The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 145

by exposing land bridges. Because there were a number of glacial-interglacial cycles
in the Pleistocene, flora and fauna would have been subject to evolutionary selection
pressures favouring those that were capable of migrating from one place to another,
often in suboptimal conditions. Many species (but not all) that survived from one cli-
matic cycle to another did so in ecological refugia. Such refugia might be latitudinal -
that is, closer or further away from the equator - or alternatively altitudinal, in moun-
tain ranges with vertical migration upwards in response to warm interglacials and
downwards in cooler glacial times.
Genetic analyses provide insights into the dynamic ecology of refugia over time.
The southern part of South America provides a good illustration of how such insights
can be gained. There the Andean mountains chain, separating Chile and Argentina,
was glaciated during the LGM while the lowlands on either side were ice-free. Pollen
analysis reveals that there were trees there throughout the LGM, suggesting that these
lowlands were refugia for temperate forest species. The question then arises as to
whether there was just one refuge during the LGM on one side of the Andes or two
distinct refugia, one on each side. If the former, then an analysis would reveal that
the two populations were genetically similar. Conversely, if there were two separate
refugia then populations would exhibit significant genetic differences. In 2000 Andrea
Premoli from Patagonia and researchers from the University of Colorado looked at
a tree species, the Patagonian cypress ( Fitzroya cupressoides ). Today it can be found
substantially spread out in lowland Chile but only in scattered populations on the
Argentinean side of the Andes. They examined 11 enzyme systems associated with
21 genetic loci in this species and found that spatially distinct populations on both
sides of the Andes did have distinct genetic differences, indicating that there were
two separate refugia.
Were life as simple as the above suggests. It would be facile to conclude that just
because one species reached out from, and retreated to, two refugia that temperate
forest species all behaved in a similar way. Fitzroya is a species that exhibits con-
siderable longevity, with individuals living in excess of 3000 years, but it has poor
regenerative powers. This indicates that it is not best suited to migrating. However,
other temperate forest species are and they may be able to relocate further afield. In
short, component species of the same ecosystem do not necessarily respond uniformly
to climate change; indeed, responses are varied.
There are many examples of genetic analysis illuminating how a population outrode
the last glacial. One European study, by the French botanist Remy Petit, examined the
chloroplast DNA of 22 widespread species of European trees and shrubs (all woody
angiosperm species) from the same 25 European forests (Petit et al., 2003). The
advantage of using chloroplast DNA is that it accompanies seeds only (not pollen,
which can travel considerably further). The forests were located in Sweden, Scotland,
Germany, England, Austria, Hungary, Italy, France, Greece, Slovakia, Croatia,
Romania and Spain. During the last glacial European forests were considerably more
restricted than those in North and South America, or for that matter South Africa,
due to the Mediterranean Sea forming a barrier to latitudinal migration. Nonetheless,
one might expect the greatest genetic diversity to be found in those trees and shrubs
in the south, as the glacial refugia would be more likely to be located there. The
reality is more complicated. True, the southern European latitudes were important
for providing refugia during the LGM and the Iberian, Italian and Balkan peninsulas