Not sure if this qualifies as pen-and-paper, but one game I am familiar with to highlight natural selection goes something like this:
Creatures - Small paper pieces of different colors from construction paper - hole-punch remnants work great, or you could cut squares
"Habitats" - Large, colorful surfaces. Whimsical maps work well for this, maybe certain posters, the important thing is to have some diversity.
This is ideally done in multiple groups, each group should start with an equal ratio of creatures of at least two different colors.
One or more people spread the Creatures around the Habitat.
Another student (or multiple) act as the "predators". They sit/stand above the board and close their eyes; then they open their eyes, and harvest the first Creature they see. Close eyes again, and repeat until 1/2 of the Creatures have been eaten.
Count up the paper pieces and note the number of each remaining; for the next Generation, the Creature makeup should be 2X the number of remaining pieces: so if there are 30 Blue and 20 Red remaining, the new generation is 60 Blue and 40 Red.
Repeat over a few generations - the result should be that different habitats lead to different proportions. You can also change things up by suddenly shifting the habitat!
You could possibly include genotypes and recessive traits for an older group (the version I suggested is effectively haploid parthenogenesis), these might be harder to keep track of.
(I don't have a citation for this game but if anyone knows it, please provide in the comments!)