Muller's ratchet

Muller's Ratchet is like a game. Every time you play the game, you can lose points. The more times you play the game, the more points you can lose, and eventually you can lose so many points that you can't play the game anymore.

In nature, Muller's Ratchet is a process that affects certain kinds of cells, like bacteria. Every time these cells reproduce, mutations (or mistakes) can happen. Most of these mutations are bad for the cell and can cause it to not work as well, which makes it harder for the cell to survive and reproduce. As more and more mutations happen, eventually the cell can no longer reproduce, and it can only lose points until it is completely gone.