Evolution, 4th Edition

432 CHAPTER 17

As you read this chapter, you will see examples of some important patterns.

Living things have profoundly affected the atmosphere and other physical aspects

of Earth. Climates and the distribution of oceans and land masses have changed

over time, affecting the geographic distributions of organisms (as we will discuss

in Chapter 18). The taxonomic composition of the living world has changed con-

tinually as new forms have originated and others have become extinct. As new

forms of life originated, the variety of ways of living increased, and a wider variety

of habitats became occupied. In fact, the evolution of new kinds of organisms cre-

ated new habitats. At several times, extinction rates have been particularly high.

Especially after these mass extinctions, the diversification of higher taxa has some-

times been rapid. Extinct taxa have sometimes been replaced by unrelated but eco-

logically similar taxa. Of the variety of forms in a higher taxon that were present in

the remote past, usually only a few have persisted in the long term, and over time,

Earth’s biota more and more resembles the biota of today. This magnificent history

has transpired over a depth of time that we find hard to comprehend.

Some Geological Fundamentals

The rocks we find at Earth’s surface originated as molten material (magma) that

is extruded from deep within Earth. Some of this extrusion occurs via volcanoes,

but much rock originates as new crust forms at mid-oceanic ridges (FIGURE 17.1).

Rock formed in this way is called igneous rock. Sedimentary rock is formed by

the deposition and solidification of sediments, which are usually formed either by

the breakdown of older rocks or by precipitation of minerals from water. Under

high temperatures and pressures, both igneous and sedimentary rocks are altered,

forming metamorphic rocks. Most fossils are found in sedimentary rocks. A few

fossils are found in other situations; for example, insects are found in amber (fos-

silized plant resin), and some mammoths and other species have been found fro-

zen in permafrost.

The lithosphere, the solid outer layer of Earth bearing both the continents and

the crust below the oceans, consists of eight major and several minor plates that

FIGURE 17.1 Plate tectonic

processes. At a mid-oceanic

ridge, rising magma creates

new lithosphere and pushes

the existing plates to either

side. When moving lithospher-

ic plates meet, one plunges un-

der the other, frequently caus-

ing earthquakes and mountain

building. Heat generated by

this process of subduction

melts the lithosphere, causing

volcanic activity.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_17.02.ai Date 12-31-2016

Mid-oceanic ridge

Volcano

Oceanic crust

Subduction trench

Asthenosphere

Deep mantle

Lithosphere

Continental

crust

Sinking

plate

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