Evolution, 4th Edition

82 CHAPTER 4

Like the genetic code, the cellular machinery for transcribing DNA and trans-

lating mRNA is almost universal across life on Earth. The DNA or mRNA from a

sea urchin is translated into a protein if it is injected into a bacterium. This univer-

sality is the basis of genetic engineering. The “golden rice” strain was developed by

inserting a bacterial gene that allows the plant to synthesize a precursor to vitamin

A [26]. A deficiency of that vitamin kills 2 million people each year, many of whom

live in countries where rice is a major part of the diet. Golden rice has the potential

to relieve much of that suffering. This advance is only possible because rice and

bacteria share genetic machinery that they both inherited from a common ancestor

that lived more than 1.5 billion years ago (see Chapter 2).

Genes make up only a small part of the genome in eukaryotes. In humans, for

example, 98 percent of the DNA does not code for any gene product. A small frac-

tion of this noncoding DNA affects how coding genes are expressed. The vast bulk

of noncoding DNA, however, does not have an obvious function. The genomes of

prokaryotes are very different, and typically only about 20 percent of their genome

is noncoding. C hapter 14 will return to the fascinating observation that so much of

the eukaryotic genome is noncoding DNA.

The Inheritance of Variation

The variation we see among individuals of a species (such as our own) are differ-

ences in phenotypes, or observable characteristics. Natural selection acts on phe-

notypes, but that process only results in evolution if at least some of the variation

in phenotypes is transmitted between generations. We resemble our parents more

than we do passing strangers on the street. The familiar patterns of inheritance

are the result of differences in the genotypes, encoded by DNA. To understand

how evolution works, it is crucial to understand how this remarkable mechanism

of inheritance works.

The basic unit of genetic inheritance is a locus (plural: loci), which is the more

formal term for what we sometimes call a gene. A locus is a section of chromosome,

often one that produces a gene product such as a protein. The DNA sequence at a

given locus often varies among the chromosomes carried by different individuals,

and if it does we say that the locus is polymorphic.

The different variants at a locus are called alleles. A specific DNA base in the

genome that varies among individuals is called a single nucleotide polymorphism,

or SNP (pronounced “snip”). The allele frequency tells us how often a variant

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

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Evolution4e_04.04.ai Date 12-20-2016 01-12-17

DNA 5 ʹ A 3 ʹ

Pre-mRNA

mRNA 1 mRNA 2

Transcription

Alternative splicing

Translation

Protein 1 Protein 2

Promoter Exon

B

Exon

C

A B

A B

C

A C

FIGURE 4.4 In the first step of assembling Intron Intron Exon

a protein, a gene’s DNA sequence is tran-

scribed into pre-mRNA. This molecule is

spliced to remove the introns (and often

some of the exons) to produce mature

mRNA. Many pre-mRNAs are spliced in more

than one way, yielding different mRNAs. The

mRNA is then translated into the protein.

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