Having been asked to demonstrate natural selection, I have created a very easy to understand simulation. It took some hours to do, but this is the kind of thing I do all the time in my work.
Imagine creatures, prey animals, that have three genes we are looking at: r, g, and b. As usual in genetic problems each gene can be dominant (G) or recessive (g), so a creature could be RgB or RGb or so on.
As you may have guessed, these stand for Red, Green, and Blue in the simulation. A creature with a dominant G will have some green in its color, same for Red (R) and Blue (B). recessive (r,g,b) have none of that color.
Prey animals can reproduce creating offspring that can have combinations of their genes only.
Here is a video of the simulation running.
The selective pressure is provided by predator creatures. A predator prefers prey animals with a G gene, ignoring the others.
What we should see if natural selection is at play is that the percentage of prey with G should decrease, not just because of predation but also reproduction can only recombine existing DNA.
Here is a video of the simulation running. Colors indicate genotype, so white circles have RGB, yell are RGb, green are rGb, and so on. Squares are predators.