The Geography of Phytochemical Races

246 6 Oceanic Islands
Fig. 6.1 Location of the Galapagos Islands in relation to the South American mainland. The
darkened island is Albermerle, Isla Isabela, in the Ecuadorian system
(Cox, 1983). Cox (1983) suggested that the earliest of the present islands emerged
about 3–5 mya. Simkin (1984) placed the age of the easternmost islands at about
3.3 million. A more recent study suggests a much longer timeline, 5–9 million years
according to dredge fi ndings (Christie et al., 1992). Those authors discuss evidence
suggesting that there has been volcanic activity in the region for the past 15–20 million
years and that it is likely that islands could have existed throughout the entire
time of hotspot activity, 80–90 million years. At the other end of the scale, the age of
the westernmost island is given as 0.50 ± 0.08 million years (Cox, 1983). Volcanic
activity continues at the present time.
Porter (1983) discussed the vascular fl ora of the archipelago, both species numbers
and possible means of dispersal. A conservative estimate of species endemism
was given as 37%. Despite their signifi cance in the biological world and their
unusual fl ora, surprisingly little in the way of comparative (bio) chemical work
seems to have been done on endemic taxa. There are important exceptions to this
generalization, perhaps the most noteworthy of which in the botanical realm is the
work on cotton by Jonathan Wendel and his colleagues (Wendel and Percy, 1991).
Only a few studies have dealt with secondary metabolites. We begin with an examination
of cyanogenic glycosides.
6.1.1 Cyanogenic Plants
A question that has concerned evolutionary biologists for many years involves the
suggestion that defense mechanisms displayed by a continental species (plant or
animal) would eventually be lost should that species become established on an
island. The reason for the loss would be the presumed absence on the island of
herbivores, or pathogens, against which the organism’s defense mechanism had
been selected in its original continental environment [see Carlquist (1974, 1980)
for discussions of this phenomenon]. Among the defensive strategies commonly
found in plants is the production of a variety of chemical substances that are either