The eye of a vertebrate or an octopus looks so complex that it can be difficult to believe it could have evolved by natural selection and it has traditionally been an argument against Darwinism by advocates of creationism.

Nilsson and Pelger simulated a model of the eye to find out how difficult its evolution really is.

The simulation does not cover the complete evolution of an eye. It takes light-sensitive cells as given and ignores the evolution of advanced perceptual skills (which are more a problem in brain, than eye, evolution). It concentrates on the evolution of eye shape and the lens; this is the problem that Darwin's critics have often pointed to, because they think it requires the simultaneous adjustment of many intricately related parts.

Nilsson and Pelger allowed the shape of the model eye to change at random, in steps of no more than 1% change at a time. This fits in with the idea that adaptive evolution proceeds in small gradual stages. The model eye then evolved in the computer, with each new generation formed from the optically superior eyes in the previous generation; changes that made the optics worse were rejected, as selection would reject them in nature.

How long did it take?

The complete evolution of an eye like that of a vertebrate or octupus took about 2000 steps.

Nilsson and Pelger used estimates of heritability and strength of selection to calculate how long the change might take; their answer was about 400,000 generations. Far from being difficult to evolve, the model shows that it is rather easy.

The work illustrates the value of building models to test our intuitions. Darwin referred to the evolution of complex organs by natural selection as presenting a problem for the imaginatio, not the reason. This computer study supports his remark.

An animation illustrates this fascinating simulation.

Figure: later stages in the evolution of the eye. In (a) the eye is protected by adding a transparent cover of skin; and part of the cellular fluid has differentiated into a lens. (b) Full, complex eye as found in octopus and squid.