There are two types of creatures but the same species, called ions, some are brown and others are pick.
Differential
The brown ions can camouflage into the trees around them but the pinks stick out which makes them easy to pick out. The brown are passing down their color to the offspring that are able to survive
Reproduction
The brown ions are being able to reproduce as they are able to survive, the pinks are able to reproduce but they aren't growing in size
Overtime
The pink ions are being spotted by predators easily and being eaten or taken away while the brown ions are camouflaging into their habitat making it harder for them to be spotted
The brown ions are being able to reproduce and increase in population while the pinks are decreasing slowly
Over time the pinks start to die while the brown ions start to thrive, reproduce and survive.