We study partition distributions in a population genetics model incorporating symmetric selection and mutation. They generalize Ewens distributions in the infinitely-many-neutral-alleles model, an explicit expression of which is known as the Ewens sampling formula. A sampling formula for the generalized model is obtained by means of calculus for Poisson and gamma processes.
"Sampling Formulae for Symmetric Selection." Electron. Commun. Probab. 10 223 - 234, 2005. https://doi.org/10.1214/ECP.v10-1159