The evolution of complex characters—Two Themes: 1)Complex characters can evolve gradually via intermediate forms (if those forms have adaptive value) 2)Cooption of pre-existing genes for new purposes We will focus first on #1. “What good is half an eye?”

Even single-celled organisms can sense light Euglena respond to light by moving flagellum Does this lowly form of vision have adaptive value? What could it be?

These are representatives of different eye types currently found in nature. They may be similar to the intermediate stages long ago passed on the way to our eye. It is unlikely to evolve a complex eye “from nothing”—i.e., Boeing 747 mutations. But incremental improvements from simple photoreceptors to more complex eyes are very likely.

Fig (a) Patch of pigmented cells-limpet Optic nerve Pigmented layer (retina) Pigmented cells (photoreceptors) Fluid-filled cavity Epithelium (c) Pinhole camera-type eye- Nautilus Optic nerve Cornea Retina Lens (e) Complex camera-type eye-squid (d) Eye with simple lens- marine snail Optic nerve Cornea Cellular mass (lens) (b) Eyecup-slit-shell mollusc Pigmented cells Nerve fibers

theory-richard-dawkins-on-eye-evolution theory-richard-dawkins-on-eye-evolution

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