Evolution, 4th Edition

492 CHAPTER 19

of webspinners (Embioptera), but 350,000 beetles (Coleoptera), with possibly a

million others awaiting description [27]. The orders of mammals range from the

single species of aardvark to more than 2280 species of rodents (FIGURE 19.1).

These contrasts raise a host of questions. Has the number of species on Earth

increased steadily since the origin of life? Do we happen to live when diversity is

at its highest point ever, or has diversity fluctuated? Have some groups dwindled

even as others have increased, and if so, why? Why are there so many more kinds

of beetles than webspinners—or almost anything else? Do groups with more spe-

cies produce new species at a higher rate, or are they more resistant to extinc-

tion? Is there any limit to the possible number of species, and has that limit been

reached?

Biodiversity can be studied from the complementary perspectives of ecology

and evolutionary history. Ecologists focus primarily on factors that operate over

short time scales to influence diversity within local habitats or regions. But factors

that operate on longer time scales also affect diversity. On a scale of millions of

years, extinction, adaptation, speciation, climate change, and geological change

create the potential for entirely new assemblages of species. Understanding factors

that have altered biodiversity in the past may help us predict how diversity will be

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_1901.ai Date 11-02-2016

(B)

(C)

(A)

(D)

(E)

(F)

FIGURE 19.1 Contrasts in species richness. (A) The single species of Ginkgoaceae (Gink-

go biloba) and (B) one of the more than 18,000 species of Orchidaceae (Ophrys apifera).

(C) The webspinners, order Embioptera, are far less diverse than (D) the beetles, order

Coleoptera (here Trachelophorus giraffa). (E) The order Tubulidentata has a single living

member, the African aardvark (Orycteropus afer). (F) The order Rodentia includes more

than 2280 species, among them this greater Egyptian jerboa, Jaculus orientalis.

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