The Geography of Phytochemical Races

3.4 South America 169
Fig. 3.13 Compounds 286–298, terpenoids from Fitzroya cupressoides. The horizontal lines at ca.
40°S and 43°30′S represent the range of occurrence of Fitzroya
to reestablish populations on their respective slopes of the mountains. The greatest
genetic variation would be expected when comparing populations from eastern
versus western slopes of the Andes. In this scenario, the mountain chain would have
served as an effective barrier to gene exchange between trees on the opposite sides
of the range.
The extent-of-the-ice effect is based on the assumption that there would be
differential effects along a north south transect. This was based on the knowledge that
the extent of ice cover was about twice as large at 42°30′ S than it was at 39°50′ S.
If multiple refugia had existed, one would expect a greater degree of genetic isolation
of populations near the present southern limit of the species than at its northern
limit. Northern populations of Fitzroya could have originated from long-distance
dispersal of propagules from a southern refugium, or they may have survived glaciation
in patches and continued to exchange genes during the period of maximum glaciation.
This scenario would lead to the prediction of greater genetic identity between
northern populations and their southern source population(s) if long-distance dispersal
had occurred, or between northern populations if there had been local survival.