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Evolution__3rd_Edition

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CHAPTER 23 / Extinction and Radiation 667<br />

Table 23.1<br />

Survival of different kinds of snail taxa through the Cretaceous–Tertiary mass extinction and at other times (showing a “background”<br />

extinction pattern). Background extinction rates vary with snail type, whereas survival through the mass extinction may have been a<br />

matter of luck. (a) Relation between the chance of generic extinction and developmental mode. The background rate of extinction is<br />

lower for snails with planktonic, than with direct, development (this evidence is the same as in Figure 23.8), but in mass extinctions<br />

snail genera of the two types have the same chance of surviving. (b) Relation between the chance of generic extinction and number of<br />

species in the genus. The background extinction rate is lower for genera that are species rich (contain three or more species) than for<br />

genera that are species poor (contain one or two species). But in mass extinctions a species-rich genus had about the same chance of<br />

going extinct as a species-poor genus; in both cases about 40% of genera went extinct and about 60% survived. n is number of genera<br />

although the genera studied at the two times are not all the same. From Jablonski (1986).<br />

(a) Extinction rate and mode of development.<br />

Background extinctions Mass extinction<br />

Median geological<br />

Mode of development n longevity (Myr) n Genera surviving (%) Genera extinct (%)<br />

Planktonic development 50 6 28 60 40<br />

Direct development 50 2 21 60 40<br />

(b) Extinction rate and species richness of genus.<br />

Background extinctions Mass extinction<br />

Median geological<br />

Species richness n longevity (Myr) Genera surviving (n) Genera extinct (n)<br />

Species poor 145 32 31 38<br />

Species rich 114 49 22 25<br />

. . . changed during a mass<br />

extinction<br />

and species-poor taxa. In contrast, in the Cretaceous–Tertiary mass extinction the<br />

difference disappeared. The conditions had altered and the form of species selection<br />

altered too.<br />

In Jablonski’s research, the extinction pattern through a mass extinction became<br />

less selective a perhaps because the extinction was so massive that almost all snail<br />

species succumbed regardless of their adaptations. But in other periods, or other<br />

taxa, the extinction patterns remained selective during mass extinctions. The form of<br />

the selectivity can even provide clues about the nature of the extinction event. For<br />

instance, a major (if not a mass) extinction occurred at the Oligocene–Eocene boundary<br />

(Figures 23.2 and 23.5). The extinction is widely thought to have been caused by<br />

global cooling. Evidence suggests that species with adaptations to warm temperatures<br />

were more likely to go extinct at that time than species adapted to cool temperatures.<br />

The selective pattern of the extinctions fits with the climatic explanation.

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