Adaptationism - Anatomical Constraints

Anatomical Constraints

Anatomical constraints are features of organism's anatomy that are prevented from change by being constrained in some way. When organisms diverge from a common ancestor and inherit certain characteristics which become modified by natural selection of mutant phenotypes, it is as if some are traits are locked in place and are unable to change in certain ways. Some textbook anatomical constraints often include examples of structures that connect parts of the body together though a physical link.

These links are hard if not impossible to break because evolution usually requires that anatomy be formed by small consecutive modifications in populations through generations. Dr. Randolph Nesse, in his book: Why We Get sick explains that the "blind spot" in the vertebrate eye caused by the nerve fibers running in front of the retina and blocking vision in parts. He says that natural selection has come up with an elaborate work-around of the eyes wobbling back-and-forth to correct for this, but vertebrates have not found the solution embodied in cephalopod eyes, where the optic nerve does not obstruct the view. He goes on to say that a likely reason for this is that in order for the nerve to migrate over evolutionary time it would have to be disconnected or "cut" at some point to complete the trip. This disconnection would not allow for the message to go from the eye to the brain and this would render those mutant individuals blind, so making the transition virtually impossible. See also: Evolution of the eye.

Other examples often cited for the same reason are the cranial nerves in tetrapods. Some of the best derived models of early vertebrate evolution have been sharks, skates, and rays (collectively chondrichthyes). In chondrichthyes the cranial nerves run from the part of the brain that interprets sensory information, and radiate out towards the organs that produce those sensations. In tetrapods however, and mammals in particular, the nerves take an elaborate winding path through the cranium. They go up down and around structures that evolved after the common ancestor with sharks. This is because when their locations shifted over time each stage had to remain connected. See also: Cranial nerves.

It is not possible to have a number of generations of organisms that are blind (as in the vertebrate eye example) or generations with areas that are not "innervated", nor is it possible that in transition the "spermatochord" becomes severed, then continue to migrate to reach around the pelvis. These would not work because the intermediates with severed connections would be poorly suited for their environments and would fail to reproduce. So these links are maintained throughout millions of years simply because departure from the status quo is inviable.