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
Prey animals can reproduce creating offspring that can have combinations
of their genes only. All prey animals at he beginning of the simulation have
a GG genotype (are green) but the R and B genes will affect how the color
appears on the screen.
Every once in a while a mutation will occur, changing a G into a g. The color
is still expressed as G but the mutation adds a recessive into the mix. Eventually
this recessive is a selective advantage, because predators only eat G types.
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.
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.