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Comment on "Buffered Tree Population Changes in a Quaternary Refugium: Evolutionary Implications"
Tzedakis et al. (1) reconfirmed the
traditional, historic biogeographical model of temperate species in
Europe. Underthis model, temperate-adapted species survived the long
unfavorableepisodes of the Pleistocene glacials in refugial areas,
and thenexpanded their ranges during interglacial periods. It has beenassumed that refugia for taxa that recolonized northern and centralEurope, were located in the Mediterranean or western Asia, andparticularly in the southern European peninsulas of Iberia, Italy,and
the Balkans. Although popular and well published recently(2,
3), this model does not provide the onlyexplanation of the
empirical data relevant to the location oftemperate refugia during the
ice ages--nor does it represent themost likely areas from which
postglacial recolonization took place.In fact, several reports have
suggested the existence of crypticrefugia in northern Europe, citing
fossil evidence from macroscopicplants and vertebrates, as well as
phylogeographic studies ofland snails, sedges and ferns, and
freshwater fish (4-11).Although these data contradict the
dominant paradigm of both palynologyand phylogeography, their
implications are difficult to ignore.
Tzedakis et al. (1) discussed the production of
endemic species in microrefugia within the greater Balkan refugium.However, the Balkans are more generally considered to be a refugiumfor
the pan-European temperate taxa (2, 3).It is therefore
difficult to explain how the southern Europeanpeninsulas could act
both as refugia for a species and as an isolatedarea that produced
allopatric endemics (12). Furthermore,recent efforts to map
mammalian species in Europe during the LatePleistocene (13)
clearly show that most, if not all,the temperate species typically
associated with deciduous woodlandswere found far to the north of the
peninsular refugia of Iberia,Italy, and the Balkans. This includes
species that are not foundabove 60°N in present-day Europe. Assuming
that dispersal fromthese northern refugial areas took place by a
northern leadingedge mode (Leptokurtic dispersal)
(14), the geneticdiversity spreading north is
unlikely to have emanated from peninsularEurope. Some species such as
the mole Talpa europaea and woodmouse Apodemus
sylvaticus have fossil records that extend as farback as the
Early Middle Pleistocene (15) (about 600ka). This
suggests that their origins as pan-European taxa areancient and that
any patterns of genetic differentiation couldhave persisted through
many glacial cycles. It is also possiblethat the different haplotypes
originated in northern Europe andwere simply pushed south into the
Mediterranean peninsulas duringthe glacials, as opposed to having
originated in southern Europe.
As mentioned by Tzedakis et al. in
(1), parapatry may have played a part in the formation of
the southern Europeanendemic species, although this may have been more
important forthe pan-European taxa, which may have experienced a much
lesserdegree of isolation. There is a general over-emphasis in the
literatureon allopatric speciation. This has led to hypotheses like
thoseof Coope (16) and Bennett
(17), which suggestthat allopatric differentiation
that occurred during the glacialswould have been "undone" by
interbreeding between emergent refugialpopulations during the
succeeding warm periods. Taking a morecomplex view of speciation that
considers the existence of northernand southern refugia, the possible
role of parapatry or sympatry,and the newly popularized view of
ecological speciation (18)will help generate a more
realistic scenario for the origins ofmodern European biota.
John R. Stewart
Department of Anthropology and AHRB Centre for the Evolutionary Analysis of Cultural Behaviour University College London Gower Street London, WC1E 6BT, UK E-mail: ucsajrs{at}ucl.ac.uk
J. R. Stewart, M. van Kolfschoten, A. Markova, R. Musil, in Neanderthals and Modern Humans in the European
Landscape During the Last Glaciation, 60,000 to 20,000 Years Ago:
Archaeological Results of the Stage 3 Project, T. H. van Andel, Ed. (McDonald Institute Monograph Series, Cambridge, UK,
in preparation).
A. J. Stuart, in Island Britain: A
Quaternary Perspective, R. C. Preece, Ed. (Geological Society
Special Publication No. 96, Geological Society, London, 1995).
15 October 2002; accepted 11 December
2002
10.1126/science.1079388 Include this information when citing this paper.
The editors suggest the following Related Resources on Science sites:
In Science Magazine
TECHNICAL COMMENTS
P. C. Tzedakis, I. T. Lawson, M. R. Frogley, G. M. Hewitt, and R. C. Preece (7 February 2003) Science299 (5608), 825b.
[DOI: 10.1126/science.1080630] |Full Text »|PDF »
