Background Science Knowledge for Teachers

Evolution is not a truly random process, nor a purely deterministic one. Over generations, individuals who are better adapted to the current environmental pressures are most likely to survive, while those more poorly adapted are less successful. Over generations, the slight offspring advantage enjoyed by these better adapted individuals causes their genes to crowd out those less fit, and the population comes to be almost exclusively dominated by the more fit genes. But this is not a static state, it is at equilibrium. All of the evolutionary pressures have to be carefully balanced. For example, male cardinals (the bird), have a strong incentive to avoid their natural predators (hawks and cats), by being drably colored and patterned as camouflage. But there is apparently a stronger pressure for them to be brightly colored to impress females. So there is competition between males to be the most brightly colored. However, if all predators of cardinals were removed, surely the male birds would become considerably brighter within a few generations, because the predators act as a negative force pushing against the brightness of the cardinals. Perhaps more intriguingly, the more pronounced change in coloration might come in female cardinals, who clearly were suffering more natural selection pressures as they were the ones who had adapted to it by becoming drab.

Thus, every species that we see is currently reacting to their current environmental pressures, just very, very slowly and at a subtle genetic level. These evolutionary forces are in addition to the plasticity at the individual level, which is an additional source of variation among individuals. Thus it can be very difficult to detect the effects of evolution at a small scale.

Evolution can be a difficult topic, because while the rules which govern it are simple, it is not easily observable because its effects only become apparent over many generations at the level of the entire population. The background information here is simple:

  1. Guppies and other fish species that live in diverse habitats, over evolutionary time, can "tune" their sensory systems to better respond to the stimuli available where they live. For example, fish living in a muddy environment need not be adapted to see blues, because that wavelength of light is mostly absorbed.
  2. Once a sensory bias is established by natural selection, sexual selection piggybacks on top of this. As females become more drawn to bright colors, males benefit from being brightly colored.
  3. However, there is a natural selection counterweight to this, as bright colors make males more likely to be eaten by predators. Thus, the population settles into an evolutionarily stable state, with males becoming bright, but not too bright, and fish becoming attracted, but not too attracted to bright colors.
  4. To teach a rule-based system with emergent properties, games are a valuable media. They let students see how order emerges from many, many, aggregated random events. The Guppy Game simulates several generations of natural selection without the need to do in house breeding cycles.