In a simulation of natural selection like the owl and mouse simulation, a new trait arising from a mutation could be modeled by introducing a new allele or gene variant into the population.
To do this, we could modify the program code to include a new mutation function that can generate a new allele with a slightly different version of the gene responsible for the trait. For example, if the trait is the color of the fur, we could introduce a new mutation that changes the color of the fur in a small way, such as making it slightly lighter or darker.
We could then introduce the mutated allele into the population by randomly selecting a few individuals from the population and assigning them the new allele. Over time, as the population reproduces, the new allele will become more common if it confers an advantage or if it is neutral.
To determine whether the new trait is advantageous, we could run the simulation for several generations and observe the changes in the frequency of the new allele. If the frequency of the new allele increases over time, it suggests that the new trait is beneficial and is being positively selected for. On the other hand, if the frequency of the new allele decreases over time, it suggests that the new trait is detrimental and is being negatively selected against.
Overall, modeling a new trait arising from a mutation in a simulation of natural selection involves modifying the program code to include a new allele, introducing the new allele into the population, and observing the changes in its frequency over time to determine whether it confers an advantage or a disadvantage.