Jökull - 01.01.2004, Page 9

Theories on migration and history of the North-Atlantic flora: a review
in the North Atlantic regions, may be examples of
such long-distance dispersal from Siberia. Moreover,
it has been suggested that parts of northern Norway
and east Greenland, where extremely disjunct Eurasi-
atic vascular plants occur, are linked to coastal areas
favourable for accumulation of drift ice and driftwood
from Siberia during an initial immigration following
the last glaciation (Johansen and Hytteborn, 2001).
NEW TECHNIQUES – THE DEBATE
CONTINUES
Much research has been conducted in the last decade
to try to resolve the question of plant migration and
history of arctic and alpine plant species. By using
molecular methods as well as results from e.g. pollen
analyses and statistical models for species richness
pattern the debate on the plant migration and the ori-
gin of the flora in the North Atlantic region has been
revived.
Through molecular techniques, it is now possi-
ble to examine genetic similarities between different
populations of species and compare them to their ge-
ographic distances (Brochmann and Steen, 1999). Bi-
ologists have been debating the genetic consequences
of survival of isolated plant populations in glacial
refugia. Many predict that higher levels of diver-
sity should characterize refugial populations relative
to their descendant populations (e.g. Gabrielsen et al.,
1997; Comes and Kadereit, 1998). Others assume a
massive loss of allelic variability in refugial popula-
tions through inbreeding, founder events and popu-
lation bottlenecks, especially in small refugial areas
and populations (Füchter et al., 2001). Even though
small and inbred plant populations often show a very
small amount of genetic diversity within populations,
the genetic distance between different populations is
most often large (Nordal et al., 1998).
Tabula Rasa
Statistical analyses of diversity patterns in the Norwe-
gian mountain flora indicate that contemporary distri-
bution patterns can be explained by climate, geology
and topography without recourse to historical vari-
ables. The glacial survival theory thus seems to be
redundant (Birks 1993, 1996).
RAPD phylogeography has been used to study
variation in two arctic/alpine species in Svalbard.
Both were interpreted in support of the tabula rasa
theory (Gabrielsen et al., 1997; Tollefsrud et al.,
1998). The results indicate that Svalbard popula-
tions of Saxifraga caespitosa and S. oppositifolia are
closely related to Norwegian mainland populations.
This close relationship cannot be explained with-
out recent long-distance dispersal across the Barents
Sea barrier and it is most likely that this dispersal
occurred after the last glaciation (Gabrielsen et al.,
1997; Tollefsrud et al., 1998).
Abbot et al., (1995) detected 5 different Cp-
haplotypes in a population of S. oppositifolia in Sval-
bard, and interpreted this as an indication of possible
survival in high arctic refugia during the last glacia-
tion. A more recent study challenges these conclu-
sions as only two Cp-DNA haplotypes were found in
S. oppositifolia in Svalbard. This result supports the
proposal that there is no molecular evidence for local
survival of S. oppositifolia within Svalbard or Norway
during the last full-glacial period (Abbott et al., 2000).
Glacial Survival
Pollen records from Iceland have been interpreted
in support of the glacial survival theory (Rundgren
and Ingólfsson 1999). Pollen records from Lake
Torfadalsvatn, N-Iceland spanning 11,300–9,000 BP
show that many of the taxa present prior to the
Younger Dryas stadial (11,000–10,000 BP) continued
to produce pollen during that cold event. Rundgren
and Ingólfsson (1999) interpreted this as evidence for
glacial survival, e.g. that many plant species with high
tolerance for climate fluctuations probably survived
the whole Weichselian in Iceland, and cite a high plant
diversity in arctic areas and present-day nunataks in
Iceland and Greenland as further support for their the-
sis.
Some recent molecular research also favours
glacial survival at high latitudes. Using isozyme
analyses, Odasz et al. (1991) measured genetic dis-
tances between geographically isolated populations of
Pedicularis dasyantha in Spitsbergen, Svalbard. They
found significant variation in allele frequencies and
interpreted the pattern of variation, and the evolu-
tion of self-compatability in an otherwise mostly self-
JÖKULL No. 54 9