Evolution, 4th Edition

T HE EvoluTion of BiologiCAl DivERsiTy 507

an increasing rate of extinction (E), or both. Several methods have been proposed to

estimate S and E, but this is a very difficult problem because the phylogeny of living

species does not reflect the past existence of species that were not ancestors of living

species [47]. For example, the fossil record shows that cetaceans (whales and dol-

phins) were much more diverse in the Miocene than they are today, and have suf-

fered more extinction that can be inferred from the phylogeny of living species [53].

Many clades show a declining rate of lineage accumulation; relatively few show

a pattern of increasing diversification [44]. Mathematical analysis and computer

simulations suggest that this pattern is likely to be caused by a decreasing rate of

speciation, not by an increasing extinction rate [55]. This pattern strongly suggests

that diversification has been diversity-dependent, but we will see that this is a

matter of some controversy. And it is likely that most new species become extinct

soon after, or even during, the lengthy process of speciation, so the roles of specia-

tion and extinction may be hard to distinguish, even conceptually [61].

Does species Diversity Reach Equilibrium?

A huge ecological literature is concerned with whether or not the number of coex-

isting species (of some group such as plants or mammals) tends toward an equi-

librium. This question is complex and not entirely resolved, but ecologists agree

that some factors tend to limit species diversity. The space that plants compete for

and the energy fluxes that organisms depend on are finite, so they can be divided

among a limited number of species populations that are

still large enough to persist. At a local level, the num-

ber of species is sometimes directly correlated with the

number in a larger region—a pool of species, of which

only a sample is found at any one place. This pattern

suggests that the number of coexisting species is limited

only by the number available to colonize a local site [11].

If the species richness in local assemblages shows little

variation despite access to more diverse species pools,

some limiting factor, such as competition for resources,

is likely to place an upper bound on the number of coex-

isting species. Both patterns have been found in differ-

ent situations. Phenomena such as competitive exclusion

of species from each other’s ranges suggest that interac-

tions among species can limit local species diversity.

The ecological factors that determine the number

of locally coexisting species may differ from those that

determine the number of species in a clade or taxonomic

group. Researchers differ as to whether or not the diver-

sity of most clades has approached limits set by compe-

tition or other diversity-dependent factors (see [29] vs.

[54]). Paleontologists have found some evidence that the

per taxon rate of increase in the number of species (or

higher taxa) is diversity-dependent: it decreases as the

number grows. For example, Michael Foote calculated

the rates of origination (S), extinction (E), and diversifi-

cation (D) of marine genera from one stratigraphic stage

to the next, then correlated these short-term changes

with the number of genera present (N) at the beginning

of the stage (FIGURE 19.18) [21]. Both the diversification

rate (D = S – E) and the origination rate (S) declined as

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_19.18.ai Date 02-02-2017

–150 –100 –50 0 50 100 150

–1.0

–0.5

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Diversication rate

P/Tr

O/S

K/Pg

Tr/J

D/C

Diversity

FIGURE 19.18 The per lineage rate of diversification of skeletonized

marine invertebrate genera during the Phanerozoic (i.e., since the

start of the Cambrian) is diversity-dependent. Each point plots the rate

of change during a stratigraphic stage against the diversity of taxa at

the start of that stage. The higher the diversity, the lower the rate of

diversification. Further analysis showed that this pattern is attributable to

the reduced rate at which new genera arise. The pattern suggests that

higher diversity imposes stronger competition and prevents new gen-

era from evolving. For statistical reasons, the points are shown on scales

that are centered at zero. Points representing mass extinction events are

labeled (O/S, end-Ordovician; D/C, late Devonian; P/Tr, end-Permian;

Tr/J, end-Triassic; K/Pg, Cretaceous/Paleogene boundary). (After [21].)

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