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SETUP, STEP, and GO are standard.

TRAIT-VALUE-DISTRIBUTION controls the initial distribution of trait values within the population upon SETUP. RANDOM sets each to a random value from 0 to 9, SET TO SLIDER sets the value for all turtles to the value set by the TRAIT-VALUE1 slider. SET TO SLIDERS sets the value for half of the turtles to the value set by TRAIT-VALUE1 and sets the value for the other half to TRAIT-VALUE2.

The GEN monitor reports the average generation of the turtles. The MEAN TRAIT VALUE monitor reports the average trait value for all turtles. The distribution of trait values is shown in the TRAIT VALUE DISTRIBUTION plot. The TRAIT DIVERSITY plot and monitor keeps track of the diversity of trait values in the population, calculated using Simpson's Diversity Index. A value of zero indicates that all turtles have the same trait value; a value of one indicates that trait values are exactly evenly distributed in the population.

Start with trait values setup randomly. Watch how the average trait value, trait value distribution, and trait diversity change (or don't change) over time. Next, set traits to a single value (try 1). Now how do the mean value and diversity change over time? Do they stabilize? If so, where?

This demonstrates the left-wall effect, which is one of the ways evolutionary trends can occur in the absence of selection pressure. With an absolute minimum value and random change, there is a tendency for the mean value to move away from the minimum, and for diversity to increase. (This model in fact demonstrates both a left-wall effect and a right-wall effect, meaning that there is a limit at both ends.) An example of a left-wall effect in the real world is the trend in animals toward larger sizes. There is no necessary advantage to being larger (thus no selective pressure), but beyond a certain limit an organism can't get any smaller, so with random change over countless generations, there has been a shift toward a larger mean body size (and an increase in the diversity of body sizes).