In a pond there are two different sized fish. Some of the fish are large and some are small.
Fishers discover the pond and the bigger fish are easier to see because of their size. The bigger fish are caught and killed and the small fish are able to survive since they can't be seen as easily.
Fishing season is over and there are very few large fish that have survived. The small fish start to reproduce and pass on the beneficial genetic trait. For many years the pond has more small fish and the people can't see them as easily so they survive.
The winter that year is very cold and the pond starts to freeze over.
The small fish become too cold and slowly die out. The larger fish live long enough to swim down to the bottom of the pond and find shelter from the cold.
The large fish survive the winter and in the spring are able to reproduce. Many of the small fish died out and their population is smaller.