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