WHAT IS IT? ----------- This is a simple model of population genetics. There are two populations, the REDS and the YELLOWS. Each has settable birth rates. The reds and yellows move around and reproduce according to their birth rates. When the carrying capacity of the terrain is exceeded, they die to maintain a relatively constant population. The model allows you to explore how differential birth rates affect the ratio of reds to yellows. HOW TO USE IT ------------- Each "tick" represents a generation in the time scale of this model. The NUMBER slider sets the carrying capacity of the terrain. The model is initialized to have a total population of NUMBER with half the population reds and half yellows. The RFERTILITY slider sets the average number of children each red has in a generation. It is set to increment in units of 0.01. The DIFF-CHILD slider sets the difference in birth rates between the red and the yellow. For example if RFERTILITY is set to 1 and DIFF-CHILD is set to 0.02, then the yellow birthrate would be 1.02. If the DIFF-CHILD slider is set to -0.01, then the yellow birth rate would be 0.99. The RCOUNT and YCOUNT monitors display the number of reds and yellows respectively. The GO button runs the model. A running plot is also displayed of the numbers of reds, yellows and total population (in green). The RUN-EXPERIMENT button lets you experiment with many trials at the same settings. That way you can see the variance of number of generations till extinction. RUNNING THE MODEL ----------------- (1) THINGS TO NOTICE -------------------- How does differential birth rates affect the population dynamics? Does the population with a higher birth rate always start off growing faster? Does the population with a lower birth rate always end up extinct? (2) THINGS TO TRY ----------------- Try running an experiment with the same settings many times. Does one population always go extinct? How does the number of generations till extinction vary? EXTENDING THE MODEL ------------------- In this model, once the carrying capacity has been exceeded, every member of the population has an equal chance of dying. Try extending the model so that reds and yellows have different saturation rates. How does the saturation rate compare with the birthrate in determining the population dynamics? In this model, the original population is set to the carrying capacity (both set to NUMBER). Would population dynamics be different if these were allowed to vary independently? STARLOGO FEATURES ----------------- Note the use of floating point slider values and slider increments for the RFERTILITY and DIFF-CHILD sliders. Note the use of a remainder "idiom" to enable the fractional part of the birth rate. setfertrem (fert - (int fert)) if (random 100) < (100 * fertrem) [hatch []] Note also, the use of grim-reaper to keep the total population relatively stable: to grim-reaper settodie (turtle-total - number) setseed1 random turtle-total if seed1 < todie [die] end