Evolution What the Fossils Say and Why it Matters

106 Evolution and the Fossil Record

interference. Although the molecular clock has had some great successes, mutation rates can
vary unpredictably, so scientists are cautious about putting too much weight on molecular
clock estimates for the age of a lineage when all the other evidence disagrees.
More importantly, the fact that 80–97 percent of the DNA in most organisms codes for
nothing at all (so far as we know) says that evolution and selection must work entirely on
the remaining few percent of the DNA that does code for something. Those remaining genes
are known as regulatory genes. They are the master switches that control the reading of the
rest of the DNA, some of which is used to make the basic structures of life (structural genes)
and therefore does not differ between organisms. So from the assertion in the 1950s that
every gene codes for one protein, we now know that most genes don’t code for anything,
and only a few regulatory genes exert almost complete control over every other gene in the
DNA. By tiny changes in those “switches” or regulatory genes, the organism can make big
evolutionary leaps.

FIGURE 4.3. (A) A famous example of a rare mutant horse that has three toes, rather than one. (B) Bone structure
of the feet of the mutant horses. On the left is a normal horse foot; in the middle is an extra toe formed by du-
plicating the central toe; on the right is an extra toe formed by enlarging the reduced side toes (splint bones),
which were functioning side toes in earlier horses. (From Marsh 1892)

(A) (B)

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