204 Evolution? The Fossils Say YES!
Once arthropods evolved, they diverged into a wide variety of body forms, from
elongate multilegged centipedes and millipedes, to six-legged insects and eight-legged
spiders, to the huge diversity of crustaceans. How did the onychophoran “kind” turn into
the millipede “kind,” the insect “kind,” the spider “kind,” the crustacean “kind,” and so
on? As we saw in chapter 4, the answer lies in the Hox genes and their ability to dramati-
cally change body form through small changes in gene regulation. We have already seen
how minor changes in the Hox genes produce homeotic mutants such as flies with legs
on their heads or with four wings (fig. 4.5). Arthropods are particularly suited to this type
of evolution because they have a modular construction with multiple segments, and each
segment bears appendages that can be easily changed from a leg to a wing to an antenna
to a pincer to mouthparts. Experiments have shown that a few Hox genes cause arthro-
pods to add or subtract segments, and other Hox genes can produce whatever appendage
is needed (fig. 8.18). Ronshaugen et al. (2002) put a shrimp Ubx Hox gene into an insect
larva and showed how this gene was responsible for suppressing the development of
limbs in insects (which have 6 legs, compared to the 10 in most crustaceans). Lewis et al.
(2000) and Pearson et al. (2005) have shown how by manipulating Hox genes you can get
just about any type or number of appendage on each segment of an arthropod, therefore
making radical changes in body plan with a simple gene change. And there are many fos-
sils that show primitive arthropods with more than two sets of wings, as in dragonflies,
showing that the creature in figure 8.18 is not imaginary (Kulakova-Peck 1978; Raff 1998).
Ironically, creationists attacked the first edition of this book by focusing just on one figure,
figure 8.18, and claiming that it’s “made up.” Because of their ignorance, they didn’t real-
ize there are fossils of insects with six, eight, and even more wings, so if anything, this
multiwinged insect is a bit too conservative.
In addition, arthropods can undergo radical changes in body form each time they shed
their exoskeleton during molting. Think about how radically the body is rearranged from
FIGURE 8.17. Some examples of marine lobopod
onychophoran fossils from the middle Cambrian
Burgess Shale of Canada. (A) Aysheaia. (B) Hallucigenia.
(Photos courtesy S. Conway Morris)
(A)(B)