish whether this was the result of comparatively recent contact between the
two main races or whether it was the result of selection. By contrast, no sharp transition
zones were observed in Eastern Europe, which was taken, tentatively at least,
to indicate that the main races had been in contact in this area for a comparatively
longer time during which there was suffi cient opportunity for gene exchange. An
effort to represent the three major zones can be seen in Fig. 2.18.
With the establishment of clear-cut fl avone races, the next obvious question was
how the pattern became established. One possibility discussed by Mastenbroek
(1983) involved selection for fl avone profi les in response to some environmental
factor or factors, climatological or edaphic being the most obvious ones. Alternatively,
the present-day distribution pattern may be the result of historical factors:
(1) migration involving long- or intermediate-distance dispersal; (2) range extension
Fig. 2.18 Distribution of Silene glycosylation types in Europe. The numbers refer to the three
major types. The zone of contact between types 1 and 2 is marked by a line of C’s