CENOZOIC BIODIVERSITY 165
something else involved? It is again tempting to
link a phase of rapid diversification to one of
rapid climate change, and several mechanisms
have been proposed whereby enhanced tropical
speciation could have been caused by changes in
ORD. These all seem entirely plausible, but
have yet to be subjected to any form of rigorous
examination.
There are also counter-arguments available in
the literature to suggest that at least Late
Pliocene-Pleistocene climatic cycles were
detrimental to the process of speciation. For
example, in the marine realm Potts (1984)
presented a model whereby Plio-Pleistocene
disturbances (principally sea-level changes)
were so frequent in the central IWP region that
evolutionary-scale processes such as speciation
would rarely be seen through to completion.
This is, in effect, precisely the opposite of the
prediction made by the Dynesius & Jansson
(2000) isolate formation model. However, it
would seem likely that most living marine inver-
tebrate taxa associated with coral reefs origi-
nated well before the Pleistocene and we should
perhaps be looking at origination events that
occurred in the mid-Miocene to mid-Pliocene
time frame (i.e. 15-3 Ma BP) (Jackson & Johnson
2000).
Evidence for a direct link between enhanced
Neogene diversification and climatic change
may be even harder to find in the terrestrial
realm. Intensive stratigraphic study of North
American Cenozoic mammal faunas has shown
very little obvious correlation with short-term
climate change (e.g. Alroy 1998; Prothero 1999);
for many prominent turnover events there is no
obvious external trigger. Palynological evidence
suggests that the vast Amazonian lowland
forests were stable throughout much of the
Cenozoic (Colinvaux & de Oliveira 2001), and
molecular phylogenetic evidence to date does
not point to any great Pleistocene radiation of
rainforest vertebrate faunas (Moritz et al 2000).
Nevertheless, a recent synthesis of phylogenetic
and geological data suggests that the species-rich
Cape Flora of South Africa is no more than
7-8 Ma old (Richardson et al 2001). In this
particular instance speciation seems to have
been driven by a combination of aridification of
a subtropical flora and imposition of regular
climate cycles.
Clearly these are complex issues that will not
be resolved either quickly or easily. However,
what is becoming clearer is the major role played
by Cenozoic plate tectonics over the last
15-20 Ma. Only when we know much more
about complex plate interactions in a number of
key areas, and how these affected parameters
such as oceanic circulation, climate change and
organism dispersal, will we be able to under-
stand more fully the nature and origin of our
major extant biodiversity patterns.
We are grateful to M. J. Benton, A. Brandt, A. Clarke,
A. Hallam and A. W. Owen for reading the manuscript
and suggesting various ways in which it could be
improved. Contribution to British Antarctic Survey
Project, Antarctic marine biodiversity: a historical
perspective'.
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