Evolution, 4th Edition

80 CHAPTER 4

is neither exceptionally big or small. Some bacteria have genomes that are thou-

sands of times smaller, with less than 200,000 bp. At the other extreme are some

plants, salamanders, and protozoa that have genomes that are hundreds of times

larger than ours. A surprising observation is that there is little correlation between

the complexity of an organism and the size of its genome. We’ll return to this puz-

zling fact in C hapter 14.

Some viruses use RNA (ribonucleic acid) rather than DNA for their genetic

material. To replicate themselves, these viruses convert their genome into DNA by

a process called reverse transcription. This DNA is then inserted into the genome

of the host cell that the virus has infected, and offspring viruses are made using

that cell’s biochemical machinery. Thus, despite the difference in the genetic mate-

rial of RNA- and DNA-based life forms, they share much of the apparatus that

expresses their genes.

An organism’s genetic material is carried by one or more chromosomes. Chro-

mosomes in eukaryotes are long strings of DNA bases bound together with pro-

teins. In diploid species such as humans, chromosomes come in pairs, one inher-

ited from each parent. In prokaryotes, chromosomes are unpaired (haploid). Genes

are segments of chromosomes that perform a function. Many code for proteins that

comprise tissues and catalyze reactions. A smaller number have other functions,

for example coding for the RNA of ribosomes and the microRNAs that are impor-

tant to gene regulation. The human genome has roughly 20,000 protein-coding

genes, some plants and fish have many more, and some bacteria have hundreds of

times fewer.

To make a protein, the cellular machinery reads a gene’s DNA in sets of three

bases. These sets, called codons, represent the amino acids that make up the pro-

tein. The genetic code is a set of rules that relates the codons to the amino acids

they represent (FIGURE 4.2). A profound and wonderful fact is that the genetic

code is shared by virtually all life on Earth, from viruses to bacteria to pineapples

to humans. This is powerful evidence that all life evolved from a single common

ancestor.

Since there are four types of DNA bases, there are 4 × 4 × 4 = 64 different

codons. But because there are only 20 types of amino acids, most amino acids are

represented by more than one codon. For example, the codons CCT, CCC, CCA,

and CCG all specify the amino acid proline. Changes to a codon that do not alter an

amino acid, for example from GAG to GAA, are called synonymous (or “silent”). In

contrast, changes to a codon that do alter an amino acid, such as GAG to GTG, are

called nonsynonymous (or “replacement”). The contrast between these two types

of changes is illustrated with the β-hemoglobin gene from humans in FIGURE 4.3.

We’ll see shortly that the nonsynonymous change in β-hemoglobin leads to inter-

esting evolutionary consequences.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_0401.ai Date 11-02-2016

FIGURE 4.1 The strawberry poison dart

frog (Dendrobates pumilio) has conspicu-

ous coloration that warns predators it is toxic.

Why this species is so variable, however,

is not understood. Central questions in

evolutionary biology include what maintains

variation, and how variation is shaped by se-

lection and other evolutionary factors. (Frog

top views from [25b].)

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