Evolution, 4th Edition

362 CHAPTER 14

Genomes large and small

The staggering diversity seen across the tree of life extends all the way down to

the level of the genome. The record for the smallest known genome is held by a

DNA virus (porcine circovirus type 1), with only 1759 bp that code for just three

proteins. The smallest genome for a bacterium is an insect symbiont called Nasuia

deltocephalinicola, with just 137 coding genes and about 112,000 bp [6]. At the other

end of the spectrum, the prize for the largest genome is held by the amoeba Chaos

chaos that you met in this chapter’s introduction—it has 400 times more DNA

than we do [18]. There is substantial variation in a genome’s DNA content even

within some species: it differs by up to 30 percent in Drosophila melanogaster [9].

The largest number of genes in any organism so far known is bread wheat, with

about 100,000 genes, which is five times more than in Homo sapiens. Our species,

with 3.2 billion DNA bases and about 20,000 protein-coding genes, has neither an

unusually large nor small genome for a multicellular organism.

These numbers bring into bright relief an apparent disparity between genome

size and any subjective measure of organismal complexity (for example, number

of types of tissues). Patterns emerge, however, if we divide organisms into major

groups of life and then plot the amount of DNA devoted to coding for proteins

against the genome’s total size (FIGURE 14.18). Across viruses and prokaryotes,

there is nearly a one-to-one relation. In animals and plants, however, the correla-

tion is much weaker: species with similar amounts of coding sequence can differ

dramatically in the sizes of their genomes.

These patterns begin to make sense with a single key insight. The genomes of

most bacteria and viruses consist almost or entirely of coding sequences. In ani-

mals and plants, by contrast, most DNA does not code for any protein. The amount

of noncoding DNA varies tremendously among species and explains much of the

scatter in the genome size of metazoans. The different types of DNA in the human

genome are seen in Figure 14.15. Less than 2 percent codes for proteins. About 25

times more—fully half of the genome—is taken up by the DNA parasites that we

will now discuss.

Genetic parasites and transposable elements

Transposable elements (TEs), also called transposons, are short sequences of DNA

that occur in many copies in the genome. The most numerous TE in the human Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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100

10

1

0.1

0.01

1 10 100 1000

1%

10%

100%

0.01 0.1

Genome size (Mb)

Coding sequences (Mb)

Bacteriophage

Eukaryote

DNA viruses

Prokaryotes

Unicellular

eukaryotes

Animals

Land

plants

FIGURE 14.18 The relation across the tree of life between a

genome’s total DNA content and the amount of its sequence

that codes for proteins. The three dashed lines indicate the

percentage of the genome devoted to coding sequences.

Viruses and prokaryotes have little or no noncoding DNA, so

nearly 100 percent of their genome is coding sequence. In

contrast, the genomes of most animals and plants are made

up largely of noncoding DNA, which varies greatly in quantity.

In some species, less than 1 percent of the genome is coding

sequence. Unicellular eukaryotes show an intermediate pat-

tern. (After [43].)

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