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Seeing myself speeding into a steepening spiral of more and more complicated models, containing turtles with more and more 'traits' interacting in more and more complex ways, I have gone back to my earlier model with only one trait and started working in a different direction.  Acting on  Paul Grobstein's suggestion, I'm working on more ways to visualize the way simple evolutionary processes (randomness and the left-wall effect) can affect the distribution of trait values in a population.  The current iteration of my model calculates and displays a 'trait diversiy index,' which measures the diversity of trait values within the population using Simpson's Diversity Index.  Next I added a plot that shows how trait values branch out in a tree-like form from one or several initial starting points.  I'm still looking for further ways to refine my model and make it more useful and powerful without necessarily cluttering up the interface with lots of options.