This population of squirrels is changed due to the natural disaster that sweeps through their habitat. Some squirrels survive and pass their genes on. Other genes are killed off in the fire.
The population of these two birds is affected when one white bird decides to travel across the river to a place where white birds haven't been present for years. This affects the alleles in both populations.
In this population of cows the mom births four cows. By her fourth one her baby ends up with a mutation that makes her a black cow with white spots unlike all the other babies that are white cows with black spots. This changes the alleles of future generations.
These fish are choosing to mate with only the other red fish in the ocean leaving the babies to also be red fish. This will eventually wipe out the purple gene alleles if it is not passed on.