Cyclism revisited: extinction and ‘Achilles’ Heels' keep diversification in check on macroevolutionary time scales
Mass extinctions of varying magnitude prune the continuous diversification predicted by Darwinian evolutionary processes. They are caused by events that are too rare to become adaptatively accommodated. Their effects depend not only on the nature and magnitude of the triggering event but also on the state of the biosphere at the particular time. This is most clearly shown by the existence of Golden Ages preceding all Phanerozoic mass extinctions. These coincide with greenhouse periods, in which doomed clades gave rise to heteromorphs, deviating in strange ways from established bauplans. When critically examined, the seemingly ?decadent? morphologies of Schindewolf's ?typolytic stages? turn out to have been highly functional. The paradoxical link between adaptive peaks and evolutionary failure can now be explained. Specialisation tends to increase vulnerability (1) by narrowing niches and (2) by the retention of clade-specific conservative features that happen to become fatal Achilles? Heels for entire clades in the face of a particular perturbation. Following extinctions, the availability of open niches favoured relatively rapid diversification of more innovative clades and their rise to ecological dominance (Schindewolf's ?typogenetic stage?). Although the long-term changes can be observed only in the fossil record, Golden Biotopes in the present biosphere show that the Darwinian process may also be promoted by ecological isolation. As a result, clade histories do resemble individual biographies, but for ecological rather than orthogenetic reasons. This insight may help us to deal with the present mass extinction caused by our own species.