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104 JOSEPH P. BOTTING
(Whitlock & MeCauley 1990; McCauley 1991).
Among the most critical parameters controlling
genetic variability is whether recolonization
occurs through population fusion (differentia-
tion decreased) or permanent fission (differenti-
ation increased). In many cases, however, it
appears that subspecific diversification results
from fission of a population, followed by
refusion, and further factors are presumed to be
operating in order to account for observed
diversification. One possibly is unidirectional
'meiotic drive," discussed by Amos & Harwood
(1998), but present understanding is very
limited. The critical point when considering the
increases in genetic diversity and potential
speciation rate is that the bloom events were
repeated. Several thousand events, each sepa-
rated by the order of decades to centuries, have
the potential to exaggerate slight probabilistic
changes, perhaps producing noticeable effects
on global diversity curves.
A further element that strongly encourages
speciation in modern faunas (e.g. cichlids:
Barlow et al 1984; Russock 1986; Knight et al,
1998; Seehausen & van Alphen 1999) is sexual
selection based on parental imprinting, which
can potentially lead to sympatric speciation in
heterogeneous populations. However, inter-
preting the reproductive habits of extinct organ-
isms is notoriously difficult, despite ambiguous
evidence that trilobites, for example, provided
some degree of guardianship over their offspring
(Davis et al. 2001). Uniformitarian assumptions
suggest that many arthropods provided the
potential for imprinting (e.g. Barnes 1987), but
direct evidence is lacking. Overall, the potential
for parental imprinting and pure sympatric
speciation by this mechanism in invertebrates
was probably limited.
The effects of subsequent mixing under stable
environmental conditions must also be con-
sidered. Taxa showing highly specific behav-
ioural traits may not fall into direct competition,
and the increased diversity may be conserved.
This is also true of inhabitants of facies with a
diverse range of ecological niches, a feature
enhanced by the development of tiering during
the Ordovician (e.g. Droser et al. 1995). Soft-
substrate shelf facies with low faunal abundance
and diversity may provide the focus for diversity
increases, but some diversity is likely to have
been lost by merging or competition during
long-term quiescence, in the absence of complex
ecosystems that could maintain niche separ-
ation. Although the centres of diversification
would have been in these environments, sus-
tained long-term diversity increases may have
initially occurred by range expansion into
shallower-water facies, where diverse physical
conditions and complex ecosystems provided
permanent habitats. (Trends of this nature
should be recognizable through comparison of
temporal ranges and morphological variability
between closely related species occurring in
different environments.) Abundant specific
niches could then have allowed preservation of
lineages that would rapidly have become extinct
in their original environment. The observed
onshore-offshore trend during the Palaeozoic
(e.g. Bottjer et al. 1988) would reflect the gradual
expansion of this complex, stable ecosystem into
deeper water, while most of the explosive diver-
sity increase at speciation centres was subse-
quently lost from the original environment.
Since the geographic mixing of distinctive
populations is critical to this process, planktic
organisms should not be significantly affected.
The point relating to preservation of novel char-
acters within the blooms would still stand, but
constant mixing of populations would reduce the
significance of these effects; diversity increases
should be expected, but to a much lesser extent
than in the benthos.
Implications for Ordovician diversity
The Ordovician Period was characterized by
widespread explosive volcanism in many parts of
the world, as discussed above. The results of ash
deposition in shelf regions represent a concep-
tual argument whereby an increased speciation
rate could have operated in volcanically active
areas, and which provides objective predictions
for several aspects of the diversity patterns
expected. The temporal, geographical and
environmental distributions should show three
basic correlations; maximum rates of diversifica-
tion should correspond to:
(a) areas and times of extensive pyroclastic
volcanism, emphasizing numerous small
eruptions rather than few large ones;
(b) benthic oxygen limitation and well-
developed stratification, leading to high-
amplitude bloom events;
(c) high eustatic sea level, increasing the shelf
area over which small eruptions could
produce significant effects.
Volcanism and sea level are usually related, on
a global scale, through increased tectonic
activity and ocean ridge volume, while their
relationship to benthic anoxia is presently un-
certain. However, all three factors particularly
characterize the Ordovician in general, and the
coincidence of all three is predicted to correlate