Evolution, 4th Edition

472 CHAPTER 18

that are still widely recognized today (FIGURE 18.2). These realms are the Palearctic

(Eurasia and northern Africa), the Nearctic (North America), the Neotropical (Cen-

tral and South America), the Ethiopian (sub-Saharan Africa), the Oriental (India and

Southeast Asia), and the Australian (Australia, New Guinea, New Zealand, and

nearby islands). These realms are more the result of Earth’s history than of its current

climate or land mass distribution. For example, Wallace’s line separates islands that,

despite their close proximity and similar climate, differ greatly in their fauna. The

islands to the west were connected to the Asian mainland during periods of low sea

level; those to the east were not.

Each biogeographic realm is inhabited by many higher taxa that are much more

diverse in that realm than elsewhere, or are even restricted to that realm. For

example, the endemic (or nearly endemic) taxa of the Neotropical realm include

the Xenarthra (anteaters and allies), platyrrhine primates (such as spider monkeys

and marmosets), most hummingbirds, a large clade of suboscine birds such as fly-

catchers and antbirds, many families of catfishes, and plant families such as the

bromeliads (FIGURE 18.3).

The borders between biogeographic realms cannot be sharply drawn because

some taxa infiltrate neighboring realms to varying degrees. In the Nearctic realm

(North America), for instance, some species are related to, and have been derived

from, Neotropical stocks: examples include an armadillo, an opossum, and Span-

ish moss (Tillandsia usneoides), a bromeliad that festoons southern trees.

Some taxa have disjunct distributions; that is, their distributions have gaps

(FIGURE 18.4). Disjunctly distributed higher taxa typically have different repre-

sentatives in each area they occupy. For example, many taxa are represented on

two or more southern continents, including lungfishes, marsupials, cichlid fishes,

and Araucaria pines. Another common disjunct pattern is illustrated by alligators

(Alligator), skunk cabbages (Symplocarpus), and tulip trees (Liriodendron), which are

among the many genera that are found both in eastern North America and in

temperate eastern Asia, but nowhere in between [43]. Understanding how taxa

became disjunctly distributed has long been a preoccupation of biogeographers.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_18.02.ai Date 12-07-2016

NEARCTIC

NEO-

TROPICAL

PALEARCTIC

ETHIOPIAN

ORIENTAL

Sahara and

Arabian deserts

Himalayas

AUSTRALIAN

Equator

Tropic of Cancer

Tropic of Capricorn

Wallace's line

FIGURE 18.2 Biogeographic

realms. The biogeographic realms

recognized by Wallace are the

Palearctic, Ethiopian, Oriental, Aus-

tralian, Nearctic, and Neotropical.

Note the position of Wallace’s line.

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