The Geography of Phytochemical Races

30 2 Examples Within Continents
earlier in the European survey, or was simply a function of longitude. The next question
addressed dealt with the effect of elevation on the expression of pigmentation,
which Baker and Jones (1986) answered with transectional studies in the Alps, the
Auvergne, and in the Pyrenees. Again, a trend was observed with higher frequencies
of the pigmented keel observed in plants at higher elevations. We now come to
the “grand survey” in which Crawford and Jones (1988), with the help of numerous
volunteers, scored no less than 125,503 plants from 1348 sites representing all of
the 100 km 2 in the United Kingdom National Grid. This very large sampling confi
rmed and extended the earlier studies, and put the west-to-east frequency trend on
much fi rmer ground. The comparatively lower frequency of pigmented keel plants
in southern England was also confi rmed.
No satisfactory explanation for this color polymorphism in terms of its selective
advantage over the whole range of the species has been forthcoming. Some local
effects were noted, however. For example, Jones et al. (1986) observed that lightkeeled
plants at two sites in Yorkshire fl owered earlier than their dark-keeled neighbors.
Similarly, at six sites near York, the frequency of dark-keeled plants increased
as the season progressed. Little advantage seems to accrue to the color morphs,
however. The authors (Crawford and Jones, 1988) also remarked that attempts to
correlate large-scale clines of this sort with climatic conditions is tempting, but
no evidence exists to support such a relationship in this species. In concluding,
they stated that, “We fi nd it hard to accept that the distribution of the keel-color
morphs could have arisen by chance. We believe, therefore, that the determination
of the selective forces involved in this polymorphism provides a major challenge to
ecological genetics.” This conclusion could serve equally well for the discussion of
Woodson’s study of fl ower color morphs in Asclepias tuberosa, which will be met
a little later.
Although the author has not had the opportunity to read either of the following
two papers, it seems appropriate to identify another color morph described in
them, this time involving Arum maculatum L. (Araceae). Both Burbidge (1903)
and Colgan (1903) reported that leaf markings in this species appeared to be more
prevalent in populations visited in England than those observed in Ireland. Without
chemical data, it is not possible to equate these markings to the chevrons commonly
seen in clover species, but the patterning in some leaves is caused by deposition
of anthocyanin derivatives in the upper epidermis of leaves, resulting in a purple
coloration. This has been shown to be the case with Collinsia parvifl ora Dougl.
ex Lindl. (Scrophulariaceae), locally known as blue-eyed Mary, a plant common in
southwestern British Columbia. Griffi ths and Ganders (1983, pp. 93–97), studied
the occurrence of this leaf marking in populations of blue-eyed Mary on Vancouver
Island, islands in the Straits of Juan de Fuca and Georgia, and a few sites on the
mainland. Populations were scored as unspotted, lightly spotted, or heavily spotted.
Populations characterized by heavy spotting were usually found in cooler, northfacing
slopes rather than on the warmer southern sides of hills and headlands. Since
this species fl owers in early spring, the authors suggested that the darker pigmented
leaves absorb more sunlight, and thus can overcome the differential in air temperature
between cooler and warmer microhabitats. Growth of plants in controlled