HUMAN BIOLOGY

prinCiples of evolution 445

hoW do fossils and biogeography contribute

to our knoWledge of evolution?

• Fossils and biogeography both provide evidence of evolution.


• The completeness of the fossil record varies, depending on
  the kinds of organisms represented, where they lived, and how
  stable their burial sites have been.


• Biogeographical patterns can provide clues to where a species
  arose. Along with evidence from plate tectonics, the patterns
  also may shed light on the routes by which some groups of
  organisms spread to new areas.

taKe-Home message

biogeography The study

of the world distribution of

plants and animals.

radiometric dating

Method that uses the rate

of decay of an isotope to

determine the age of rock

samples.

of Earth’s crust (Figure 23.6A). From

studying evidence of such move-

ments, we know that early in our

planet’s history all present-day con-

tinents, including Africa and South

America, were parts of a massive

“supercontinent” called Pangea

(Figure  23.6B). By determining the

locations of plates at different times

in Earth’s history, researchers can shed light on possible

dispersal routes for some groups of organisms and when

(in geological history) the movements took place.

How do we know a fossil’s age? Sedimentary rocks that
contain fossils are dated by way of their position relative
to nearby volcanic rocks. The age of the volcanic rocks is
determined by radiometric dating. This method tracks
the radioactive decay of an isotope of some element that
had been trapped inside the rock when the rock formed.
Like the ticking of a perfect clock, the decay rate is con-
stant. Radiometric dating is about 90 percent accurate.

Biogeography provides other clues about
ancient evolutionary events

Biogeography—the study of the world distri bution of
plants and animals—also can shed light on past evolution-
ary changes. Biogeography asks why certain species (and
higher groupings) occur where they do. For example, why
do Australia, Tasmania, and New Guinea have species of
monotremes (egg-laying mammals such as the duckbill
platypus), while such animals are absent from other parts
of the world where the living conditions are similar? And
why do the tropics have the greatest diversity of life forms?
The simplest explanation for such biogeographical pat-
terns is that species occur where they do either because
they evolved there from ancestral species or because they
dispersed there from elsewhere.
Charles Darwin probably would have been fascinated
to learn of modern plate tectonics, the movement of plates

PACIFIC

PLATE

AFRICAN

PLATE

SOMALI

PLATE

EURASIAN

PLATE

INDO-

AUSTRALIAN

PLATE

PHILIPPINE

PLATE

PACIFIC

PLATE

NAZCA

PLATE

SOUTH

AMERICAN

PLATE

NORTH

AMERICAN

PLATE

ANTARCTIC PLATE

COCOS

PLATE

B 420 mya 260 mya 65 mya 10 mya

Figure 23.6 Movements of Earth’s crustal

plates help explain the geographical

distribution of species. A A general view

of Earth’s crustal plates. B About 240 million

years ago (mya) Earth’s land masses were

joined in a massive supercontinent, Pangea.

By about 40 million years ago plate movements

had split Pangea into isolated land masses,

including Africa, South America, Australia,

and Eurasia. (© Cengage Learning)

A

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