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Emergence 2009: Final Projects, V


Biology 361 = Computer Science 361
Bryn Mawr College, Spring 2009


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Download/view: NonSelective.nlogo


This model demonstrates some basic trends in evolution which occur without selection pressure.

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).

Each turtle has a variable that stores its trait value. Turtles get older with every tick, and when they have lived for forty ticks, they reproduce, making turtles with trait values distributed normally around their own trait value. They die after reproducing, and the number of offspring they produce is based on the number of living turtles, which keeps the population stable. Any turtle that is given a trait value outside the range from 0 to 9 (which happens when a turtle with a trait value of 0 or 9 reproduces) dies, producing the left- and right-wall effects. The movement of turtles is random, but doesn't actually do anything particular in this version of the model except make it slightly more interesting to watch, but it is crucial in version that incorporates selection.


Models created using NetLogo.


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