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

178 | HOW THE MIND WORKShave built in by guesswork, tradition, or trial and error. The innate specsinclude how many units there are, how they are connected, what the initialconnection weights are, and how much the weights should benudged up and down on each learning episode. Simulated Evolutiongives the networks a big head start in their learning careers.So evolution can guide learning in neural networks. Surprisingly,learning can guide evolution as well. Remember Darwin's discussion of"the incipient stages of useful structures"—the what-good-is-half-an-eyeproblem. The neural-network theorists Geoffrey Hinton and StevenNowlan invented a fiendish example. Imagine an animal controlled by aneural network with twenty connections, each either excitatory (on) orneutral (off). But the network is utterly useless unless all twenty connectionsare correctly set. Not only is it no good to have half a network; it isno good to have ninety-five percent of one. In a population of animalswhose connections are determined by random mutation, a fitter mutant,with all the right connections, arises only about once every million (2 20 )genetically distinct organisms. Worse, the advantage is immediately lostif the animal reproduces sexually, because after having finally found themagic combination of weights, it swaps half of them away. In simulationsof this scenario, no adapted network ever evolved.But now consider a population of animals whose connections cancome in three forms: innately on, innately off, or settable to on or off bylearning. Mutations determine which of the three possibilities (on, off,learnable) a given connection has at the animal's birth. In an avelrage animalin these simulations, about half the connections are learnable, theother half on or off. Learning works like this. Each animal, as it lives itslife, tries out settings for the learnable connections at random until ithits upon the magic combination. In real life this might be figuring outhow to catch prey or crack a nut; whatever it is, the animal senses itsgood fortune and retains those settings, ceasing the trial and error. Fromthen on it enjoys a higher rate of reproduction. The earlier in life the animalacquires the right settings, the longer it will have to reproduce at thehigher rate.Now with these evolving learners, or learning evolvers, there is anadvantage to having less than one hundred percent of the correct network.Take all the animals with ten innate connections. About one in athousand (2 10 ) will have all ten correct. (Remember that only one ; in a millionwowlearning animals had all twenty of its innate connections correct.)That well-endowed animal will have some probability of attaining the