Evolution What the Fossils Say and Why it Matters

The Evolution of Evolution 105

uracil). As the DNA is transcribed by tRNA, it interprets each three-letter sequence as the code
for one of the 20 amino acids (plus a few codes are used to stop the transcription of DNA).
Notice in figure 4.2 that of the 64 possible combinations of three letters, many of them specify
the same amino acid. It is usually the first two letters of the triplet that count, and the third
letter makes no difference. For example, if the first two nucleotides are cytosine and uracil, it
produces the amino acid leucine, no matter what the third letter is. Clearly, most mutations in
the third-letter position (every third nucleotide in the DNA) are invisible to natural selection
and must be neutral as a result.
From these discoveries, geneticists have come to realize that many mutations are adap-
tively neutral and continue to occur without interference from natural selection. This has
led to the discovery of the molecular clock. When molecular biologists began to compare the
DNA of closely related organisms, they found that there seemed to be a regular, predictable
amount of change in their DNA that depended only upon how long ago the two lineages had
been separated. When they calibrated their divergence points on the molecular family tree
with the fossil record, they found that they could determine how long ago various lineages
branched off, even in the absence of fossil evidence. All of this works because so much of
the genome is invisible to selection and can constantly change by random mutation without

FIGURE 4.2. The genetic code. Each protein is specified by a three-letter “triplet” codon combination of adenine,
guanine, cytosine, or uracil. Note how most amino acids can be specified by just the first two letters, and
the third letter makes no difference—it is adaptively neutral, and most mutations at this locus are silent and
nonselective.

Phenylalanine

U

Phenylalanine

Leucine

Leucine

Leucine

Leucine

Leucine

Leucine

Isoleucine

Isoleucine

Isoleucine

Methionine

Valine

Valine

Valine

Valine

Serine

C

Serine

Serine

Serine

Proline

Proline

Proline

Proline

Threonine

Threonine

Threonine

Threonine

Alanine

Alanine

Alanine

Alanine

Tyrosine

A

Tyrosine

Stop

Stop

Histidine

Histidine

Glutamine

Glutamine

Asparagine

Asparagine

Lysine

Lysine

Aspartic acid

Aspartic acid

Glutamic acid

Glutamic acid

The genetic code, which specifies by three letters in the genome (A = adenine; C = cytosine;

G = guanine; U = uracil) any one of 20 amino acids, or a stop command.

Cysteine

G

Cysteine

Stop

Tryptophan

Arginine

Arginine

Arginine

Arginine

Serine

Serine

Arginine

Arginine

Glycine

Glycine

Glycine

Glycine

U

Third base

in the codon

C A G U C A G U C A G U C A G

Second base in the codon

U

First base in

the codon

C

A

G

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