Biology Now, 2e

236 ■ CHAPTER 13 Adaptation and Species

EVOLUTION

such as a river, a canyon, or a mountain chain,

isolates two populations of a single species

(Figure 13.6). Such geographic isolation can

also occur when a few members of a species colo-

nize a region that is difficult to reach, such as an

island located far outside the usual geographic

range of the species.

Geographically isolated populations are

disconnected genetically; there is little or no

gene flow between them. Without gene flow, the

other mechanisms of evolution we saw in Chap-

ter 12—mutation, genetic drift, and natural

selection—can more easily cause populations to

diverge from one another. If populations remain

isolated long enough, they can evolve into new

species. The formation of new species from

geographically isolated populations is called

allopatric speciation (allo, “other”; patric,

“country”), as shown in Figure 13.7.

But what happens when there isn’t a phys-

ical barrier between two populations? In the

ocean, for example, plants and animals can drift

The Kaibab

squirrel is confined

to the North Rim of

the Grand Canyon.

Abert’s squirrel

lives on the

South Rim and

other southern

locations, all the

way into Mexico

Figure 13.6

The Grand Canyon is a geographic barrier for squirrels

The Kaibab squirrel population became isolated from the Abert’s squirrel population when the

Colorado River cut the Grand Canyon—which is as deep as 6,000 feet in some places. With gene flow

between them blocked, probably beginning about 5 million years ago, the two populations eventually

accumulated enough genetic differences to become two distinct species.

Q1: What is the definition of gene flow? How was gene flow blocked between these species?

Q2: Name as many types of geographic barriers as you can. Which do you think would be the

best at blocking gene flow?

Q3: Are geographic barriers universal for all species? If not, name a geographic barrier that

might block gene flow for one species but not another.

The lizards that adapted to the island of Pod

Mrcˇaru are physically different enough from

their cousins on Pod Kopište to be considered

a different species, says Herrel, yet their DNA

sequences are close to identical, so he believes

they might still be able to interbreed. Speciation

is the process by which one species splits to form

two species or more. Fundamentally, speciation

occurs because of genetic divergence, the accu-

mulation of differences in the DNA sequences of

genes in two or more populations of organisms

over time; as a result, the populations become

more and more genetically dissimilar.

The two populations of lizards may continue

to diverge, becoming more and more different

because of their geographic isolation from one

another, and may eventually become two unar-

guably distinct species. This is one of the most

common ways that new species form: individu-

als of a single population become geographically

separated from one another. This process can

begin when a newly formed geographic barrier,