Darwin argued that all extinction is selective: species not able to compete with other species die out. In one of the passages quoted above (ref. 1, p. 337), Darwin expressed confidence that if ancient species could be re-created today and put in competition with their modern counterparts, the old species would be "beaten and exterminated." This is definitely not the current view, and major research programs are now being devoted to determining the extent, if any, of selectivity in past extinctions. A common (though by no means proven) view is that the victims of extinction are in no way different from the survivors, except for the fact of their extinction. Simpson was clearly moving in this direction when he suggested in Tempo and Mode (5) that the mammals were the lucky recipients of space vacated by the dinosaurs. Taxonondc Selectivity. Much of current extinction research attempts to identify taxonomic selectivity. Do some taxonomic groups suffer significantly more species extinction in an extinction episode than other groups? These studies can take advantage of the availability of taxonomic data bases, such as those compiled by Sepkoski for marine genera (8) and families (15), and thus have the benefit of large samples. The approach carries the tacit assumption that genealogical relatedness implies similarity of physiology, ecology, or other attributes that determine susceptibility to extinction. Taxonomic selectivity has been documented, but the effect is generally quite small and requires massive samples for confirmation. For example, when extinction rates for several taxonomic groups are compared with the mean for all groups, about 10% differ from the mean at a 0.05 significance level, whereas 5% would be expected by chance. Similarly, about 2% of the tests are significant at the 0.01 level. Thus, taxonomic selectivity is present but minor.
Occasionally, pronounced taxonomic selectivity has been found. The dinosaur extinction is such a case. In the latest Cretaceous of western North America, Clemens (16) tabulated 117 genera offossil mammals, amphibians, reptiles, and fish; 50 of these (43%) died out at or near the end of the Cretaceous, including all 22 dinosaur genera. The null hypothesis that all genera shared the same probability of extinction (0.43) can be rejected easily, and this demonstrates a clear bias against dinosaur survival. But such cases are relatively rare. In the same data set, for example, 8 of 24 mammalian genera died out (33%), but because of the small sample size, it is impossible to demonstrate that this extinction rate is significantly lower than the mean for all groups. Small sample sizes have plagued many studies of selectivity, giving rise to generalizations that are widely accepted but not supportable statistically. For example, it is often claimed that the amphibians survived the K-T event with little difficulty. In the Clemens (16) data set, only one-third of the amphibian genera went extinct (equal to the mammalian rate), but the sample size (12 genera) is, again, much too small for statistical significance.
Selectivity for Specific Traits. Somewhat more success has been achieved by focusing directly on aspects of physiology, behavior, habitat, and biogeography. Jablonski (17) showed, for example, that marine mollusks with planktonic larvae survive longer during "background" times than those which develop directly from the egg. This result is reasonable because species with a planktonic stage have greater dispersal capabilities and can attain wider geographic distributions, and thus are more likely to survive stresses that eliminate species in small areas. Whereas Jablonski has confirmed that widespread species are significantly less likely to go extinct during most geologic intervals, he has also shown that this protection breaks down at times of severe mass extinction, when broad geographic range at the genus level becomes important (18).
It is commonly thought that tropical organisms suffer more extinction than those in higher latitudes. Although this is supported by anecdotal data for several extinction events, a recent study based on a global data base (3514 occurrences of 340 genera) found no recognizable geographic pattern in extinction of bivalve mollusks at the end of the Cretaceous, once reef-dwelling rudists were omitted (19). Approximately 50%6 of all genera died out, regardless of geographic position. Unpublished follow-up studies show a lack of habitat selectivity for bivalves and gastropods of the Gulf Coast during the same interval (D. Jablonski, personal communication). It may be that extinction is selective when overall extinction rates are low (so-called background extinction), but not when rates are high.
Large body size is thought to increase the risk of extinction. Indeed, many apparently good examples exist, including the now-extinct dinosaurs, ammonites, eurypterids, mammoths and mastodons, and rudist clams (20). In the terrestrial realm, at least, decreased survival can be related easily to body size through demographic considerations (small populations, large home range, low birth rates, etc.). But the issue is clouded by the lack of rigorously controlled statistical analysis and by the fact that in several cases (e.g., eurypterids and ammonites), the largest species did not exist late in the group's range.
Species Selection. A special case of selective extinction involves differential origination and extinction of species in an evolving clade. In theory, species carrying a favored trait should survive longer and thus have greater opportunity for speciation than less well-adapted species. The expected result is an increase in frequency of species carrying the favored trait. The species-selection idea is close to Darwin's view of selective extinction among closely related species. Unfortunately, well-documented cases of species selection are few. Also, there has been vigorous disagreement among evolutionary biologists and paleobiologists on whether species selection could ever be an important force in evolution. On the one hand, it is argued that species selection can alter frequencies of alternative traits, even eliminating some, but cannot be responsible for complex adaptations such as eyes and limbs (21). The counterargument is that adaptations, originating at the population level by natural selection, may be sustained, even "cloned," by species selection, thus making possible further improvements in the trait by natural selection operating within the descendent species. By the latter scenario, species selection plays a useful and possibly indispensable part in the evolution of those adaptations (including complex organs) that require more time than is available during the life span of a single species. Again, it must be emphasized that there are too few authenticated cases of species selection to build a strong case for or against its role in evolution.
Units of Selectivity. To the extent that selective extinction occurs, it may operate at several alternative hierarchical levels. In the dinosaur extinction, all species of two major orders (Saurischia and Ornithischia) were eliminated. Presumably these species had something in common that made them all susceptible to the environmental stresses of the terminal Cretaceous. Selectivity at this high level is the only way by which highly diverse groups of species (classes or orders) have a measurable probability of being eliminated completely. It can be shown easily that if the determinants of extinction were at the species level, independent of membership in a larger group, diverse clades would never die out (22). But the fossil record contains ample instances of large, successful clades going extinct, either gradually (e.g., trilobites) or suddenly (e.g., ammonites).
Summary. Extinction is evidently selective at certain times and places but the effects tend to be subtle and require careful analysis of large data bases. Darwin's contention that all extinction is selective cannot be sustained, although this may reflect only our inability to recognize complex patterns in an imperfect fossil record.
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