Evolution, 4th Edition

THE GEoGRAPHy of EvoluTion 475

habitat. The separated populations diverge, and they often become different subspe-

cies, species, or higher taxa. For example, in many fishes, shrimps, and other marine

animal groups, the closest relative of a species on the Pacific side of the Isthmus of

Panama is a species on the Caribbean side of the isthmus. Each pair of species has

descended from a broadly distributed ancestral species that was sundered by the rise

of the isthmus during the Pliocene, about 3 Mya (FIGURE 18.7) [22, 23, 30]. Vicari-

ance sometimes accounts for the presence of related taxa in disjunct areas.

In many cases, dispersal, vicariance, and extinction together explain distribu-

tions. For example, during the Pleistocene glaciations, species shifted their ranges

by dispersing into new regions. Some northern, cold-adapted species became dis-

tributed far to the south. When the climate became warmer, these species recolo-

nized northern regions, and southern populations became extinct, except for popu-

lations of some such species that survived on cold mountaintops (FIGURE 18.8).

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A. nuttingi A. millsae

FIGURE 18.7 The snapping shrimps Alpheus nuttingi,

found on the Atlantic side of the Isthmus of Panama, and

A. millsae, found on the Pacific side, are sister species

that evolved from a common ancestor that became di-

vided into two populations as the isthmus formed. Their

geographic distributions illustrate vicariance. (Courtesy of

Arthur Anker.)

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Range of

S. cernua

FIGURE 18.8 The disjunct

distribution of a saxifrage (Saxi-

fraga cernua) in northern and

mountainous regions of the

Northern Hemisphere. Relict

populations have persisted

at high elevations following

the species’ retreat from the

southern regions that it oc-

cupied during glacial periods.

(After [2]; photo courtesy of Egil

Michaelsen and the Norwegian

Botanical Association.)

18_EVOL4E_CH18.indd 475 3/22/17 1:39 PM