Hi Bruno, it always makes me really happy to hear that this little simulator is being used for educational purposes, thanks for that!
The "fitness" score in this simulation is perfectly correlated to the creature's proficiency at the task. For example, the fitness score for the running task is computed as the horizontal distance from the start divided by a maximum distance value that a creature would have to reach in order to receive a perfect score of 100%. This maximum distance has been arbitrarily chosen by me. It obviously also scales with the simulation time of each generation, so that increasing that time doesn't make it any easier for the creatures to achieve a higher fitness score.
During the reproduction step, two creatures are selected and their chromosomes are recombined using 1-point crossover, which produces two new offspring. This step is repeated until the new generation has reached the chosen population size. Creatures with a higher fitness score have a higher chance of being selected as parents for reproduction and since the same creature can also be selected multiple times, the fitness score of a creature effectively contributes to the number of offspring that will carry part of the genetic information of this parent creature into the next generation.
I might allow the user to select between different selection and recombination strategies in a future update, in order to offer a few more options for everybody to experiment with.