In order to elaborate further on the differentiation of species and organisms within species after the phylotypic stage and its relation to Hox genes, I thought it might be useful to discuss the evolution of wing number and form. In Sean B. Carroll's book, Endless Forms Most Beautiful, he describes this process by using the gill-to-wing theory. The gill-to-wing theory has much evidence that supports the evolution from crustacean gill branches to insect wings. Of course, there are many transitional evolutionary steps that this change required. For example, a mayfly nymph, a primitive winged adult, and a modern lepidopteran all have very different types and numbers of wings. Specifically, the mayfly nymph does not have any wings on T1, whereas the primitive winged adult has reduced T1 structures and no wings on the abdomen.
So, how does this differentiation of wing number and type occur across these species? According to Carroll, it is much related to my previous post that related this specilaization to Hox genes. In fact, the addition of more Hox gene binding sites allowed for the promotion and differentiation of wing development. Therefore, it is evident that Hox genes and their binding sites play a major role in determining the structures and forms of unique species.