Evolution__3rd_Edition

..
CHAPTER 23 / Extinction and Radiation 667
Table 23.1
Survival of different kinds of snail taxa through the Cretaceous–Tertiary mass extinction and at other times (showing a “background”
extinction pattern). Background extinction rates vary with snail type, whereas survival through the mass extinction may have been a
matter of luck. (a) Relation between the chance of generic extinction and developmental mode. The background rate of extinction is
lower for snails with planktonic, than with direct, development (this evidence is the same as in Figure 23.8), but in mass extinctions
snail genera of the two types have the same chance of surviving. (b) Relation between the chance of generic extinction and number of
species in the genus. The background extinction rate is lower for genera that are species rich (contain three or more species) than for
genera that are species poor (contain one or two species). But in mass extinctions a species-rich genus had about the same chance of
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
although the genera studied at the two times are not all the same. From Jablonski (1986).
(a) Extinction rate and mode of development.
Background extinctions Mass extinction
Median geological
Mode of development n longevity (Myr) n Genera surviving (%) Genera extinct (%)
Planktonic development 50 6 28 60 40
Direct development 50 2 21 60 40
(b) Extinction rate and species richness of genus.
Background extinctions Mass extinction
Median geological
Species richness n longevity (Myr) Genera surviving (n) Genera extinct (n)
Species poor 145 32 31 38
Species rich 114 49 22 25
. . . changed during a mass
extinction
and species-poor taxa. In contrast, in the Cretaceous–Tertiary mass extinction the
difference disappeared. The conditions had altered and the form of species selection
altered too.
In Jablonski’s research, the extinction pattern through a mass extinction became
less selective a perhaps because the extinction was so massive that almost all snail
species succumbed regardless of their adaptations. But in other periods, or other
taxa, the extinction patterns remained selective during mass extinctions. The form of
the selectivity can even provide clues about the nature of the extinction event. For
instance, a major (if not a mass) extinction occurred at the Oligocene–Eocene boundary
(Figures 23.2 and 23.5). The extinction is widely thought to have been caused by
global cooling. Evidence suggests that species with adaptations to warm temperatures
were more likely to go extinct at that time than species adapted to cool temperatures.
The selective pattern of the extinctions fits with the climatic explanation.