The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 146

were all important locations. But pollen and fossil analysis also indicates that sev-
eral tree species were localised in small, favourable spots at the southern edge of
the then cold, dry steppe-tundra in eastern, central and south-western Europe. With
post-glacial warming some of the surviving populations expanded (horizontally),
some migrated altitudinally (vertically up mountains) and some unable to migrate
in time became extinct. One would expect that those populations that had been able
to migrate and persist would show the greatest genetic diversity. One might also
anticipate a gradual decrease in diversity away from source populations. Again the
reality was more complex.
Petit and colleagues (2003) analysed their data in two ways. First they tested the
genetic diversity of species within populations, and second they looked at the genetic
divergence of populations in forests from all remaining populations in other forests.
This last is an indication of how strongly the separate populations are related to each
other.
It transpires that for the most part there is a greater genetic diversity in southern
forests which suggests that their population persisted within the region throughout the
last glacial. There was, though, one exception and that was on the Iberian peninsula,
where populations exhibited low genetic diversity. This may well be because that
peninsula seems to have been exposed to particularly severe climatic episodes of
cold and aridity, and because the Pyrenees formed a particularly formidable barrier.
(The lower LGM sea level did not provide extensive land bridges there, as there were
between the British Isles and mainland Europe or Italy and the Balkan peninsula.)
The second thing discovered was that the genetic divergence of populations from all
remaining populations was highest not in the low-latitude refugia but across France,
Germany, Austria and Hungary.
What appears to have happened is that species were constrained to southern refu-
gia during the glacial, and a few fortunate higher-latitude refugia. However, after the
glacial each refuge made its own contribution to the genetic diversity of the current
interglacial higher-latitude forests on land in between the refugia. Interglacial pop-
ulations of trees close to glacial refugia retained the genetic make-up of the refuge
populations. Populations further away and equidistant from refugia tended to share
source-refugia population genes. However, with the rapid transition from glacial to
interglacial conditions and back it was the populations of trees that were closest to
refugia that were able to migrate back while the mixed-gene populations furthest
away tended to die out. Consequently, the refuge populations tended to retain their
genetic identity.
Of course, as with any study, one has to be aware of limitations and this analysis
only looked at diversity within species (intraspecific diversity) and not interspecific
biodiversity. Furthermore, whereas this scenario applies in this circumstance and
with these species, it does not necessarily apply to other species or circumstances,
such as with the refuge (or refugia) being further away.
Nonetheless, genetic analysis does provide some insights. One such possible infer-
ence applied to humans came in 2009 from work by Jeremy Searle and colleagues
published in the Proceedings of the Royal Society of London Series B . There are
two competing views as to the origin of the Celts, whose descendents today inhabit
Cornwall, Ireland and Scotland in the British Isles and who have some cultural con-
nections with some in Brittany in France. The traditional view is that the ancestors