Historicity and constraint
This book is about natural selection, with all discussion constrained by acceptance of the two other doctrinal bases of modern biology, mechanism and historicity. In accepting mechanism I assume that all the laws of chemistry and physics are obeyed by living organisms, in their individual activities, in the workings of their minutest parts, and in their ecological interactions. This chapter will deal with the more generally unappreciated doctrine of historicity and with related evolutionary constraints (phylogenetic, developmental, genetic) on the action of natural selection.
As noted in Chapter 1, every organism shows features that are functionally arbitrary or even maladaptive. I illustrated this idea with two examples from mammalian necks, the number of cervical vertebrae and the crossing of digestive and respiratory systems. Here I will present two others, a classic anatomical example and another less widely appreciated.
My chosen classic is the vertebrate eye. It was used by Paley (Appendix) as a particularly forceful part of his theological argument from design. As he claimed, the eye is surely a superbly fashioned optical instrument. It is also something else, a superb example of maladaptive historical legacy. The retina consists of a series of special layers in the functionally appropriate sequence. A layer of light-sensitive cells (rods and cones) stimulate nerve endings from one or more layers of ganglion cells that carry out initial stages of information processing. From these ganglia, nerve fibers converge to form the main trunk of the optic nerve, which conveys the information to the brain. All layers are served by blood capillaries that provide their metabolic requirements. Unfortunately for Paley's argument, the retina is upside down. The rods and cones are the bottom layer, and light reaches them only after passing through the nerves and blood vessels.