The Geography of Phytochemical Races

112 2 Examples Within Continents
along the transect with the four Californian populations clearly distinguishable from
the Arizona populations. The total combined chromene-benzofuran fraction of the
California populations was several times larger than the corresponding fraction in
the Arizona populations. With regard to individual compounds, encecalin was the
predominant compound in the four California populations, with amounts ranging
from about 90 to nearly 150 µmol g −1 dry weight. The Palm Springs population
differed from all the rest with a concentration of euparin of about 75 µmol/g dry
weight, while in all of the other nine populations, concentrations of this compound
did not exceed about 25 µmol g −1 dry weight. No explanation for these striking differences
was offered.
An earlier study of E. farinosa by Kyhos (1967) suggested that fl oral color variation
within the species might be linked to environmental factors. Encelia farinosa
comprises two varieties, E. farinosa var. farinosa and E. farinosa var. phenicodonta
(S. F. Blake) I. M. Johnston. (A third variety, E. f. var. radicans, now considered
E. radicans Brandegee, occurs mainly in the Cape region of Baja California and does
not concern us here.) Variety phenicodonta occurs in most of Baja California extending
around the northern shore of the Gulf of California, in the Colorado River Valley,
and in scattered sites at higher elevations in southern California. Variety farinosa
occurs in much of the northern Sonora Desert. The two varieties are most readily
distinguishable on the basis of fl oral pigmentation: var. farinosa is characterized by
yellow ray and disk fl orets, whereas var. phenicodonta has brownish-purple disk
fl orets (a color usually associated with anthocyanins). Furthermore, the color differences
appear to be inherited as a single genetic dominant. Kyhos (1967) observed
differences in disk fl oret color as he followed a transect from Rice, California,
southeastward toward Blythe, California, and then across the Colorado River and
east into Arizona. In the vicinity of Rice, only var. farinosa was observed. About
26 miles northwest of Blythe, individuals of var. phenicodonta began to appear. Over
the next mile, their proportion rose to 5% and then to 38% over the next 6 miles.
Within about 10 miles of Blythe the frequency of var. phenicodonta had risen to
70%. The census could not be sustained in the immediate vicinity of Blythe owing to
extensive agricultural development, but just east of the Colorado River the frequency
of var. phenicodonta was 100%. There then followed a steady decrease so that at the
13–14 mile mark (along US Rt. 60) the frequency had dropped to 7%. Further east
only var. farinosa was seen. Very similar counts were recorded on other transects
(e.g., in the vicinity of Yuma, Arizona). The only apparent exception to the appearance
of var. phenicodonta along watercourses is its occurrence at several sites at
higher elevations where frequencies approaching 20% were recorded. The predominance
of var. phenicodonta near the coast, along watercourses, and at higher, moister
sites in otherwise desert locales strongly suggested a higher moisture requirement
for this variety. Kyhos (1967) reasoned that the correlation between higher moisture
and frequency of occurrence of var. phenicodonta might indicate a sensitivity of
this variety to higher temperatures, but no correlation with maximum, minimum, or
mean temperatures was evident from a study of local meteorological records. The
possibility that different fl oral colors might be the product of selection for pollinator
preference was also tested, but the major pollinator, the beetle Tanaops abdominalis,