The Ecology Book

THE LIVING EARTH 213

Wegener’s theory was not well

received at first. In 1943, George

Gaylord Simpson, one of the most

influential paleontologists in the

US, criticized the theory. He argued

that the fossil record could be

explained by static continents linked

and unlinked by periodic flooding.

Evidence and evolution

Despite early doubts, evidence for

the plate tectonics theory grew. A

series of discoveries established that

the seafloor was spreading and that

new oceanic crust was constantly

being created. We now understand

that the movement of the tectonic

plates is driven by convection

currents carrying heat from deep

inside the planet to the surface.

Once Wegener’s theory was

accepted, the fossil evidence made

much better sense. Continental drift

has had a profound influence on how

species have evolved. For example,

if a continent splits apart, the two

separated populations of a species

can start to evolve in completely

different directions. On the other

hand, if two continents collide, or a

bridge of land forms between them,

different species begin to mix and

compete, and some may become

extinct as a result. ■

Marsupials in America and Australia

Marsupials are nonplacental

mammals whose young complete

their gestation feeding from their

mother’s teats, typically in a

pouch on the belly. Now found

only in the Americas (mainly

South and Central) and Australia,

they are thought to have evolved

in North America 100 million

years ago. They spread to South

America and diversified into many

different species.

Several groups later moved

into what is now Antarctica and

on into southern Australia. It is

thought that they traveled via a

belt of vegetation straddling the

three areas, which were once all

part of the southern landmass

called Gondwana.

By 55 million years ago,

the continents had separated,

and marsupial species began

to evolve differently. The only

known Antarctic marsupial

fossils, found on Seymour Island

in 40-million-year-old rocks,

resemble South American

marsupials of the same period,

but not those of Australia.

Marsupials are strongly identified

with Australia, yet they evolved in

America and are still also found there.

Three types of plate boundary

Divergent Convergent Transform

See also: Island biogeography 144–149 ■ The distribution of species over space and time 162–163 ■ Macroecology 185

■ Metapopulations 186–187 ■ Biogeography 200–201

Tectonic plates can move in three

different ways, forming different types

of boundary. When plates diverge, new

oceanic crust is formed. When they

converge, new mountains form. When

plates slide past each other, the rift is

known as a transform fault.

Plates move apart

Plates slide

Plates collide alongside each other

The forces which

displace continents

are the same

as those which

produce great

fold-mountain ranges.

Alfred Wegener

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