The Geography of Phytochemical Races

188 4 Intercontinental Disjunctions
Western Australia, as one might have expected. This observation was in agreement
with conclusions drawn on the basis of morphological differences (Ornduff, 1974).
4.2.4 Icacinaceae
One of the most serious contributors to chemical ecology-chemical geography has
been the Brazilian natural product chemist Otto Gottlieb (see Gottlieb and Kubitzki,
1983; Gottlieb, 1986, 1990). It has been his goal to examine chemical profi les of
target taxa, not just from the point of view of presence or absence of a suite of
compounds, but rather to investigate the levels of complexity involved in the processes
by which the compounds are elaborated. Among other things, his work has
involved searches for regular changes in oxidation level of a set of compounds, or
for situations where there has been a switch between different biochemical pathways
(e.g., shikimate to acetate/malonate), or differences in the way plants protect
component compounds from degradation. Following these analyses is not always
easy, with point scores being tallied for certain structural aspects within each class
of compounds and the calculation of various indices of evolutionary advancement.
Detailed consideration is given to the steps involved in forming end products,
with recognition given not only to compounds that are specialized or “advanced”
because they have had substituents added to the base structure, but also to those that
have been modifi ed in ways that make them appear simpler then the base structure,
that is, the loss of a functional group originally present. An example of this is seen
below, where loss of a carbon atom through oxidation leads to a derived (advanced)
molecule.
The example to be described, admittedly one whose chemistry is diffi cult, is,
nonetheless, typical of the approach. In the case of Icacinaceae, Kaplan et al. (1991)
studied the increase in complexity of terpenoid compounds of selected members of
the family as a function of where, in the geographic range of the family, the various
genera occur. Although the work was set in a taxonomic context—using chemical
features to assess the proper placement of the family—our interest lies in the chemical
changes that appear to be associated with geography.
Icacinaceae are a moderate-sized, primarily tropical family consisting of 52
genera, many of which are monotypic, and 300 species. Additional background on
taxonomic problems surrounding the family can be found in the 1991 paper by
Kaplan et al. Chemical data have been used by Dahlgren (1980) to assess these
relationships with related families, but the application involved simple presence or
absence data (iridoids) and did not touch upon the dynamic nature of the pathways
involved.
The genera and relevant chemistry of Icacinaceae involved in this example are
given in Fig. 4.8, using the format of Kaplan et al. (1991) wherein the four sets of
genera, grouped according to chemical similarity, are listed according to geographic
areas. The geographic areas represented range from eastern Australia (Queensland)
westward through Melanesia, southeastern Asia, India, and Africa to South America.