Spineless Wonders of Evolution 205a caterpillar to an adult moth or butterfly, or from a maggot to an adult fly. Thus we can
experimentally show that macroevolutionary transition from one body form to another with
a completely different number of segments and appendages is a very easy process. No won-
der arthropods are the most successful, abundant, and diverse organisms on earth. After we
humans are long gone, the cockroaches and other insects will still rule the earth, as they have
for over 300 million years so far.
In summary, the molecular-anatomical-embryological phylogeny of the animal king-
dom (fig. 5.7) links mollusks and annelids; we have transitional forms, both fossil and
living, between these two phyla. It also links nematodes and arthropods, and we have
transitional forms, both fossil and living, between these two phyla as well. Using Hox
genes, we can demonstrate how radical changes in body plans are controlled by relatively
simple genetic mechanisms and allow macroevolutionary changes to take place. As we
shall see in the next chapter, we also have abundant transitional forms that link the ver-
tebrates and the echinoderms and show us the earliest ancestry of the vertebrates from
soft-bodied ancestors.
FIGURE 8.18. The evolutionary mechanism by which Hox genes allow arthropods to make drastic changes in
their number and arrangements of segments and appendages, producing macroevolutionary changes with a
few simple mutations (see fig. 4.6). (Drawing by Carl Buell)