Knife, Fork, Gill and Wing

My thoughts upon reading Endless Forms Most Beautiful, by Sean B. Carroll

In the middle ages, says Prof Carroll, Europeans started eating with two knives, using one to spear their food and another to cut it. This redundancy opened an opportunity for one knife evolve to be more specialized for spearing, while sacrificing the cutting function. This division into knife and fork was followed by many more in sufficiently posh dining facilities - a veritable Cambrian explosion of cutlery.

A similar redundancy made room for arthropod specializations. Early arthropods had so-called biramous, or forked limbs, lots of them, but each fairly similar to all the others. Each consisted of a lower, leg-like limb used for walking, digging, or swimming, and an upper gill branch, used for absorbing oxygen.

The most dramatic arthropod changes have taken place in those who moved onto land. In the case of insects, only six legs have been retained, but many others have been converted to other uses: mouth parts, antenna, genitalia, etc. The gill branches took at least equally dramatic turns: in insects, two pairs of them became wings. In spiders, some became book lungs, others spinnerets, and still others, sex organs. The evolution can be traced because the developmental tool kit genes are conserved while the targets change dramatically.

Tool kit genes, recall, are turned on by a set of molecular switches, which are in turn controlled by the geography specifying genes. Such a gene can acquire new functions while keeping the old by acquiring a new switch. Thus, the distal-less gene whose original function was to declare "make a limb here", acquired a new function to "make a butterfly wing eyespot here" by adding a switch that was sensitive to the detailed geography of the developing butterfly wing.

Context is everything, though. How does distal-less tell developing scale cells to make suitable colors for eyespots instead of another leg in the middle of the wing? That requires still other genes and switches that know about the geographic location on the butterfly, and hence know that in this context the message is "form an eyespot" instead of "make a leg."