Evolution, 4th Edition

T HE EvoluTion of BiologiCAl DivERsiTy 499

they require specific environments and so may have patchier dis-

tributions. Second, species with broad geographic ranges tend to

have a lower risk of extinction because they are not extinguished

by local environmental changes [24]. They also have lower rates

of speciation [34], probably because they have a high capacity for

dispersal and perhaps broader environmental tolerances.

Major groups characteristically differ in how long species

persist before they become extinct. For example, the average

duration of a species of bivalve (clams and relatives) is 23 My,

whereas it is only 10 My for a gastropod (snails and relatives)

and 7 My for a sea urchin [43, 69]. Leigh Van Valen wondered

if within any such group, older species might be better adapted

than young ones (because they had had more times to adapt)

and less prone to extinction [73]. The result would be a declin-

ing extinction rate [41]. This can be determined by plotting the

fraction of taxa (e.g., the fraction of genera in a family) that

survive for different lengths of time. This approach is different

from asking whether or not extinction rates have changed over

the course of geological time (e.g., whether they were lower in

the Jurassic than in the Devonian). If new taxa have the same

probability of extinction as older ones, then the proportion of

component taxa surviving to increasingly greater ages should

decline exponentially. Plotted logarithmically, the survivorship

curve would become a straight line. If taxa become increasingly

resistant to causes of extinction as they age, the logarithmic plot

should be upwardly concave, with a long tail (FIGURE 19.9A).

When Van Valen plotted taxon survivorship in this way,

he found a surprising result: rather straight curves, implying that the probabil-

ity of extinction is roughly constant (FIGURE 19.9B). There was no evidence that

these animals became more resistant to extinction over time. Instead, this is what

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Origination probability

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Extinction probability

0.1

0.2

0.3

0.4

0.5

0.6

0.0

Living orders Extinct orders

FIGURE 19.8 Groups of marine organisms vary in volatility.

The rate (probability) of origination of new families within

an order, per time interval, is correlated with the rate (prob-

ability) of extinction of families. More volatile orders, with

higher rates, have higher turnover, and so are more likely to

decrease greatly or become extinct, as the trilobites (upper

drawing) did. Most living orders, such as orders of gastropods

(lower drawing), have low rates of origination and extinction

compared with extinct orders (After [25].)

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Evolution4e_19.09.ai Date 02-02-2017

1000

Age of taxon Age of taxon (My)

Number of surviving taxa

(A) Hypothetical curves

10

1

100

0 50 100 150

1000

Number of surviving taxa

(B) Ammonoids

10

1

100

Families

Genera

Age of taxon (My)

0 50

Genera surviving (%)

(C) Crinoids

10

100

Camerates

Non-pinnulates

Constant

extinction rate

Decreasing

extinction rate

FIGURE 19.9 Taxonomic survivorship curves. Each curve or

series of points represents the number of taxa that persisted in

the fossil record for a given duration, irrespective of when they

originated during geological time. (A) Hypothetical survivorship

curves. In a semilogarithmic plot, the curve is linear if the prob-

ability of extinction is constant. It is concave if the probability

of extinction declines as a taxon ages, as it might if adaptation

lowered the long-term probability of extinction. (B) Taxonomic

survivorship curves for families and genera of ammonoids. The

plot for families suggests an extinction rate that is constant with

age, whereas the plot for genera suggests that older survivors

might have a lower rate of extinction. (C) Taxonomic survivorship

curves for two groups of Paleozoic crinoids (sea lilies), sessile

echinoderms that filter plankton with branched arms. Older

genera in these two groups, which differ in their filter structure,

have lower extinction rates. (B after [73]; C after [6].)

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