133845.pdf

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