Mass Extinction and Other Challenges from the Fossil Record

A number of other challenges to the view that macroevo-lution was simply extrapolated microevolutionary change have come from paleontology. While the process of spe-ciation and its genetic explanation forge a strong conceptual link between microevolution and many of the morphological changes observed in fossil lineages, there are a number of events in the history of life that seem so striking and catastrophic that certainly no population genetic models, even with fluctuating selection intensity or strong frequency dependence, can account for them.

The first such case is that of mass extinction. It has been known since the inception of geology as a field of scientific inquiry that the major periods in geological time are defined stratigraphically by complexes of fossil organisms that are distinct from those in higher or lower strata. These complexes can be defined over many timescales, with eras being subdivided into periods and periods being subdivided into lower units still, most of which are defined by the presences or absences of characteristic flaural and faunal assemblages. One reason that such boundaries defined by distinct sets of organisms exist is that the history of life has been defined by catastrophic mass extinction events.

For example, it has been estimated that the mass extinction at the Permian-Triassic boundary was responsible for the extinction of nearly 97% of marine invertebrate species (^85% of genera) and strongly impacted terrestrial plants, insects, and vertebrates as well. The extinction at the Cretaceous-Tertiary boundary took a somewhat smaller but still catastrophic toll of c. 50% of marine invertebrate genera, as well as (famously) leading to the extinction of the dinosaurs and many other terrestrial animals and plants. Other significant mass extinctions characterized the Late Devonian and the Late Triassic, with lesser ones defining other stratigraphic boundaries. In the 1980s, D. Raup and J. J. Sepkoski argued from surveys of various fossil taxa that mass extinctions are periodic, on ^26-million-year cycles. Since then the notion has been attacked both on statistical grounds (i.e., a frequent lack of resolution of dating below 5-million-year intervals) and on the basis that no single geophysical cause accounts for all or even most mass extinctions.

While much debate surrounds the causes of mass extinctions, it appears that no one causal explanation can account for the half dozen or so documented mass dyings. Based on evidence of elevated iridium levels in the boundary strata, it is likely a comet impact that was responsible for the K-T boundary extinctions. On the other hand, there is little evidence that extraterrestrial impacts had anything to do with the much greater P-T extinction, which may have been driven by drastic changes in sea level, volcanism, and changes in methane and carbon dioxide levels in both terrestrial and marine environments. The earliest documented mass extinction, one in the Late Proterozoic (the Varangian ice age of c. 850 million years ago), which hit unicellular eukaryotes in the such as acritarchs, may have been the most catastrophic of all, as some geologists believe that it was caused by a severe and global cooling event ('snowball Earth') that froze most of the oceans and nearly wiped out all living organisms apart from those sustained near volcanic vents and similar warm refugia.

Regardless of specific causes, it is clear that such massive, sudden extinctions of species cannot be modeled meaningfully at the microevolutionary level, and what lineages persist versus which ones become extinct can be arguable due to matters of chance rather than matters of adaptation or fitness. It seems to be the case that specialist taxa are more vulnerable than generalist taxa, which again is a statement of 'selection' properties above the species level, one that introduces an intrinsically biological, rather than simply extrinsic, explanation for the outcomes of mass extinctions.

Whether ecological catastrophes caused by abiotic events can be considered evolutionary processes as such is a matter of semantics, but it is clear that to address questions such as 'why are brachiopods prevalent in the Paleozoic while bivalve mollusks are prevalent in the Mesozoic and Paleozoic?' or 'why are there no living trilobites or ammonites.?' arguments based on superior or inferior Darwinian fitness are not the most meaningful way to seek answers. For such questions, an understanding of geology and random mass extinction is probably more informative than an understanding of Darwinian fitness in Mendelian populations.

But what of the other aspect of biostratigraphy? Geological boundaries are defined not only by the extinction of groups of taxa, but by the relatively sudden origin of many others. The extinction of large amphibians and pelycosaurs in the Permian was followed by a dinosaur radiation in the Mesozoic, just as the extinction of dinosaurs was followed by a radiation of mammals in the Cenozoic. The history of life contains many even more spectacular 'adaptive radiations' - the most dramatic being the Cambrian explosion, which was characterized by the rapid appearance of most of the major metazoan phyla in the Early Cambrian after the near absence of any but the most basal invertebrate taxa in the latest Proterozoic.

As with mass extinctions, such rapid evolutionary radiations are often due to major environmental changes (biotic or abiotic) in Earth history. For example, among the many competing explanations for the Cambrian explosion, one of the leading hypotheses involves the likely increase in atmospheric free oxygen in the Late Proterozoic. Other major radiations are to a large part due to the ecological niches vacated by the mass extinction of competing taxa; for example, large Cenozoic mammals filled niches once occupied by dinosaurs.

The issue of whether adaptive radiations are due to processes outside the scope of classical Neo-Darwinism is a question having two facets. The first is whether the phenomenon of high rates of speciation and adaptive evolution can be explained through classic Neo-Darwinian (population genetic) models; the second is whether the extensive changes in morphology characterizing the origin of higher taxa (as in the Cambrian explosion) require special genetic or developmental explanations.

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