Evolution, 4th Edition

T HE EvoluTion of BiologiCAl DivERsiTy 497

What might account for these declines? Our first thought might be that species

become better adapted by natural selection over time, and so should become less

vulnerable to changing environments. But natural selection, having no foresight,

cannot prepare species for novel changes in the environment. If the environmen-

tal changes that threaten extinction are numerous in kind, we should not expect

much carryover of “extinction resistance” from one change to the next. So extinc-

tion rates should vary randomly over time if changes in the environment occur at

random.

Two other hypotheses have been suggested for the decline in extinction rates.

The average number of species per genus and per family seems to be greater in

the Cenozoic than in earlier eras. This would lower the probability of extinction

of higher taxa because a genus or family does not become extinct until all its con-

stituent species are extinct [1, 19]. Another explanation is that some clades are more

volatile than others: they have a higher turnover rate, evolving new families and los-

ing others before the entire clade becomes extinct. The extinction of such taxa leaves

the less volatile taxa, those that have longer life spans and lower extinction rates.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_19.06.ai Date 12-09-2016

Q: I left in balloon text as this seemed helpful. OK?

Pelagic

Semi-

infaunal

Deep

infaunal

Shallow

infaunal

Cambrian

Tiering

Feeding

Erect

Surcial

Suspension

Surface deposit

Mining

Predatory

Grazing

Absorptive

Other

Suspension

Surface deposit

Mining

Predatory

Grazing

Absorptive

Other

Holocene

Feeding Motility

Nonmotile, unattachedNonmotile, attached

Facultative, unattachedFacultative, attached

Fully motile, slow

Fully motile, fast

Each “bin” in the ecospace

represents a way of life.

Many more bins are occupied

now than in the Cambrian.

FIGURE 19.6 The use of “ecospace” by marine animals in the Cambrian compared

with the present. Each layer represents the vertical space used by animals, from open

ocean (pelagic) to deep in the sediment (deep infaunal). In each layer, the “bins” from

left to right represent different modes of feeding, and those from front to rear rep-

resent different habits of movement (motility). Far more bins, or ways of life, are filled

(indicated by green) by animals now than in the Cambrian. (After [9].)

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