Natural Selection Simulation: Color, Predation, and Population Change
An eighth-grade class models natural selection with paper dots on fabric. Each team scatters 50 green dots and 50 brown dots on a 1 m × 1 m cloth. A 'bird' picks up 20 dots in 15 seconds using tweezers. Survivors reproduce: each surviving dot is replaced by two dots of the same color, keeping the population at 100. On green cloth, the survivors after generation 1 are 32 green and 8 brown; on brown cloth, they are 9 green and 31 brown. After three generations on green cloth, the population is 78 green and 22 brown. One student says, 'The green dots learned to hide better, so they survived.' Another says, 'The birds are intentionally choosing brown dots to keep the population balanced.' The teacher asks the class to interpret the simulation using variation, selection pressure, and differential reproduction, and to predict what would happen if the cloth color changed every generation.
1. Identify the selection pressure in the simulation and explain why the green-cloth population changed.
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The selection pressure is the 'bird'-the student using tweezers-because it removes dots that are easier to see. On green cloth, green dots have lower contrast against the background, so more survive: 32 green and 8 brown after generation 1. Brown dots are more visible and are eaten at a higher rate.
The survivors then reproduce, and each survivor is replaced by two dots of the same color. Because more green dots survive, the next generation starts with more green dots. Repeating this cycle for three generations produces 78 green and 22 brown. The population changes because of differential survival and reproduction, not because any individual dot changes color.
2. Critique the statements that the green dots learned to hide and that the birds chose brown dots to keep the population balanced.
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The 'learned to hide' statement is incorrect because paper dots do not behave, learn, or acquire traits during their lifetime. The variation in color already existed at the start. The green dots survived because their color made them less visible on green cloth, and that heritable variation was passed to the next generation.
The 'birds chose to keep the population balanced' statement is also incorrect. The birds did not intend to balance anything; students simply picked the dots they could see most easily. On green cloth, that meant brown dots were removed more often. On brown cloth, the result reversed: 9 green and 31 brown survived, showing that the environment determines which variation is advantageous. Balancing is an outcome of the sampling, not a goal.
3. Predict what would happen if the cloth color changed every generation and explain your reasoning.
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If the cloth alternates between green and brown each generation, the advantage reverses each time. On green cloth, green dots survive better; on brown cloth, brown dots survive better. Neither color can be consistently favored, so the population is unlikely to become fixed for one color. Instead, both colors may persist, and their proportions may oscillate depending on which background was used most recently.
This prediction follows from the mechanism: selection acts on the match between dot color and background. When the environment changes direction each generation, directional selection does not continue long enough to eliminate the other color. The simulation would need to track actual generation-by-generation numbers to see whether the oscillation dampens or continues.