Evolution__3rd_Edition

..
Many species show similar genetic
patterns in space ...
. . . creating suture zones
Molecular dates suggest speciation
preceded the ice age
CHAPTER 17 / Evolutionary Biogeography 499
The ice age movements of species have had evolutionary consequences. In Europe,
many species survived the cold by retreating to the southern extremes of the continent.
Species such as bears and hedgehogs retreated into Spain, Italy, and the Balkans during
the peak of the ice age. These local populations, surviving adverse conditions, are called
refuges or refugia. The small populations in the different refuges would have evolved
genetic differences, either by selection or drift. The populations in Spain, Italy, and the
Balkans diverged. Then when the ice cap retreated north, all three populations
expanded north too. This has had two detectable consequences.
One is that, within a species, approximately three genetic types can be distinguished
(Figure 17.4a). If we look at the molecular phylogeny of European hedgehogs, we see
three relatively distinct clades in east, central, and west Europe. The three are
descended from the Balkan, Italian, and Spanish refugial populations, respectively
(Figure 17.4b). Secondly, many different European species form hybrid zones in similar
places. (A hybrid zone a see Section 14.9.1, p. 409 a is a region where two distinct
forms of a species meet up and interbreed.) The reason is that many species formed
refuges in a similar set of places, and expanded northwards at the same time.
In one species after another, west European and east European populations expanded
northwards and met up in a north–south line through central Europe (Figure 17.4c).
A suture zone is an area where many species form hybrid zones, and Figure 17.4c
illustrates the suture zones of Europe. On the interpretation given here, suture zones
have a historic explanation. An alternative explanation would be environmental: that
suture zones form at the sites of major environmental discontinuities. But for European
suture zones, the historic explanation is widely accepted. Analogous suture zones
seem to exist in North America, such as the one in northern Florida (Remington 1968;
Hewitt 2000).
The genetic changes in the fragmented refugial populations were not probably
enough to produce full speciation. The European hedgehogs, for instance, are currently
divided into two species. However, the molecular clock suggests the two split 3 million
years ago or more, rather than 20,000 years ago as we would expect if they speciated in
the most recent ice age. It was once suggested, following the ideas of Haffer (1969), that
the latest ice age was a time when many modern species pairs evolved. Haffer suggested
that the fragmentation of ranges accelerated the process of allopatric speciation, creating
what was called a “speciation pump” that contributed to modern biodiversity.
Haffer stimulated research, but the results of that research have not supported his
ideas. Evidence from molecular clocks, for instance, suggests that the speciation events
that produced many modern species are too old to fit Haffer’s hypothesis, nor do speciation
rates seem to go up during ice ages. However, the periods of glacial refuges may
have been accelerated times of genetic divergence between populations within some
species. Although the latest ice age did not produce a burst of speciation, it may have
helped to finish off speciation between populations that had already diverged, or
started the divergence between populations that could lead to speciation in the future.
Refuges are not only formed during ice ages. The same principle is at work, in
an inverted form, in species that now have local distributions but were more widely
distributed in past climatic conditions. The Nevadan deserts contain the vestiges of
former large lake systems, and the desert pupfish (Cyprinodon) occupies some of the
scattered remaining waterholes (Brown 1971). The 20 or so isolated populations of