Imagine that we have a large set of Legos and two children, one very industrious and the other very judgmental. The industrious little guy starts building, sticking Lego pieces together in no particular order. Every once in a while, his sister, who doesn't think highly of this undirected chaos of construction, grabs the Lego creation and gives a numerical rating expressing her feelings towards the latest Lego masterpiece. After she passes her judgment, her brother, either depressed or ecstatic, starts on a new design.
Eventually, he begins to run out of pieces and is forced to begin disassembling the old Lego objects to provide parts for the new ones, and naturally, he prefers to destroy the poorly rated structures first. As time goes on, assuming that he doesn't lose the desire to please his sibling, he will begin to learn what designs are acceptable to her. The Lego creations will become more complex and progressively higher rated, since he will be able to reuse entire old structures in the construction of new ones. (Credit to Eric Beinhocker's The Origin of Wealth for this example.)
Now picture another situation. Say we have a group of very lazy fish who like to eat but not to swim. They have long arms that they use to grab their prey, eliminating the need to exert themselves. One day a fish comes along that likes to swim, especially when it's hungry. This fish discovers that, despite being lazy slobs, these other guys are very tasty. The new fish tells its friends, and soon the happy sedentary community has a problem.
Of course, as you may have discovered playing tag in elementary school, when you're being chased all you have to do is outrun the slowest member of your group. The laziest fish start getting eaten and stop having children. Those that don't get eaten are, in general, ever so slightly faster – and they continue reproducing. Some children are slower than their parents and quickly turn into a snack, but others are faster and survive. Over time, the average speed of the lazy fish increases. They may, as a group, start developing new methods of propulsion – maybe their food-grabbing arms start to transform into flapping fins. You get the idea. The population was pressured, and it evolved.
Neither the Lego creations or the lazy fish chose to evolve; there was no design or pre-determined direction of change, just the simple process of the least successful individuals being destroyed, and the most successful surviving, with very small random changes being introduced into each new generation. This process is not limited to biological applications, though it's normally described in that context. The Lego creations were evolving too; the builder had no goal other than to satisfy the existing criteria for “fitness,” determined here by the preferences of his sister rather than by physics or the eating behavior of other animals.
Evolution can be thought of as an algorithm – a recipe of sorts with the goal of finding designs that are successful in their environments. Only a basic set of conditions must be present for evolution to operate. We need raw materials, like Legos or organic molecules. We need a method of self-replication, such as the ability to produce children or a useful little boy who constructs new versions of yourself. Along with replication, we need a method of introducing random variation, since without this, no new, novel designs – like fins – could ever be produced. Most importantly, we need a judgmental environment to provide some measure of fitness and a mechanism to promote those who are well adapted and remove those who are not. With these simple ingredients, we have evolution – the most successful construction technique ever discovered. (Just look at you!)
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