In evolutionary genetics, Muller's rachet describes a process where deleterious (harmful) mutations accumulate because of the absence of recombination, so this happens when there is no sexual reproduction.
This happens because the offspring inherits the mutation of their parent, but is also subject to mutation, so they add up every generation.
Muller (the scientist that first described this phenomenon) suggested that this is why sexual reproduction is favored over asexual reproduction, and why other forms of recombination exist.