VI.13 Causes and Consequences of Extinction Michael J. Benton OUTLINE 1. SPECIES EXTINCTION 1. Species extinction Extinction is the disappearance of a species or larger 2. Some definitions: Extinction styles and taxon. The geographic scale can be local or global. The magnitudes concern here is with the latter, corresponding to the 3. Mass extinctions complete disappearance of a genetic lineage worldwide, 4. Declining extinction risk and resetting the clock not the local disappearance of a species by emigration or 5. Extinction and the drivers of macroevolution environmental change. In the global case, as has often been said, extinction is forever. Species extinction is a normal part of evolution, but there The extinction of species is inevitable. Each species have been many times in the earth’s history when higher- has a duration, which is not predetermined but may be than-expected numbers of extinctions have occurred. characteristic of the wider taxon. A common assertion, During sudden extinction events, and especially during developed by George Gaylord Simpson and Steven mass extinctions, major physical environmental crises Stanley, is that mammals evolve at 10 times the rate of have wiped out large portions of life. The fact that se- clams, which means they originate and go extinct at rates lectivity during extinction events differs from natural differing by an order of magnitude. This declaration selection suggests that higher-level macroevolutionary could be an artifact of how human taxonomists identify processes have continually affected the evolution of life. morphospecies (see chapter VI.1)—perhaps they sub- divide mammalian species 10 times as finely as they do those of bivalves, possibly responding to the evident vi- GLOSSARY sual differences among mammalian species while miss- Court Jester. The model of macroevolution that con- ing the less visible species-specific cues in mollusk shells. centrates on changes in the physical environment as Nonetheless, assuming that speciesof mammals and mol- the main drivers (cf. Red Queen). lusks are somehow equivalent sections of the tree of life, Ecospace. A combination of habitat and ecological activ- then there are broad differences in mean species dura- ity at any scale. tions through geologic time, and therefore also in species Macroevolution. Evolution above the species level. extinction rates (table 1). Mass Extinction. The sudden, worldwide loss of many Such wide differences in species extinction rates and species of diverse ecologies. macroevolutionary rates have clear implications for the Morphospecies. A species defined on the assumption interpretation of times of intense extinction, such as that all members share the same morphology, and extinction events and mass extinctions in the past, and other species show different external form. the current biodiversity crisis (see chapter VIII.6): one Pseudoextinction. “Extinction” of a species when it would expect the fast-evolving species to be more liable evolves into another species. to extinction and, indeed, more likely to recover fol- Red Queen. The model of macroevolution that con- lowing the crisis than the slowly evolving species. For centrates on biotic interactions as the main drivers example, the ammonoids, a long-lived group of mol- (cf. Court Jester). lusks, typically had short species durations but suf- Taxon. A species or larger division of the tree of life. fered near-complete wipeout during four mass extinction 580 Speciation and Macroevolution Table 1. Estimated mean durations of fossil species, to be called a new species. In most molecular phylogenetic taken from various sources approaches, species terminate at the present day, and the issue of extinction does not arise. When fossil taxa are Mean incorporated into phylogenies, they are generally treated Group duration (My) as discrete entities that terminate with a definitive ex- Reef corals 25 tinction. In densely sampled fossil records, however, some Bivalves 23 species apparently evolve directly into others, and the Benthic foraminifera 21 extinction of the older parts of the lineage is termed a Bryozoans 12 pseudoextinction because the gene pool of the popula- Gastropods 10 tions that constitute the original lineage continues into the Planktonic foraminifera 10 replacing species. The relative prevalence of such pseu- Echinoids 7 doextinctions is hard to determine: it could be argued that Crinoids 6.7 they are in fact rare, and quoted examples are based on Monocot plants 4 nonobjective interpretations of sequences of rather simple Horses 4 fossils through numerous sampling horizons. Conversely, Dicot plants 3 Freshwater fish 3 critics of cladistics have claimed that such transitional Birds 2.5 successions of species are relatively common and repre- Mammals 1.7 sent a challenge to the cladistic method because it can Primates 1 identify only species that arise by splitting. Insects 1.5 If species extinction is the end of a lineage, the term extinction is also more widely used by evolutionists and Source: Summarized by McKinney 1997. paleontologists to denote the end of a clade or para- Note: Marine groups show longer durations (6.7–25 My) than phyletic group. For example, the “extinction of the dino- terrestrial groups (1–4 My). saurs” means the end of all nonavian Dinosauria, in other words, the set of clades that includes all animals popularly events, yet recovered rather rapidly in comparison with called dinosaurs but not including the dinosaurian sub- other marine invertebrates. clade Aves (Avialae), the birds. In this case, the extinction The prevalence of extinction— its inevitability—for of the dinosaurs does mean the termination of a large all species is obvious to evolutionary biologists and pa- number of clades, such as Ornithischia and Sauropodo- leontologists, but perhaps less so to nonscientists, and is morpha, and among the theropods, Ceratosauria, Car- germane to wider discussions about the current biodi- nosauria, Troodontidae, and Dromaeosauridae. In other versity crisis. Clarity is needed on three issues in this cases, however, the term extinction isappliedtoevenless context: lineage extinction at some point is the norm, cohesive groups that may share some general ecological species differ innately in their extinction risk by wider characteristics, such as body size or geographic region. An clade membership, and these points are distinct from the example is the end-Pleistocene extinction of large mam- immediate risk of extinction of any named species ac- mals in the Northern Hemisphere, sometimes termed the cording to current ecological threats. “extinction of megafauna,” meaning some, but not all, The aim here is not to discuss extinction as it man- large animals, in some, but not all, parts of the world. ifests itself in the context of natural selection or phylo- Paleontologists divide extinctions into three catego- geography (see chapters II.5, III.1, III.6, III.7, and VI.4), ries: background extinctions, extinction events, and mass nor in terms of its role in rates of evolution and species extinctions, each of which is a useful concept in particular selection (see chapters VI.11, VI.12, and VI.14) and in contexts, but between which there are no sharp divisions. the current biodiversity crisis (see chapter VIII.6) but at Background extinction is the sum of all normal spe- the macroevolutionary level and in two broad contexts: cies terminations during a defined time interval (time first, as a part of the debate about biotic and abiotic bin). The termination of any particular lineage is not pre- drivers of evolution, and second, in terms of the role of dictable, but the mean rate across a large clade, across extinction events in punctuating the history of life. a region, or worldwide for all life is predictable. Hence, all things being equal, global extinction is a stochastic process, and its rate should be predictable, dependent 2. SOME DEFINITIONS: EXTINCTION STYLES on the standing crop of species and their distribution AND MAGNITUDES through major clades (each of which has a characteristic The extinction of a species may occur according to one of mean extinction rate). two patterns in phylogenetic terms: the species termi- Extinction events are times when many species go nates without leaving any issue, or it evolves sufficiently extinct for a shared reason. Extinction events can be of Causes and Consequences of Extinction 581 Table 2. The “big five” mass extinctions, with principal victims and possible causes Event Ma Victims Possible cause End-Ordovician 444 Nautiloids, trilobites, brachiopods, crinoids, bryozoans, Glaciation corals Late Devonian 372 Trilobites, brachiopods, bivalves, corals, nautiloids, LIP, ocean anoxia sponges, crinoids, fishes (ostracoderms*, placoderms*) End-Permian 252 Brachiopods, blastoids*, trilobites*, crinoids, rugose* LIP, ocean anoxia and tabulate* corals, pareiasaurs*, synapsids End-Triassic 201 Bivalves, ammonoids, gastropods, conodonts*, basal LIP, ocean anoxia archosaurs* End-Cretaceous 66 Dinosaurs*, pterosaurs*, plesiosaurs*, mosasaurs*, Meteorite impact; LIP ammonites*, belemenites*, bivalves (rudists*), gastropods, corals, foraminifera, nannoplankton Note: LIP, large igneous province—basaltic eruptions *Groups that entirely died out all magnitudes, but the term is usually reserved for those method was statistically unreasonable because the error smaller events that do not qualify as mass extinctions. bars included negative extinction rates, which cannot Under this assumption, extinction events may