Steven Pinker -- How the Mind Works - Hampshire High Italian ...

164 J HOW THE MIND WORKSthan its rivals, it can increase its representation in a population from 0.1percent to 99.9 percent in just over four thousand generations, A hypotheticalmouse subjected to a selection pressure for increased size that isso weak it cannot be measured could nonetheless evolve to the $ize of anelephant in only twelve thousand generations.More recently, computer simulations from the new field of ArtificialLife have shown the power of natural selection to evolve organisms withcomplex adaptations. And what better demonstration than everyone'sfavorite example of a complex adaptation, the eye? The computer scientistsDan Nilsson and Susanne Pelger simulated a three-layer slab of virtualskin resembling a light-sensitive spot on a primitive organism. It wasa simple sandwich made up of a layer of pigmented cells on the bottom,a layer of light-sensitive cells above it, and a layer of translucent cellsforming a protective cover. The translucent cells could undergo randommutations of their refractive index: their ability to bend light, Which inreal life often corresponds to density. All the cells could undergo smallmutations affecting their size and thickness. In the simulation, the cellsin the slab were allowed to mutate randomly, and after each round ofmutation the program calculated the spatial resolution of an image projectedonto the slab by a nearby object. If a bout of mutations improvedthe resolution, the mutations were retained as the starting point for thenext bout, as if the slab belonged to a lineage of organisms whose survivaldepended on reacting to looming predators. As in real evolution,there was no master plan or project scheduling. The organism could notput up with a less effective detector in the short run even if its patiencewould have been rewarded by the best conceivable detector in the longrun. Every change it retained had to be an improvement.Satisfyingly, the model evolved into a complex eye right on the computerscreen. The slab indented and then deepened into a cup; the transparentlayer thickened to fill the cup and bulged out to form a cornea.Inside the clear filling, a spherical lens with a higher refractive indexemerged in just the right place, resembling in many subtle details theexcellent optical design of a fish's eye. To estimate how long it would takein real time, rather than in computer time, for an eye to unfold, Nilssonand Pelger built in pessimistic assumptions about heritability, variation inthe population, and the size of the selective advantage, and even forcedthe mutations to take place in only one part of the "eye" each generation.Nonetheless, the entire sequence in which flat skin became a complexeye took only four hundred thousand generations, a geological instant.