Evolution, 4th Edition

The people around you differ in the color of their hair, the shape of their

noses, the length of their fingers, and in countless other ways. If you knew

their DNA sequences, you would also see that they differ from you at millions

of places in their genomes. Variation among individuals is universal across all

species on Earth (FIGURE 4.1).

Unlike physics and chemistry, evolution depends on variation. All electrons

are identical, but no two living organisms are. Without that variation, evolution—

and so life itself—would not be possible.

Understanding how traits and genes vary and how this variation is inherited is

fundamental to understanding evolution. Genetics also provides us with a vast

trove of information about the history of life on Earth and about the evolutionary

factors acting on living species. Before delving into those topics, this chapter

starts with a short review of key concepts in genetics that should be familiar to

you from earlier courses in biology.

The machinery of Inheritance

The genetic material of almost all organisms on Earth is DNA (deoxyribonucleic

acid). This is a very long molecule made up of pairs of bases. Each base takes

one of four forms: adenine (A), guanine (G), cytosine (C), or thymine (T). In each

pair of bases, an A is matched with a T, or a C is matched with a G. The aver-

age chromosome in humans has more than 100 million base pairs (abbreviated

bp), and the entire human genome consists of 3.2 billion bp. Our genome

Mutation rates

Mutation and Variation

4

The zigzag nerite (Neritina communis), a marine snail that lives among

mangroves in the Western Pacific, has extremely variable shells.

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