Recombination restriction between evolving sex chromosomes leads to the 
degeneration of the chromosome that is present only in the heterogametic sex 
(the Y chromosome in XY species). The evolutionary forces driving Y chromosome 
degeneration, however, are still under debate and include positive and negative 
selection models. In a recent study, we showed that the rate of accumulation of 
loss-of-function mutations on the neo-Y chromosome of Drosophila miranda is 
compatible with the process of Muller's ratchet, the stochastic loss of the best 
mutational class of individuals from a small asexual population. Purifying 
selection at amino acid sites can accelerate the ratchet, and the speed of 
degeneration depends on the number of genes still present on the evolving Y 
chromosome. Our study shows that Y chromosome degeneration does not require the 
action of selective sweeps at linked sites, and can take place under realistic 
parameters of purifying selection only.