1922b), but in the Precambrian it seems to have been a general phenomenon characteristic of a group of prokaryotic microorganisms that dominated the Earth's biota, possibly even as early as 3.5 Ga ago (Schopf, 1993). Why have cyanobacteria evidently changed so little over their exceedingly long evolutionary history?

To understand the underlying causes of cyanobacterial hypobradytely, it is instructive to review Simpson's thoughtful analysis in Tempo and Mode, for although he was unaware of the Precambrian prokaryotic fossil record, Simpson was much interested in slowly evolving (bradytelic) Phanerozoic lineages. In addition to noting (but dismissing) the possibility that "asexual reproduction (as inhibiting genetic variability)" might be conducive to slow evolution (Simpson, 1944, p. 137), he singled out two principal factors: large population size, and ecologic versatility, an exceptional degree of adaptation "to some ecological position or zone with broad … selective limits … a particular, continuously available environment" (Simpson, 1944, pp. 138, 140, 141). Because unusually slow evolution involves "not only exceptionally low rates of [evolutionary change] but also survival for extraordinarily long periods of time" (p. 138), and because "more specialized phyla tend to become extinct before less specialized," Simpson proposed ''the rule of the survival of the relatively unspecialized" (Simpson, 1944, pp. 138, 143).

Although intended by Simpson to apply to Phanerozoic organisms, chiefly animals, these same considerations (with the addition of asexual reproduction) apply to Precambrian cyanobacteria. First, with regard to reproduction, cyanobacteria are strictly asexual, lacking even the parasexual processes known to occur in some other prokaryotes. Given the remarkable longevity of the cyanobacterial lineage and moderate or even low rates of mutation, however, the absence of sexually generated genetic variability cannot be the sole explanation for their hypobradytely. Second, like virtually all free-living microorganisms, cyanobacteria typically occur in local populations of large size. Coupled with their ease of dispersal (via water currents, wind, and hurricanes, for example) and for many species a resulting very wide (essentially cosmopolitan) geographic distribution, their large populations can also be presumed to have played a role in their evolutionary stasis (Figure 1). Third, and probably most important, however, is the ecologic versatility of the group.

Summarized in Table 2 are known ranges of survivability (and of growth under natural conditions) for modern oscillatoriaceans and