The pocket mice that evolved on dark lava rock had a selective pressure associated with being light or dark—owls and other predators could more easily see the light pocket mice than the dark pocket mice (see "Pocket Mouse and Predation" animation). By using a mathematical formula from population genetics called the Hardy-Weinberg equation, we can calculate how quickly a given advantageous trait will spread through a population. The main variable we will consider is called s (selection coefficient). It is a measure of how much of an advantage is given to the individual with that trait. An s equal to 0.01 is a very modest 1% selective advantage—for every 100 light-morph offspring that survive, 101 dark morphs survive. In this case, the dark variation will make up 95% of the population in about 1,000 generations. With s equal to 0.1—a more significant but still moderately modest 10% advantage—the population becomes almost all dark colored in only 100 generations, illustrating how a rapid evolutionary change can occur even when the selective advantage is relatively low.


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