And prions don't have descent with modification. It's a regular mammal brain protein that got accidentally constructed backwards. There's nothing to pass on, nothing to modify.
> Prion replication is subject to epimutation and natural selection just as for other forms of replication, and their structure varies slightly between species.[19]
[19] is https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2848070