The Evolution of Evolution 109The most important recent development, however, has been the discovery of the master
regulatory genes, known as the homeotic genes (especially the “Hox” genes). These genes
are found in nearly all multicellular organisms and regulate the fundamental development
of the body plan and how major organ systems develop. They were first discovered with
experiments on fruit flies that had unusual mutations. Some had legs growing on their heads
instead of antennae (fig. 4.5A); this is known as the “antennipedia” mutation. Some flies
developed two pairs of wings instead of the usual single pair (fig. 4.5B). Normal flies have
tiny knob-like balancing organs called halteres where the second pair of wings would be, but
these mutant flies have apparently changed their regulatory genes so that ancestral wings
appeared instead of the halteres.
From these early discoveries, molecular biologists have identified most of the Hox
genes in a number of organisms and found that nearly all animals (including flies, mice,
and humans) use a very similar set of Hox genes, with slight variations, subtractions, and
additions. Each Hox gene is responsible for the development of part of the organism and
all its normal organ systems (fig. 4.6). Small changes in the Hox genes can put different
appendages on a segment of a fly (like the leg where the antenna would go or the wing
where a haltere belongs) or even multiply the number of segments. Clearly, then, a tiny
change in Hox genes can make a big evolutionary difference. In the arthropods (the “jointed
legged” animals, such as insects, spiders, scorpions, and crustaceans), for example, a small
FIGURE 4.4. The Mexican salamander Ambystoma, known to the Aztecs as the axolotl. In normal conditions, it
retains its larval gills into sexual maturity, enabling it to remain aquatic. However, if the water becomes stag-
nant, it completes its metamorphosis into an adult salamander with lungs, so it can crawl out and find a new
lake to live in. By using different parts of its natural developmental cycle, it has great evolutionary flexibility.
(After Dumeril 1867)