The Evolution of Evolution 107We can see the importance of these regulatory genes when something goes wrong, and
a bizarre atavistic mutant, or “evolutionary throwback” occurs. Humans still have the genes
for the long tail of our monkey ancestors, and every once in a while, the suppression of those
genes fails and a human is born with an external tail (fig. 15.9). A simple failure in the tran-
scription in the genes of a horse, and you get a horse with three toes (fig. 4.3). The side toes
are poorly developed, but they still resemble the condition of the ancestral horses, which
had two functional side toes. This experiment shows that the genes for the ancestral side toes
are not lost in modern horses, only suppressed by the regulatory genes, and when there is a
mistake in regulation, these ancient features reappear. Such freakish “horned horses” were
thought to have great powers, and Julius Caesar rode one into battle.
The most striking example was an experiment that showed that birds still have the
genes for teeth, even though no living bird has teeth. The embryonic mouth tissues of a
chick were grafted into the mouth area of a developing mouse. When the mouse grew teeth,
they were not normal mouse teeth, but conical peg-like teeth similar to those of the ear-
liest toothed birds, or the dinosaurian ancestors of birds. All it took was the removal of
the regulatory genes that a chick would normally have (by grafting tissues into a mouse)
and the long-suppressed genes for reptilian teeth carried by all birds finally emerged. Other
embryonic studies have managed to change the genes that code for the birds’ short, stumpy,
bony tail, resulting in the development of a long bony tail like a dinosaur. Another genetic
modification experiment in developing chickens gave rise to chickens with dinosaur-like
feet, not bird feet. Yet another experiment produced a bird with a dinosaurian snout with
teeth instead of a normal beak. Birds have nearly all their old dinosaurian genes residing in
their genome, just not expressed.
Macroevolution and Evo/Devo
You have loaded yourself with an unnecessary difficulty in adopting Natura non facit
saltum [Nature does not make leaps] so unreservedly.
—Thomas Henry Huxley, in an 1859 letter to Charles DarwinThe importance of regulatory genes goes far beyond neutralism and junk DNA. It raises the
question again of whether microevolution, which is so successful at making small changes
(such as the number of bristles or wing veins in a fruit fly or the length of the beak of a
Galapagos finch) is sufficient to explain macroevolution (the development of large-scale
changes in evolution, such as new body plans). If you just keep accumulating tiny microevo-
lutionary changes through time, would this produce wholly novel organisms?
This debate goes back to the earliest days of evolutionary biology. Darwin was a con-
vinced gradualist, but his friend and defender Huxley warned him (in the quotation at the
start of this section) that he need not tie his evolutionary ideas to gradualism or rule out evo-
lutionary “leaps” to new body forms. When neo-Darwinism became dominant in the 1940s
and 1950s, Richard Goldschmidt, a German-born geneticist at Berkeley, protested the strict
gradualist position. He argued from his studies of gypsy moths that the changes required
to build new body plans and new species were not the same as those he found within the
normal variation within a species. Goldschmidt argued that some sort of large-scale genetic
change was needed (a “systemic mutation” in his words) to jar species out of their normal