The evolution of diversity in ancient ecosystems: a review

Abstract

On a perfect planet, such as might be acceptable to a physicist, one might predict that from its origin the diversity of life would grow exponentially until the carrying capacity, however defined, was reached. The fossil record of the Earth, however, tells a very different story. One of the most striking aspects of this record is the apparent evolutionary longueur, marked by the Precambrian record of prokaryotes and primitive eukaryotes, although our estimates of microbial diversity may be seriously incomplete. Subsequently there were various dramatic increases in diversity, including the Cambrian ‘explosion’ and the radiation of Palaeozoic–style faunas in the Ordovician. The causes of these events are far from resolved. It has also long been appreciated that the history of diversity has been punctuated by important extinctions. The subtleties and nuances of extinction as well as the survival of particular clades have to date, however, received rather too little attention, and there is still a tendency towards blanket assertions rather than a dissection of these extraordinary events. In addition, some but perhaps not all mass extinctions are characterized by long lag–times of recovery, which may reflect the slowing waning of extrinsic forcing factors or alternatively the incoherence associated with biological reassembly of stable ecosystems. The intervening periods between the identified mass extinctions may be less stable and benign than popularly thought, and in particular the frequency of extraterrestrial impacts leads to predictions of recurrent disturbance on timescales significantly shorter than the intervals separating the largest extinction events. Even at times of quietude it is far from clear whether biological communities enjoy stability and interlocked stasis or are dynamically reconstituted at regular intervals. Finally, can we yet rely on the present depictions of the rise and falls in the levels of ancient diversity? Existing data is almost entirely based on Linnean taxa, and the application of phylogenetic systematics to this problem is still in its infancy. Not only that, but even more intriguingly the pronounced divergence in estimates of origination times of groups as diverse as angiosperms, diatoms and mammals in terms of the fossil record as against molecular data point to the possibilities of protracted intervals of geological time with a cryptic diversity. If this is correct, and there are alternative explanations, then some of the mystery of adaptive radiations may be dispelled, in as much as the assembly of key features in the stem groups could be placed in a gradualistic framework of local adaptive response punctuated by intervals of opportunity.

Jupiter – friend or foe? I: The asteroids

“…As well as disrupting the landscape, larger impacts had effects that led to climate changes, usually short-lived, that in turn have led to the extinction of a large proportion of species in the biosphere (Morris 1998).…”

Biotic Transitions in Global Marine Diversity

“…1A): (i) intervals of increasing diversity focused mainly in the early Paleozoic and the postPaleozoic, (ii) intervals of decreased diversity and rebound associated with mass extinctions, and (iii) broad transitions in dominance among higher taxa. These attributes, and possible relationships among them, have led to two different theories (3,4) to explain the major features of Phanerozoic diversification. One theory holds that mass extinctions constitute a distinct class of phenomena that transcended ongoing evolutionary patterns and processes operating at other times and governed long-term, global biotic transitions through wholesale removal of incumbents (5,6).…”

Coevolutionary dynamics of adaptive radiation for food‐web development

“…Thus, it may be predicted that rapid evolutionary diversification tends to be accompanied by mass extinction. A widespread view is that external factors are the main causes of mass extinction detected in fossil records (Morris 1998), in which rapid radiation is expected after the extinctions. However, there are empirical studies that support the simultaneous occurrence of the radiation of mammals and the mass extinctions of dinosaurs (Springer et al 2003).…”