CONCEPT 4-3 85
These long-term climate changes have a major ef-
fect on biological evolution by determining where
different types of plants and animals can survive and
thrive and by changing the locations of different types
of ecosystems such as deserts, grasslands, and forests
(Concept 4-3). Some species became extinct because the
climate changed too rapidly for them to survive, and
new species evolved to fill their ecological roles.
Another force affecting natural selection has been
catastrophic events such as collisions between the
earth and large asteroids. There have probably been
many of these collisions during the earth’s 4.5 billion
Figure 4-6 Over millions of years, the earth’s continents have moved very slowly on several gigantic tectonic
plates. This process plays a role in the extinction of species, as land areas split apart, and also in the rise of new
species when isolated land areas combine. Rock and fossil evidence indicates that 200–250 million years ago, all
of the earth’s present-day continents were locked together in a supercontinent called Pangaea (top left). About
180 million years ago, Pangaea began splitting apart as the earth’s tectonic plates separated, eventually resulting
in today’s locations of the continents (bottom right). Question: How might an area of land splitting apart cause
the extinction of a species?
Figure 4-7 Changes in ice coverage
in the northern hemisphere during the
past 18,000 years. Question: What
are two characteristics of an animal
and two characteristics of a plant that
natural selection would have favored
as these ice sheets (left) advanced?
(Data from the National Oceanic and
Atmospheric Administration)18,000
years before
presentLegend
Continental ice
Sea ice
Land above sea levelModern day
(August)Northern Hemisphere
Ice coverageyears. Such impacts have caused widespread destruc-
tion of eco systems and wiped out large numbers of
species. But they have also caused shifts in the loca-
tions of ecosystems and created opportunities for the
evolution of new species. On a long-term ba-
sis, the four scientific principles of sustainability
(see back cover), especially biodiversity (Fig-
ure 4-2) have enabled life on earth to adapt to drastic
changes in environmental conditions (Science Focus,
p. 86). In other words, we live on a habitable planet.
(See The Habitable Planet, Video 1, http://www.learner.org/
resources/series209.html.)120 ° 80 ° 40 ° 80 ° 120 °225 million years agoP
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