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?”

http://www.youtube.com/watch?v=J3sgoy22Ij4

Even single-celled organisms can sense lightEuglena 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. 25-24

(a) Patch of pigmented cells-limpet

Opticnerve Pigmented

layer (retina)

Pigmented cells(photoreceptors)

Fluid-filled cavity

Epithelium

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

CorneaCellularmass(lens)

(b) Eyecup-slit-shell mollusc

Pigmentedcells

Nerve fibers Nerve fibers

• http://richarddawkins.net/videos/527044-bang-goes-the-theory-richard-dawkins-on-eye-evolution

http://www.pbs.org/wgbh/nova/evolution/intelligent-design-trial.

htmlWatch Behe at 1:06

• http://cosmiclog.msnbc.msn.com/_news/2011/04/08/6433919-intelligent-design-in-tenn-schools